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Digital electronics Document Transcript

  • 1. Basics of ElectronicsElectronicsRelationship between Voltage, Current ResistanceAll materials are made up from atoms, and all atoms consist of protons, neutrons and electrons. Protons, have apositive electrical charge. Neutrons have no electrical charge while Electrons, have a negative electrical charge.Atoms are bound together by powerful forces of attraction existing between the atoms nucleus and the electronsin its outer shell. When these protons, neutrons and electrons are together within the atom they are happy andstable. However, if we separate them they exert a potential of attraction called a potential difference. If wecreate a circuit or conductor for the electrons to drift back to the protons the flow of electrons is called a current.The electrons do not flow freely through the circuit, the restriction to this flow is called resistance. Then allbasic electrical or electronic circuit consists of three separate but very much related quantities, Voltage, ( v ),Current, ( i ) and Resistance, ( Ω ).VoltageVoltage, ( V ) is the potential energy of an electrical supply stored in the form of an electrical charge. Voltagecan be thought of as the force that pushes electrons through a conductor and the greater the voltage the greater isits ability to "push" the electrons through a given circuit. As energy has the ability to do work this potentialenergy can be described as the work required in joules to move electrons in the form of an electrical currentaround a circuit from one point or node to another. The difference in voltage between any two nodes in a circuitis known as the Potential Difference, p.d. sometimes called Voltage Drop.The Potential difference between two points is measured in Volts with the circuit symbol V, or lowercase "v",although Energy, E lowercase "e" is sometimes used. Then the greater the voltage, the greater is the pressure (orpushing force) and the greater is the capacity to do work.A constant voltage source is called a DC Voltage with a voltage that varies periodically with time is called anAC voltage. Voltage is measured in volts, with one volt being defined as the electrical pressure required to forcean electrical current of one ampere through a resistance of one Ohm. Voltages are generally expressed in Voltswith prefixes used to denote sub-multiples of the voltage such as microvolts ( μV = 10-6 V ), millivolts ( mV =10-3 V ) or kilovolts ( kV = 103 V ). Voltage can be either positive or negative.Batteries or power supplies are mostly used to produce a steady D.C. (direct current) voltage source such as 5v,12v, 24v etc in electronic circuits and systems. While A.C. (alternating current) voltage sources are available fordomestic house and industrial power and lighting as well as power transmission. The mains voltage supply inthe United Kingdom is currently 230 volts a.c. and 110 volts a.c. in the USA. General electronic circuits operateon low voltage DC battery supplies of between 1.5V and 24V d.c. The circuit symbol for a constant voltagesource usually given as a battery symbol with a positive, + and negative, - sign indicating the direction of thepolarity. The circuit symbol for an alternating voltage source is a circle with a sine wave inside.Voltage Symbols A simple relationship can be made between a tank of water and a voltage supply. The higher the water tankabove the outlet the greater the pressure of the water as more energy is released, the higher the voltage thegreater the potential energy as more electrons are released. Voltage is always measured as the differencebetween any two points in a circuit and the voltage between these two points is generally referred to as the"Voltage drop". Any voltage source whether DC or AC likes an open or semi-open circuit condition but hatesany short circuit condition as this can destroy it.Electrical CurrentElectrical Current, ( I ) is the movement or flow of electrical charge and is measured in Amperes, symbol i, forintensity). It is the continuous and uniform flow (called a drift) of electrons (the negative particles of an atom)around a circuit that are being "pushed" by the voltage source. In reality, electrons flow from the negative (-ve)terminal to the positive (+ve) terminal of the supply and for ease of circuit understanding conventional currentflow assumes that the current flows from the positive to the negative terminal. Generally in circuit diagrams theflow of current through the circuit usually has an arrow associated with the symbol, I, or lowercase i to indicatethe actual direction of the current flow. However, this arrow usually indicates the direction of conventionalcurrent flow and not necessarily the direction of the actual flow.In electronic circuits, a current source is a circuit element that provides a specified amount of current forexample, 1A, 5A 10 Amps etc, with the circuit symbol for a constant current source given as a circle with anarrow inside indicating its direction. Current is measured in Amps and an amp or ampere is defined as theK. Adisesha Page 1
  • 2. Basics of Electronicsnumber of electrons or charge (Q in Coulombs) passing a certain point in the circuit in one second, (t inSeconds). Current is generally expressed in Amps with prefixes used to denote micro amps ( μA = 10-6A ) ormilli amps ( mA = 10-3A ). Note that electrical current can be either positive in value or negative in valuedepending upon its direction of flow.Current that flows in a single direction is called Direct Current, or D.C. and current that alternates back andforth through the circuit is known as Alternating Current, or A.C.. Whether AC or DC current only flowsthrough a circuit when a voltage source is connected to it with its "flow" being limited to both the resistance ofthe circuit and the voltage source pushing it. Also, as AC currents (and voltages) are periodic and vary withtime the "effective" or "RMS", (Root Mean Squared) value given as Irms produces the same average power lossequivalent to a DC current Iaverage . Current sources are the opposite to voltage sources in that they like shortor closed circuit conditions but hate open circuit conditions as no current will flow.ResistanceThe Resistance, ( R ) of a circuit is its ability to resist or prevent the flow of current (electron flow) throughitself making it necessary to apply a greater voltage to the electrical circuit to cause the current to flow again.Resistance is measured in Ohms, Greek symbol ( Ω, Omega ) with prefixes used to denote Kilo-ohms ( kΩ =103Ω ) and Mega-ohms ( MΩ = 106Ω ). Note that Resistance cannot be negative in value only positive.Resistance can be linear in nature or non-linear in nature. Linear resistance obeys Ohms Law and controls orlimits the amount of current flowing within a circuit in proportion to the voltage supply connected to it andtherefore the transfer of power to the load. Non-linear resistance, does not obey Ohms Law but has a voltagedrop across it that is proportional to some power of the current. Resistance is pure and is not affected byfrequency with the AC impedance of a resistance being equal to its DC resistance and as a result can not benegative. resistance is always positive. Also, resistance is an attenuator which has the ability to change thecharacteristics of a circuit by the effect of load resistance or by temperature which changes its resistivity.For very low values of resistance, for example milli-ohms, ( mΩ´s ) it is sometimes more easier to use thereciprocal of resistance ( 1/R ) rather than resistance ( R ) itself. The reciprocal of resistance is calledConductance, symbol ( G ) and represents the ability of a conductor or device to conduct electricity. In otherwords the ease by which current flows. High values of conductance implies a good conductor such as copperwhile low values of conductance implies a bad conductor such as wood. The standard unit of measurementgiven for conductance is the Siemen, symbol (S).Quantity Symbol Unit of Measure AbbreviationVoltage V or E Volt VCurrent I Amp AResistance R Ohms ΩOhms LawThe relationship between Voltage, Current and Resistance in any DC electrical circuit was firstly discovered bythe German physicist Georg Ohm, (1787 - 1854). Georg Ohm found that, at a constant temperature, theelectrical current flowing through a fixed linear resistance is directly proportional to the voltage applied acrossit, and also inversely proportional to the resistance. This relationship between the Voltage, Current andResistance forms the bases of Ohms Law and is shown below.Ohms Law RelationshipBy knowing any two values of the Voltage, Current or Resistance quantities we can use Ohms Law to find thethird missing value. Ohms Law is used extensively in electronics formulas and calculations so it is "veryimportant to understand and accurately remember these formulas".To find the Voltage, ( V )[V=IxR] V (volts) = I (amps) x R (Ω)To find the Current, ( I )[I=V÷R] I (amps) = V (volts) ÷ R (Ω)To find the Resistance, ( R )[R=V÷I] R (Ω) = V (volts) ÷ I (amps)K. Adisesha Page 2
  • 3. Basics of ElectronicsIt is sometimes easier to remember Ohms law relationship by using pictures. Here the three quantities of V, Iand R have been superimposed into a triangle (affectionately called the Ohms Law Triangle) giving voltage atthe top with current and resistance at the bottom. This arrangement represents the actual position of eachquantity in the Ohms law formulas.Ohms Law Triangleand transposing the above equation gives us the following combinations of the same equation:Then by using Ohms Law we can see that a voltage of 1V applied to a resistor of 1Ω will cause a current of 1Ato flow and the greater the resistance, the less current will flow for any applied voltage. Any Electrical device orcomponent that obeys "Ohms Law" that is, the current flowing through it is proportional to the voltage across it(I α V), such as resistors or cables, are said to be "Ohmic" in nature, and devices that do not, such as transistorsor diodes, are said to be "Non-ohmic" devices.Power in Electrical CircuitsElectrical Power, (P) in a circuit is the amount of energy that is absorbed or produced within the circuit. Asource of energy such as a voltage will produce or deliver power while the connected load absorbs it. Thequantity symbol for power is P and is the product of voltage multiplied by the current with the unit ofmeasurement being the Watt (W) with prefixes used to denote milliwatts (mW = 10-3W) or kilowatts (kW =103W). By using Ohms law and substituting for V, I and R the formula for electrical power can be found as:To find the Power (P)[P=VxI] P (watts) = V (volts) x I (amps) Also,[ P = V ÷ R ] P (watts) = V2 (volts) ÷ R (Ω) Also, 2[ P = I2 x R ] P (watts) = I2 (amps) x R (Ω)Again, the three quantities have been superimposed into a triangle this time called the Power Triangle withpower at the top and current and voltage at the bottom. Again, this arrangement represents the actual position ofeach quantity in the Ohms law power formulas.The Power TriangleOne other point about Power, if the calculated power is positive in value for any formula the component absorbsthe power, but if the calculated power is negative in value the component produces power, in other words it is asource of electrical energy. Also, we now know that the unit of power is the WATT but some electrical devicessuch as electric motors have a power rating in Horsepower or hp. The relationship between horsepower andwatts is given as: 1hp = 746W.Ohms Law Pie ChartWe can now take all the equations from above for finding Voltage, Current, Resistance and Power andcondense them into a simple Ohms Law pie chart for use in DC circuits and calculations.K. Adisesha Page 3
  • 4. Basics of ElectronicsOhms Law Pie ChartExample No1For the circuit shown below find the Voltage (V), the Current (I), the Resistance (R) and the Power (P).Voltage [ V = I x R ] = 2 x 12Ω = 24VCurrent [ I = V ÷ R ] = 24 ÷ 12Ω = 2AResistance [ R = V ÷ I ] = 24 ÷ 2 = 12 ΩPower [ P = V x I ] = 24 x 2 = 48WPower within an electrical circuit is only present when BOTH voltage and current are present for example, Inan Open-circuit condition, Voltage is present but there is no current flow I = 0 (zero), therefore V x 0 is 0 so thepower dissipated within the circuit must also be 0. Likewise, if we have a Short-circuit condition, current flowis present but there is no voltage V = 0, therefore 0 x I = 0 so again the power dissipated within the circuit is 0.As electrical power is the product of V x I, the power dissipated in a circuit is the same whether the circuitcontains high voltage and low current or low voltage and high current flow. Generally, power is dissipated inthe form of Heat (heaters), Mechanical Work such as motors, etc Energy in the form of radiated (Lamps) oras stored energy (Batteries).Energy in Electrical CircuitsElectrical Energy that is either absorbed or produced is the product of the electrical power measured in Wattsand the time in Seconds with the unit of energy given as Watt-seconds or Joules.Although electrical energy is measured in Joules it can become a very large value when used to calculate theenergy consumed by a component. For example, a single 100 W light bulb connected for one hour will consumea total of 100 watts x 3600 sec = 360,000 Joules. So prefixes such as kilojoules (kJ = 103J) or megajoules (MJ= 106J) are used instead. If the electrical power is measured in "kilowatts" and the time is given in hours thenthe unit of energy is in kilowatt-hours or kWh which is commonly called a "Unit of Electricity" and is whatconsumers purchase from their electricity suppliers.Electrical Units of MeasureThe standard SI units used for the measurement of voltage, current and resistance are the Volt [ V ], Ampere[ A ] and Ohms [ Ω ] respectively. Sometimes in electrical or electronic circuits and systems it is necessary touse multiples or sub-multiples (fractions) of these standard units when the quantities being measured are verylarge or very small. The following table gives a list of some of the standard units used in electrical formulas andcomponent values.K. Adisesha Page 4
  • 5. Basics of ElectronicsStandard Electrical Units Parameter Symbol Measuring Unit Description Voltage Volt V or E Unit of Electrical Potential V = I × R Current Ampere I or i Unit of Electrical Current I = V ÷ R Resistance Ohm R or Ω Unit of DC Resistance R = V ÷ I Conductance Siemen G or ℧ Reciprocal of Resistance G = 1 ÷ R Capacitance Farad C Unit of Capacitance C = Q ÷ V Charge Coulomb Q Unit of Electrical Charge Q = C × V Inductance Henry L or H Unit of Inductance VL = -L(di/dt) Power Watts W Unit of Power P = V × I or I2 × R Impedance Ohm Z Unit of AC Resistance Z2 = R2 + X2 Frequency Hertz Hz Unit of Frequency ƒ = 1 ÷ TMultiples and Sub-multiplesThere is a huge range of values encountered in electrical and electronic engineering between a maximum valueand a minimum value of a standard electrical unit. For example, resistance can be lower than 0.01Ωs or higherthan 1,000,000Ωs. By using multiples and submultiples of the standard unit we can avoid having to write toomany zeros to define the position of the decimal point. The table below gives their names and abbreviations. Prefix Symbol Multiplier Power of Ten Terra T 1,000,000,000,000 1012 Giga G 1,000,000,000 109 Mega M 1,000,000 106 kilo k 1,000 103 none none 1 100 centi c 1/100 10-2 milli m 1/1,000 10-3 micro µ 1/1,000,000 10-6 nano n 1/1,000,000,000 10-9 Pico P 1/1,000,000,000,000 10-12So to display the units or multiples of units for Resistance, Current or Voltage we would use as an example: 1kV = 1 kilo-volt - which is equal to 1,000 Volts. 1mA = 1 milli-amp - which is equal to one thousandths (1/1000) of an Ampere. 47kΩ = 47 kilo-ohms - which is equal to 47 thousand Ohms. 100uF = 100 micro-farads - which is equal to 100 millionths (1/1,000,000) of a Farad. 1kW = 1 kilo-watt - which is equal to 1,000 Watts. 1MHz = 1 mega-hertz - which is equal to one million Hertz.To convert from one prefix to another it is necessary to either multiply or divide by the difference between thetwo values. For example, convert 1MHz into kHz.Well we know from above that 1MHz is equal to one million (1,000,000) hertz and that 1kHz is equal to onethousand (1,000) hertz, so one 1MHz is one thousand times bigger than 1kHz. Then to convert Mega-hertz intoKilo-hertz we need to multiply mega-hertz by one thousand, as 1MHz is equal to 1000 kHz. Likewise, if weneeded to convert kilo-hertz into mega-hertz we would need to divide by one thousand. A much simpler andquicker method would be to move the decimal point either left or right depending upon whether you need tomultiply or divide.K. Adisesha Page 5
  • 6. Basics of ElectronicsAs well as the "Standard" electrical units of measure shown above, other units are also used in electricalengineering to denote other values and quantities such as:• Wh − The Watt-Hour, The amount of electrical energy consumed in the circuit by a load of one watt drawing power for one hour, eg a Light Bulb. It is commonly used in the form of kWh (Kilowatt- hour) which is 1,000 watt-hours or MWh (Megawatt-hour) which is 1,000,000 watt-hours.• dB − The Decibel, The decibel is a one tenth unit of the Bel (symbol B) and is used to represent gain either in voltage, current or power. It is a logarithmic unit expressed in dB and is commonly used to represent the ratio of input to output in amplifier, audio circuits or loudspeaker systems. For example, the dB ratio of an input voltage (Vin) to an output voltage (Vout) is expressed as 20log10 (Vout/Vin). The value in dB can be either positive (20dB) representing gain or negative (- 20dB) representing loss with unity, ie input = output expressed as 0dB.• θ− Phase Angle, The Phase Angle is the difference in degrees between the voltage waveform and the current waveform having the same periodic time. It is a time difference or time shift and depending upon the circuit element can have a "leading" or "lagging" value. The phase angle of a waveform is measured in degrees or radians.• ω− Angular Frequency, Another unit which is mainly used in a.c. circuits to represent the Phasor Relationship between two or more waveforms is called Angular Frequency, symbol ω. This is a rotational unit of angular frequency 2πƒ with units in radians per second, rads/s. The complete revolution of one cycle is 360 degrees or 2π, therefore, half a revolution is given as 180 degrees or π rad.• τ− Time Constant, The Time Constant of an impedance circuit or linear first-order system is the time it takes for the output to reach 63.7% of its maximum or minimum output value when subjected to a Step Response input. It is a measure of reaction time.Kirchoffs Circuit LawWe saw in the Resistors tutorial that a single equivalent resistance, ( RT ) can be found when two or moreresistors are connected together in either series, parallel or combinations of both, and that these circuits obeyOhms Law. However, sometimes in complex circuits such as bridge or T networks, we can not simply useOhms Law alone to find the voltages or currents circulating within the circuit. For these types of calculationswe need certain rules which allow us to obtain the circuit equations and for this we can use Kirchoffs CircuitLaw.In 1845, a German physicist, Gustav Kirchoff developed a pair or set of rules or laws which deal with theconservation of current and energy within electrical circuits. These two rules are commonly known as: KirchoffsCircuit Laws with one of Kirchoffs laws dealing with the current flowing around a closed circuit, KirchoffsCurrent Law, (KCL) while the other law deals with the voltage sources present in a closed circuit, KirchoffsVoltage Law, (KVL).Kirchoffs First Law - The Current Law, (KCL)Kirchoffs Current Law or KCL, states that the "total current or charge entering a junction or node is exactlyequal to the charge leaving the node as it has no other place to go except to leave, as no charge is lost withinthe node". In other words the algebraic sum of ALL the currents entering and leaving a node must be equal tozero, I(exiting) + I(entering) = 0. This idea by Kirchoff is commonly known as the Conservation of Charge.Kirchoffs Current LawHere, the 3 currents entering the node, I1, I2, I3 are all positive in value and the 2 currents leaving the node, I4and I5 are negative in value. Then this means we can also rewrite the equation as;K. Adisesha Page 6
  • 7. Basics of Electronics I1 + I2 + I3 - I4 - I5 = 0The term Node in an electrical circuit generally refers to a connection or junction of two or more currentcarrying paths or elements such as cables and components. Also for current to flow either in or out of a node aclosed circuit path must exist. We can use Kirchoffs current law when analysing parallel circuits.Kirchoffs Second Law - The Voltage Law, (KVL)Kirchoffs Voltage Law or KVL, states that "in any closed loop network, the total voltage around the loop isequal to the sum of all the voltage drops within the same loop" which is also equal to zero. In other words thealgebraic sum of all voltages within the loop must be equal to zero. This idea by Kirchoff is known as theConservation of Energy.Kirchoffs Voltage LawStarting at any point in the loop continue in the same direction noting the direction of all the voltage drops,either positive or negative, and returning back to the same starting point. It is important to maintain the samedirection either clockwise or anti-clockwise or the final voltage sum will not be equal to zero. We can useKirchoffs voltage law when analyzing series circuits.When analysing either DC circuits or AC circuits using Kirchoffs Circuit Laws a number of definitions andterminologies are used to describe the parts of the circuit being analyzed such as: node, paths, branches, loopsand meshes. These terms are used frequently in circuit analysis so it is important to understand them. Circuit - a circuit is a closed loop conducting path in which an electrical current flows. Path - a line of connecting elements or sources with no elements or sources included more than once. Node - a node is a junction, connection or terminal within a circuit were two or more circuit elements are connected or joined together giving a connection point between two or more branches. A node is indicated by a dot. Branch - a branch is a single or group of components such as resistors or a source which are connected between two nodes. Loop - a loop is a simple closed path in a circuit in which no circuit element or node is encountered more than once. Mesh - a mesh is a single open loop that does not have a closed path. No components are inside a mesh. Components are connected in series if they carry the same current. Components are connected in parallel if the same voltage is across them.Example No1Find the current flowing in the 40Ω Resistor, R3The circuit has 3 branches, 2 nodes (A and B) and 2 independent loops.Using Kirchoffs Current Law, KCL the equations are given as;K. Adisesha Page 7
  • 8. Basics of ElectronicsAt node A : I1 + I2 = I3At node B : I3 = I1 + I2Using Kirchoffs Voltage Law, KVL the equations are given as;Loop 1 is given as : 10 = R1 x I1 + R3 x I3 = 10I1 + 40I3Loop 2 is given as : 20 = R2 x I2 + R3 x I3 = 20I2 + 40I3Loop 3 is given as : 10 - 20 = 10I1 - 20I2As I3 is the sum of I1 + I2 we can rewrite the equations as;Eq. No 1 : 10 = 10I1 + 40(I1 + I2) = 50I1 + 40I2Eq. No 2 : 20 = 20I2 + 40(I1 + I2) = 40I1 + 60I2We now have two "Simultaneous Equations" that can be reduced to give us the value of both I1 and I2Substitution of I1 in terms of I2 gives us the value of I1 as -0.143 AmpsSubstitution of I2 in terms of I1 gives us the value of I2 as +0.429 AmpsAs : I3 = I1 + I2The current flowing in resistor R3 is given as : -0.143 + 0.429 = 0.286 Ampsand the voltage across the resistor R3 is given as : 0.286 x 40 = 11.44 voltsThe negative sign for I1 means that the direction of current flow initially chosen was wrong, but never the lessstill valid. In fact, the 20v battery is charging the 10v battery.Application of Kirchoffs Circuit LawsThese two laws enable the Currents and Voltages in a circuit to be found, ie, the circuit is said to be "Analysed",and the basic procedure for using Kirchoffs Circuit Laws is as follows: 1. Assume all voltages and resistances are given. ( If not label them V1, V2,... R1, R2, etc. ) 2. Label each branch with a branch current. ( I1, I2, I3 etc. ) 3. Find Kirchoffs first law equations for each node. 4. Find Kirchoffs second law equations for each of the independent loops of the circuit. 5. Use Linear simultaneous equations as required to find the unknown currents.As well as using Kirchoffs Circuit Law to calculate the various voltages and currents circulating around alinear circuit, we can also use loop analysis to calculate the currents in each independent loop which helps toreduce the amount of mathematics required by using just Kirchoffs laws. In the next tutorial about DC Theorywe will look at Mesh Current Analysis to do just that.Circuit AnalysisIn the previous tutorial we saw that complex circuits such as bridge or T-networks can be solved usingKirchoffs Circuit Laws. While Kirchoff´s Laws give us the basic method for analysing any complex electricalcircuit, there are different ways of improving upon this method by using Mesh Current Analysis or NodalVoltage Analysis that results in a lessening of the maths involved and when large networks are involved thisreduction in maths can be a big advantage.For example, consider the circuit from the previous section.Mesh Analysis CircuitOne simple method of reducing the amount of maths involved is to analyse the circuit using Kirchoffs CurrentLaw equations to determine the currents, I1 and I2 flowing in the two resistors. Then there is no need to calculatethe current I3 as its just the sum of I1 and I2. So Kirchhoff’s second voltage law simply becomes: Equation No 1 : 10 = 50I1 + 40I2 Equation No 2 : 20 = 40I1 + 60I2therefore, one line of maths calculation have been saved.Mesh Current AnalysisA more easier method of solving the above circuit is by using Mesh Current Analysis or Loop Analysis whichis also sometimes called Maxwell´s Circulating Currents method. Instead of labeling the branch currents weK. Adisesha Page 8
  • 9. Basics of Electronicsneed to label each "closed loop" with a circulating current. As a general rule of thumb, only label inside loops ina clockwise direction with circulating currents as the aim is to cover all the elements of the circuit at least once.Any required branch current may be found from the appropriate loop or mesh currents as before usingKirchoff´s method.For example: : i1 = I1 , i2 = -I2 and I3 = I1 - I2We now write Kirchoffs voltage law equation in the same way as before to solve them but the advantage of thismethod is that it ensures that the information obtained from the circuit equations is the minimum required tosolve the circuit as the information is more general and can easily be put into a matrix form.For example, consider the circuit from the previous section.These equations can be solved quite quickly by using a single mesh impedance matrix Z. Each element ON theprincipal diagonal will be "positive" and is the total impedance of each mesh. Where as, each element OFF theprincipal diagonal will either be "zero" or "negative" and represents the circuit element connecting all theappropriate meshes. This then gives us a matrix of:Where: [ V ] gives the total battery voltage for loop 1 and then loop 2. [ I ] states the names of the loop currents which we are trying to find. [ R ] is called the resistance matrix.and this gives I1 as -0.143 Amps and I2 as -0.429 AmpsAs : I3 = I1 - I2The current I3 is therefore given as: -0.143 - (-0.429) = 0.286 AmpsWhich is the same value of 0.286 amps, we found using Kirchoff´s circuit law in the previous tutorial.Mesh Current Analysis Summary.This "look-see" method of circuit analysis is probably the best of all the circuit analysis methods with the basicprocedure for solving Mesh Current Analysis equations is as follows: 1. Label all the internal loops with circulating currents. (I1, I2, ...IL etc) 2. Write the [ L x 1 ] column matrix [ V ] giving the sum of all voltage sources in each loop. 3. Write the [ L x L ] matrix, [ R ] for all the resistances in the circuit as follows; o R11 = the total resistance in the first loop. o Rnn = the total resistance in the Nth loop. o RJK = the resistance which directly joins loop J to Loop K. 4. Write the matrix or vector equation [V] = [R] x [I] where [I] is the list of currents to be found.As well as using Mesh Current Analysis, we can also use node analysis to calculate the voltages around theloops, again reducing the amount of mathematics required using just Kirchoffs laws.Nodal Voltage AnalysisAs well as using Mesh Analysis to solve the currents flowing around complex circuits it is also possible to usenodal analysis methods too. Nodal Voltage Analysis complements the previous mesh analysis in that it isequally powerful and based on the same concepts of matrix analysis. As its name implies, Nodal VoltageAnalysis uses the "Nodal" equations of Kirchoffs first law to find the voltage potentials around the circuit. Byadding together all these nodal voltages the net result will be equal to zero. Then, if there are "N" nodes in theK. Adisesha Page 9
  • 10. Basics of Electronicscircuit there will be "N-1" independent nodal equations and these alone are sufficient to describe and hencesolve the circuit.At each node point write down Kirchoffs first law equation, that is: "the currents entering a node are exactlyequal in value to the currents leaving the node" then express each current in terms of the voltage across thebranch. For "N" nodes, one node will be used as the reference node and all the other voltages will be referencedor measured with respect to this common node.For example, consider the circuit from the previous section.Nodal Voltage Analysis CircuitIn the above circuit, node D is chosen as the reference node and the other three nodes are assumed to havevoltages, Va, Vb and Vc with respect to node D. For example;As Va = 10v and Vc = 20v , Vb can be easily found by:Again is the same value of 0.286 amps, we found using Kirchoffs Circuit Law in the previous tutorial.From both Mesh and Nodal Analysis methods we have looked at so far, this is the simplest method of solvingthis particular circuit. Generally, nodal voltage analysis is more appropriate when there are a larger number ofcurrent sources around. The network is then defined as: [ I ] = [ Y ] [ V ] where [ I ] are the driving currentsources, [ V ] are the nodal voltages to be found and [ Y ] is the admittance matrix of the network whichoperates on [ V ] to give [ I ].Nodal Voltage Analysis Summary.The basic procedure for solving Nodal Analysis equations is as follows: 1. Write down the current vectors, assuming currents into a node are positive. ie, a (N x 1) matrices for "N" independent nodes. 2. Write the admittance matrix [Y] of the network where: o Y11 = the total admittance of the first node. o Y22 = the total admittance of the second node. o RJK = the total admittance joining node J to node K. 3. For a network with "N" independent nodes, [Y] will be an (N x N) matrix and that Ynn will be positive and Yjk will be negative or zero value. 4. The voltage vector will be (N x L) and will list the "N" voltages to be found.Thevenins TheoremIn the previous 3 tutorials we have looked at solving complex electrical circuits using Kirchoffs Circuit Laws,Mesh Analysis and finally Nodal Analysis but there are many more "Circuit Analysis Theorems" available tocalculate the currents and voltages at any point in a circuit. In this tutorial we will look at one of the morecommon circuit analysis theorems (next to Kirchoff´s) that has been developed, Thevenins Theorem.Thevenins Theorem states that "Any linear circuit containing several voltages and resistances can be replacedby just a Single Voltage in series with a Single Resistor". In other words, it is possible to simplify any "Linear"circuit, no matter how complex, to an equivalent circuit with just a single voltage source in series with aK. Adisesha Page 10
  • 11. Basics of Electronicsresistance connected to a load as shown below. Thevenins Theorem is especially useful in analyzing power orbattery systems and other interconnected circuits where it will have an effect on the adjoining part of the circuit.Thevenins equivalent circuit.As far as the load resistor RL is concerned, any "one-port" network consisting of resistive circuit elements andenergy sources can be replaced by one single equivalent resistance Rs and equivalent voltage Vs, where Rs isthe source resistance value looking back into the circuit and Vs is the open circuit voltage at the terminals.For example, consider the circuit from the previous section.Firstly, we have to remove the centre 40Ω resistor and short out (not physically as this would be dangerous) allthe emf´s connected to the circuit, or open circuit any current sources. The value of resistor Rs is found bycalculating the total resistance at the terminals A and B with all the emf´s removed, and the value of the voltagerequired Vs is the total voltage across terminals A and B with an open circuit and no load resistor Rs connected.Then, we get the following circuit.Find the Equivalent Resistance (Rs)Find the Equivalent Voltage (Vs)We now need to reconnect the two voltages back into the circuit, and as VS = VAB the current flowing aroundthe loop is calculated as:so the voltage drop across the 20Ω resistor can be calculated as:VAB = 20 - (20Ω x 0.33amps) = 13.33 volts.Then the Thevenins Equivalent circuit is shown below with the 40Ω resistor connected.K. Adisesha Page 11
  • 12. Basics of Electronicsand from this the current flowing in the circuit is given as:Which again, is the same value of 0.286 amps, we found using Kirchoff´s circuit law in the previous tutorial.Thevenins theorem can be used as a circuit analysis method and is particularly useful if the load is to take aseries of different values. It is not as powerful as Mesh or Nodal analysis in larger networks because the use ofMesh or Nodal analysis is usually necessary in any Thevenin exercise, so it might as well be used from the start.However, Thevenins equivalent circuits of Transistors, Voltage Sources such as batteries etc, are very usefulin circuit design.Thevenins Theorem SummaryThe basic procedure for solving a circuit using Thevenins Theorem is as follows: 1. Remove the load resistor RL or component concerned. 2. Find RS by shorting all voltage sources or by open circuiting all the current sources. 3. Find VS by the usual circuit analysis methods. 4. Find the current flowing through the load resistor RL.Norton TheoremIn some ways Nortons Theorem can be thought of as the opposite to "Thevenins Theorem", in that Theveninreduces his circuit down to a single resistance in series with a single voltage. Norton on the other hand reduceshis circuit down to a single resistance in parallel with a constant current source. Nortons Theorem states that"Any linear circuit containing several energy sources and resistances can be replaced by a single ConstantCurrent generator in parallel with a Single Resistor". As far as the load resistance, RL is concerned this singleresistance, RS is the value of the resistance looking back into the network with all the current sources opencircuited and IS is the short circuit current at the output terminals as shown below.Nortons equivalent circuit.The value of this "constant current" is one which would flow if the two output terminals where shorted togetherwhile the source resistance would be measured looking back into the terminals, (the same as Thevenin).For example, consider our now familiar circuit from the previous section.To find the Nortons equivalent of the above circuit we firstly have to remove the centre 40Ω load resistor andshort out the terminals A and B to give us the following circuit.K. Adisesha Page 12
  • 13. Basics of ElectronicsWhen the terminals A and B are shorted together the two resistors are connected in parallel across their tworespective voltage sources and the currents flowing through each resistor as well as the total short circuit currentcan now be calculated as:with A-B Shorted OutIf we short-out the two voltage sources and open circuit terminals A and B, the two resistors are now effectivelyconnected together in parallel. The value of the internal resistor Rs is found by calculating the total resistance atthe terminals A and B giving us the following circuit.Find the Equivalent Resistance (Rs)Having found both the short circuit current, Is and equivalent internal resistance, Rs this then gives us thefollowing Nortons equivalent circuit.Nortons equivalent circuit.Ok, so far so good, but we now have to solve with the original 40Ω load resistor connected across terminals Aand B as shown below.Again, the two resistors are connected in parallel across the terminals A and B which gives us a total resistanceof:The voltage across the terminals A and B with the load resistor connected is given as:Then the current flowing in the 40Ω load resistor can be found as:K. Adisesha Page 13
  • 14. Basics of Electronicswhich again, is the same value of 0.286 amps, we found using Kirchoff´s circuit law in the previous tutorials.Nortons Theorem SummaryThe basic procedure for solving a circuit using Nortons Theorem is as follows: 1. Remove the load resistor RL or component concerned. 2. Find RS by shorting all voltage sources or by open circuiting all the current sources. 3. Find IS by placing a shorting link on the output terminals A and B. 4. Find the current flowing through the load resistor RL.Maximum Power TransferWe have seen in the previous tutorials that any complex circuit or network can be replaced by a single energysource in series with a single internal source resistance, RS. Generally, this source resistance or even impedanceif inductors or capacitors are involved is of a fixed value in Ohm´s. However, when we connect a loadresistance, RL across the output terminals of the power source, the impedance of the load will vary from anopen-circuit state to a short-circuit state resulting in the power being absorbed by the load becoming dependenton the impedance of the actual power source. Then for the load resistance to absorb the maximum powerpossible it has to be "Matched" to the impedance of the power source and this forms the basis of MaximumPower Transfer.The Maximum Power Transfer Theorem is another useful analysis method to ensure that the maximumamount of power will be dissipated in the load resistance when the value of the load resistance is exactly equalto the resistance of the power source. The relationship between the load impedance and the internal impedanceof the energy source will give the power in the load. Consider the circuit below.Thevenins Equivalent Circuit.In our Thevenin equivalent circuit above, the maximum power transfer theorem states that "the maximumamount of power will be dissipated in the load resistance if it is equal in value to the Thevenin or Norton sourceresistance of the network supplying the power" in other words, the load resistance resulting in greatest powerdissipation must be equal in value to the equivalent Thevenin source resistance, then RL = RS but if the loadresistance is lower or higher in value than the Thevenin source resistance of the network, its dissipated powerwill be less than maximum. For example, find the value of the load resistance, RL that will give the maximumpower transfer in the following circuit.Example No1. Where: RS = 25Ω RL is variable between 0 - 100Ω VS = 100vThen by using the following Ohms Law equations:We can now complete the following table to determine the current and power in the circuit for different valuesof load resistance.K. Adisesha Page 14
  • 15. Basics of ElectronicsTable of Current against Power RL I P RL I P 0 0 0 25 2.0 100 5 3.3 55 30 1.8 97 10 2.8 78 40 1.5 94 15 2.5 93 60 1.2 83 20 2.2 97 100 0.8 64Using the data from the table above, we can plot a graph of load resistance, RL against power, P for differentvalues of load resistance. Also notice that power is zero for an open-circuit (zero current condition) and also fora short-circuit (zero voltage condition).Graph of Power against Load ResistanceFrom the above table and graph we can see that the Maximum Power Transfer occurs in the load when theload resistance, RL is equal in value to the source resistance, RS so then: RS = RL = 25Ω. This is called a"matched condition" and as a general rule, maximum power is transferred from an active device such as a powersupply or battery to an external device occurs when the impedance of the external device matches that of thesource. Improper impedance matching can lead to excessive power use and dissipation.Transformer Impedance MatchingOne very useful application of impedance matching to provide maximum power transfer is in the output stagesof amplifier circuits, where the speakers impedance is matched to the amplifier output impedance to obtainmaximum sound power output. This is achieved by using a matching transformer to couple the load to theamplifiers output as shown below.Transformer CouplingThe maximum power transfer can be obtained even if the output impedance is not the same as the loadimpedance. This can be done using a suitable "turns ratio" on the transformer with the corresponding ratio ofload impedance, ZLOAD to output impedance, ZOUT matches that of the ratio of the transformers primary turns tosecondary turns as a resistance on one side of the transformer becomes a different value on the other. If the loadimpedance, ZLOAD is purely resistive and the source impedance is purely resistive, ZOUT then the equation forfinding the maximum power transfer is given as:Where: NP is the number of primary turns and NS the number of secondary turns on the transformer. Then byvarying the value of the transformers turns ratio the output impedance can be "matched" to the sourceimpedance to achieve maximum power transfer. For example,Example No2.If an 8Ω loudspeaker is to be connected to an amplifier with an output impedance of 1000Ω, calculate the turnsratio of the matching transformer required to provide maximum power transfer of the audio signal. Assume theamplifier source impedance is Z1, the load impedance is Z2 with the turns ratio given as N.K. Adisesha Page 15
  • 16. Basics of ElectronicsGenerally, small transformers used in low power audio amplifiers are usually regarded as ideal so any lossescan be ignored.Star Delta TransformationWe can now solve simple series, parallel or bridge type resistive networks using Kirchoff´s Circuit Laws,mesh current analysis or nodal voltage analysis techniques but in a balanced 3-phase circuit we can use differentmathematical techniques to simplify the analysis of the circuit and thereby reduce the amount of mathsinvolved which in itself is a good thing. Standard 3-phase circuits or networks take on two major forms withnames that represent the way in which the resistances are connected, a Star connected network which has thesymbol of the letter, Υ (wye) and a Delta connected network which has the symbol of a triangle, Δ (delta). If a3-phase, 3-wire supply or even a 3-phase load is connected in one type of configuration, it can be easilytransformed or changed it into an equivalent configuration of the other type by using either the Star DeltaTransformation or Delta Star Transformation process.A resistive network consisting of three impedances can be connected together to form a T or "Tee"configuration but the network can also be redrawn to form a Star or Υ type network as shown below.T-connected and Equivalent Star NetworkAs we have already seen, we can redraw the T resistor network to produce an equivalent Star or Υ typenetwork. But we can also convert a Pi or π type resistor network into an equivalent Delta or Δ type network asshown below.Pi-connected and Equivalent Delta Network.Having now defined exactly what is a Star and Delta connected network it is possible to transform the Υ intoan equivalent Δ circuit and also to convert a Δ into an equivalent Υ circuit using a the transformation process.This process allows us to produce a mathematical relationship between the various resistors giving us a StarDelta Transformation as well as a Delta Star Transformation.These transformations allow us to change the three connected resistances by their equivalents measuredbetween the terminals 1-2, 1-3 or 2-3 for either a star or delta connected circuit. However, the resultingnetworks are only equivalent for voltages and currents external to the star or delta networks, as internally thevoltages and currents are different but each network will consume the same amount of power and have the samepower factor to each other.K. Adisesha Page 16
  • 17. Basics of ElectronicsDelta Star TransformationTo convert a delta network to an equivalent star network we need to derive a transformation formula forequating the various resistors to each other between the various terminals. Consider the circuit below.Delta to Star Network.Compare the resistances between terminals 1 and 2.Resistance between the terminals 2 and 3.Resistance between the terminals 1 and 3.This now gives us three equations and taking equation 3 from equation 2 gives:Then, re-writing Equation 1 will give us:Adding together equation 1 and the result above of equation 3 minus equation 2 gives:From which gives us the final equation for resistor P as:Then to summarize a little the above maths, we can now say that resistor P in a Star network can be found asEquation 1 plus (Equation 3 minus Equation 2) or Eq1 + (Eq3 - Eq2).K. Adisesha Page 17
  • 18. Basics of ElectronicsSimilarly, to find resistor Q in a star network, is equation 2 plus the result of equation 1 minus equation 3or Eq2 + (Eq1 - Eq3) and this gives us the transformation of Q as:and again, to find resistor R in a Star network, is equation 3 plus the result of equation 2 minus equation 1or Eq3 + (Eq2 - Eq1) and this gives us the transformation of R as:When converting a delta network into a star network the denominators of all of the transformation formulas arethe same: A + B + C, and which is the sum of ALL the delta resistances. Then to convert any delta connectednetwork to an equivalent star network we can summarized the above transformation equations as:Delta to Star Transformations EquationsIf the three resistors in the delta network are all equal in value then the resultant resistors in the equivalent starnetwork will be equal to one third the value of the delta resistors, giving each branch in the star network as:RSTAR = 1/3RDELTAExample No1Convert the following Delta Resistive Network into an equivalent Star Network.Star Delta TransformationWe have seen above that when converting from a delta network to an equivalent star network that the resistorconnected to one terminal is the product of the two delta resistances connected to the same terminal, forexample resistor P is the product of resistors A and B connected to terminal 1. By rewriting the previousformulas a little we can also find the transformation formulas for converting a resistive star network to anequivalent delta network giving us a way of producing a star delta transformation as shown below.Star to Delta Network.The value of the resistor on any one side of the delta, Δ network is the sum of all the two-product combinationsof resistors in the star network divide by the star resistor located "directly opposite" the delta resistor beingfound. For example, resistor A is given as:K. Adisesha Page 18
  • 19. Basics of Electronicswith respect to terminal 3 and resistor B is given as:with respect to terminal 2 with resistor C given as:with respect to terminal 1.By dividing out each equation by the value of the denominator we end up with three separate transformationformulas that can be used to convert any Delta resistive network into an equivalent star network as given below.Star Delta Transformation EquationsStar Delta Transformations allow us to convert one circuit type of circuit connection to another in order for usto easily analyise a circuit and one final point about converting a star resistive network to an equivalent deltanetwork. If all the resistors in the star network are all equal in value then the resultant resistors in the equivalentdelta network will be three times the value of the star resistors and equal, giving: RDELTA = 3RSTARSEMICONDUCTOR DEVICESSemiconductor BasicsIf Resistors are the most basic passive component in electrical or electronic circuits, then we have to considerthe Signal Diode as being the most basic "Active" component. However, unlike a resistor, a diode does notbehave linearly with respect to the applied voltage as it has an exponential I-V relationship and therefore cannot be described simply by using Ohms law as we do for resistors. Diodes are basic unidirectionalsemiconductor devices that will only allow current to flow through them in one direction only, acting more likea one way electrical valve, (Forward Biased Condition). But, before we have a look at how signal or powerdiodes work we first need to understand the semiconductors basic construction and concept.Diodes are made from a single piece of Semiconductor material which has a positive "P-region" at one end anda negative "N-region" at the other, and which has a resistivity value somewhere between that of a conductor andan insulator. But what is a "Semiconductor" material?, firstly lets look at what makes something either aConductor or an Insulator.ResistivityThe electrical Resistance of an electrical or electronic component or device is generally defined as being theratio of the voltage difference across it to the current flowing through it, basic Ohm´s Law principals. Theproblem with using resistance as a measurement is that it depends very much on the physical size of thematerial being measured as well as the material out of which it is made. For example, If we were to increase thelength of the material (making it longer) its resistance would also increase. Likewise, if we increased itsdiameter (making it fatter) its resistance would then decrease. So we want to be able to define the material insuch a way as to indicate its ability to either conduct or oppose the flow of electrical current through it no matterwhat its size or shape happens to be. The quantity that is used to indicate this specific resistance is calledResistivity and is given the Greek symbol of ρ, (Rho). Resistivity is measured in Ohm-metres, ( Ω-m ) and isthe inverse to conductivity.If the resistivity of various materials is compared, they can be classified into three main groups, Conductors,Insulators and Semi-conductors as shown below.Resistivity ChartK. Adisesha Page 19
  • 20. Basics of Electronics Notice also that there is a very small margin between the resistivity of the conductors such as silver and gold, compared to a much larger margin for the resistivity of the insulators between glass and quartz. The resistivity of all the materials at any one time also depends upon their temperature.ConductorsFrom above we now know that Conductors are materials that have a low value of resistivity allowing them toeasily pass an electrical current due to there being plenty of free electrons floating about within their basic atomstructure. When a positive voltage potential is applied to the material these "free electrons" leave their parentatom and travel together through the material forming an electron drift. Examples of good conductors aregenerally metals such as Copper, Aluminium, Silver or non metals such as Carbon because these materials havevery few electrons in their outer "Valence Shell" or ring, resulting in them being easily knocked out of theatoms orbit. This allows them to flow freely through the material until they join up with other atoms, producinga "Domino Effect" through the material thereby creating an electrical current.Generally speaking, most metals are good conductors of electricity, as they have very small resistance values,usually in the region of micro-ohms per metre with the resistivity of conductors increasing with temperaturebecause metals are also generally good conductors of heat.InsulatorsInsulators on the other hand are the exact opposite of conductors. They are made of materials, generally non-metals, that have very few or no "free electrons" floating about within their basic atom structure because theelectrons in the outer valence shell are strongly attracted by the positively charged inner nucleus. So if apotential voltage is applied to the material no current will flow as there are no electrons to move and whichgives these materials their insulating properties. Insulators also have very high resistances, millions of ohms permetre, and are generally not affected by normal temperature changes (although at very high temperatures woodbecomes charcoal and changes from an insulator to a conductor). Examples of good insulators are marble, fusedquartz, p.v.c. plastics, rubber etc.Insulators play a very important role within electrical and electronic circuits, because without them electricalcircuits would short together and not work. For example, insulators made of glass or porcelain are used forinsulating and supporting overhead transmission cables while epoxy-glass resin materials are used to makeprinted circuit boards, PCBs etc.Semiconductor BasicsSemiconductors materials such as silicon (Si), germanium (Ge) and gallium arsenide (GaAs), have electricalproperties somewhere in the middle, between those of a "conductor" and an "insulator". They are not goodconductors nor good insulators (hence their name "semi"-conductors). They have very few "fee electrons"because their atoms are closely grouped together in a crystalline pattern called a "crystal lattice". However, theirability to conduct electricity can be greatly improved by adding certain "impurities" to this crystalline structurethereby, producing more free electrons than holes or vice versa. By controlling the amount of impurities addedto the semiconductor material it is possible to control its conductivity. These impurities are called donors oracceptors depending on whether they produce electrons or holes respectively. This process of adding impurityatoms to semiconductor atoms (the order of 1 impurity atom per 10 million (or more) atoms of thesemiconductor) is called Doping.The most commonly used semiconductor basics material by far is silicon. Silicon has four valence electrons inits outermost shell which it shares with its neighbouring silicon atoms to form full orbitals of eight electrons.The structure of the bond between the two silicon atoms is such that each atom shares one electron with itsneighbour making the bond very stable. As there are very few free electrons available to move around theK. Adisesha Page 20
  • 21. Basics of Electronicssilicon crystal, crystals of pure silicon (or germanium) are therefore good insulators, or at the very least veryhigh value resistors.Silicon atoms are arranged in a definite symmetrical pattern making them a crystalline solid structure. A crystalof pure silica (silicon dioxide or glass) is generally said to be an intrinsic crystal (it has no impurities) andtherefore has no free electrons. But simply connecting a silicon crystal to a battery supply is not enough toextract an electric current from it. To do that we need to create a "positive" and a "negative" pole within thesilicon allowing electrons and therefore electric current to flow out of the silicon. These poles are created bydoping the silicon with certain impurities. The diagram above shows the structure and lattice of a normal pure crystal of Silicon.N-type Semiconductor BasicsIn order for our silicon crystal to conduct electricity, we need to introduce an impurity atom such as Arsenic,Antimony or Phosphorus into the crystalline structure making it extrinsic (impurities are added). These atomshave five outer electrons in their outermost orbital to share with neighbouring atoms and are commonly called"Pentavalent" impurities. This allows four out of the five orbital electrons to bond with its neighbouring siliconatoms leaving one "free electron" to become mobile when an electrical voltage is applied (electron flow). Aseach impurity atom "donates" one electron, pentavalent atoms are generally known as "donors".Antimony (symbol Sb) or Phosphorus (symbol P), are frequently used as a pentavalent additive as they have51 electrons arranged in five shells around their nucleus with the outermost orbital having five electrons. Theresulting semiconductor basics material has an excess of current-carrying electrons, each with a negativecharge, and is therefore referred to as an "N-type" material with the electrons called "Majority Carriers" whilethe resulting holes are called "Minority Carriers".When stimulated by an external power source, the electrons freed from the silicon atoms by this stimulation arequickly replaced by the free electrons available from the doped Antimony atoms. But this action still leaves anextra electron (the freed electron) floating around the doped crystal making it negatively charged. Then asemiconductor material is classed as N-type when its donor density is greater than its acceptor density, in otherwords, it has more electrons than holes thereby creating a negative pole. The diagram above shows the structure and lattice of the donor impurity atom Antimony.P-Type Semiconductor BasicsIf we go the other way, and introduce a "Trivalent" (3-electron) impurity into the crystalline structure, such asAluminium, Boron or Indium, which have only three valence electrons available in their outermost orbital, thefourth closed bond cannot be formed. Therefore, a complete connection is not possible, giving thesemiconductor material an abundance of positively charged carriers known as "holes" in the structure of thecrystal where electrons are effectively missing.K. Adisesha Page 21
  • 22. Basics of ElectronicsAs there is now a hole in the silicon crystal, a neighbouring electron is attracted to it and will try to move intothe hole to fill it. However, the electron filling the hole leaves another hole behind it as it moves. This in turnattracts another electron which in turn creates another hole behind it, and so forth giving the appearance that theholes are moving as a positive charge through the crystal structure (conventional current flow). This movementof holes results in a shortage of electrons in the silicon turning the entire doped crystal into a positive pole. Aseach impurity atom generates a hole, trivalent impurities are generally known as "Acceptors" as they arecontinually "accepting" extra or free electrons.Boron (symbol B) is commonly used as a trivalent additive as it has only five electrons arranged in three shellsaround its nucleus with the outermost orbital having only three electrons. The doping of Boron atoms causesconduction to consist mainly of positive charge carriers resulting in a "P-type" material with the positive holesbeing called "Majority Carriers" while the free electrons are called "Minority Carriers". Then a semiconductorbasics material is classed as P-type when its acceptor density is greater than its donor density. Therefore, a P-type semiconductor has more holes than electrons. The diagram above shows the structure and lattice of the acceptor impurity atom Boron.Semiconductor Basics SummaryN-type (e.g. add Antimony)These are materials which have Pentavalent impurity atoms (Donors) added and conduct by "electron"movement and are called, N-type Semiconductors.In these types of materials are: 1. The Donors are positively charged. 2. There are a large number of free electrons. 3. A small number of holes in relation to the number of free electrons. 4. Doping gives: o positively charged donors. o negatively charged free electrons. 5. Supply of energy gives: o negatively charged free electrons. o positively charged holes.P-type (e.g. add Boron)These are materials which have Trivalent impurity atoms (Acceptors) added and conduct by "hole" movementand are called, P-type Semiconductors.In these types of materials are: 1. The Acceptors are negatively charged. 2. There are a large number of holes. 3. A small number of free electrons in relation to the number of holes. 4. Doping gives: negatively charged acceptors. positively charged holes. 5. Supply of energy gives: positively charged holes. negatively charged free electrons.and both P and N-types as a whole, are electrically neutral on their own.K. Adisesha Page 22
  • 23. Basics of ElectronicsAntimony (Sb) and Boron (B) are two of the most commony used doping agents as they are more feelyavailable compared to others and are also classed as metalloids. However, the periodic table groups together anumber of other different chemical elements all with either three, or five electrons in their outermost orbitalshell. These other chemical elements can also be used as doping agents to a base material of either Silicon (S) orGermanium (Ge) to produce different types of basic semiconductor materials for use in electronic componentsand these are given below.Periodic Table of Semiconductors Elements Group 13 Elements Group 14 Elements Group 15 3-Electrons in Outer Shell 4-Electrons in Outer Shell 5-Electrons in Outer Shell (Positively Charged) (Neutrally Charged) (Negatively Charged) (5) Boron ( B ) (6) Carbon ( C ) (13) Aluminium ( Al ) (14) Silicon ( Si ) (15) Phosphorus ( P ) (31) Gallium ( Ga ) (32) Germanium ( Ge ) (33) Arsenic ( As ) (51) Antimony ( Sb )The PN junctionIn the previous tutorial we saw how to make an N-type semiconductor material by doping it with Antimony andalso how to make a P-type semiconductor material by doping that with Boron. This is all well and good, butthese semiconductor N and P-type materials do very little on their own as they are electrically neutral, but whenwe join (or fuse) them together these two materials behave in a very different way producing what is generallyknown as a PN Junction.When the N and P-type semiconductor materials are first joined together a very large density gradient existsbetween both sides of the junction so some of the free electrons from the donor impurity atoms begin to migrateacross this newly formed junction to fill up the holes in the P-type material producing negative ions. However,because the electrons have moved across the junction from the N-type silicon to the P-type silicon, they leavebehind positively charged donor ions (ND) on the negative side and now the holes from the acceptor impuritymigrate across the junction in the opposite direction into the region were there are large numbers of freeelectrons. As a result, the charge density of the P-type along the junction is filled with negatively chargedacceptor ions (NA), and the charge density of the N-type along the junction becomes positive. This chargetransfer of electrons and holes across the junction is known as diffusion.This process continues back and forth until the number of electrons which have crossed the junction have alarge enough electrical charge to repel or prevent any more carriers from crossing the junction. The regions onboth sides of the junction become depleted of any free carriers in comparison to the N and P type materialsaway from the junction. Eventually a state of equilibrium (electrically neutral situation) will occur producing a"potential barrier" zone around the area of the junction as the donor atoms repel the holes and the acceptoratoms repel the electrons. Since no free charge carriers can rest in a position where there is a potential barrierthe regions on both sides of the junction become depleted of any more free carriers in comparison to the N andP type materials away from the junction. This area around the junction is now called the Depletion Layer.The PN junctionThe total charge on each side of the junction must be equal and opposite to maintain a neutral charge conditionK. Adisesha Page 23
  • 24. Basics of Electronicsaround the junction. If the depletion layer region has a distance D, it therefore must therefore penetrate into thesilicon by a distance of Dp for the positive side, and a distance of Dn for the negative side giving a relationshipbetween the two of Dp.NA = Dn.ND in order to maintain charge neutrality also called equilibrium.PN junction DistanceAs the N-type material has lost electrons and the P-type has lost holes, the N-type material has become positivewith respect to the P-type. Then the presence of impurity ions on both sides of the junction cause an electricfield to be established across this region with the N-side at a positive voltage relative to the P-side. The problemnow is that a free charge requires some extra energy to overcome the barrier that now exists for it to be able tocross the depletion region junction.This electric field created by the diffusion process has created a "built-in potential difference" across thejunction with an open-circuit (zero bias) potential of:Where: Eo is the zero bias junction voltage, VT the thermal voltage of 26mV at room temperature, ND and NAare the impurity concentrations and ni is the intrinsic concentration.A suitable positive voltage (forward bias) applied between the two ends of the PN junction can supply the freeelectrons and holes with the extra energy. The external voltage required to overcome this potential barrier thatnow exists is very much dependent upon the type of semiconductor material used and its actual temperature.Typically at room temperature the voltage across the depletion layer for silicon is about 0.6 - 0.7 volts and forgermanium is about 0.3 - 0.35 volts. This potential barrier will always exist even if the device is not connectedto any external power source.The significance of this built-in potential across the junction, is that it opposes both the flow of holes andelectrons across the junction and is why it is called the potential barrier. In practice, a PN junction is formedwithin a single crystal of material rather than just simply joining or fusing together two separate pieces.Electrical contacts are also fused onto either side of the crystal to enable an electrical connection to be made toan external circuit. Then the resulting device that has been made is called a PN junction Diode or Signal Diode.The Junction DiodeThe effect described in the previous tutorial is achieved without any external voltage being applied to the actualPN junction resulting in the junction being in a state of equilibrium. However, if we were to make electricalconnections at the ends of both the N-type and the P-type materials and then connect them to a battery source,an additional energy source now exists to overcome the barrier resulting in free charges being able to cross thedepletion region from one side to the other. The behaviour of the PN junction with regards to the potentialbarrier width produces an asymmetrical conducting two terminal device, better known as the Junction Diode.A diode is one of the simplest semiconductor devices, which has the characteristic of passing current in onedirection only. However, unlike a resistor, a diode does not behave linearly with respect to the applied voltageas the diode has an exponential I-V relationship and therefore we can not described its operation by simplyusing an equation such as Ohms law.If a suitable positive voltage (forward bias) is applied between the two ends of the PN junction, it can supplyfree electrons and holes with the extra energy they require to cross the junction as the width of the depletionK. Adisesha Page 24
  • 25. Basics of Electronicslayer around the PN junction is decreased. By applying a negative voltage (reverse bias) results in the freecharges being pulled away from the junction resulting in the depletion layer width being increased. This has theeffect of increasing or decreasing the effective resistance of the junction itself allowing or blocking current flowthrough the diode.Then the depletion layer widens with an increase in the application of a reverse voltage and narrows with anincrease in the application of a forward voltage. This is due to the differences in the electrical properties on thetwo sides of the PN junction resulting in physical changes taking place. One of the results produces rectificationas seen in the PN junction diodes static I-V (current-voltage) characteristics. Rectification is shown by anasymmetrical current flow when the polarity of bias voltage is altered as shown below.Junction Diode Symbol and Static I-V Characteristics.But before we can use the PN junction as a practical device or as a rectifying device we need to firstly bias thejunction, ie connect a voltage potential across it. On the voltage axis above, "Reverse Bias" refers to an externalvoltage potential which increases the potential barrier. An external voltage which decreases the potential barrieris said to act in the "Forward Bias" direction.There are two operating regions and three possible "biasing" conditions for the standard Junction Diode andthese are: 1. Zero Bias - No external voltage potential is applied to the PN-junction. 2. Reverse Bias - The voltage potential is connected negative, (-ve) to the P-type material and positive, (+ve) to the N-type material across the diode which has the effect of Increasing the PN-junction width. 3. Forward Bias - The voltage potential is connected positive, (+ve) to the P-type material and negative, (-ve) to the N-type material across the diode which has the effect of Decreasing the PN-junction width.Zero Biased Junction DiodeWhen a diode is connected in a Zero Bias condition, no external potential energy is applied to the PN junction.However if the diodes terminals are shorted together, a few holes (majority carriers) in the P-type material withenough energy to overcome the potential barrier will move across the junction against this barrier potential.This is known as the "Forward Current" and is referenced as IFLikewise, holes generated in the N-type material (minority carriers), find this situation favourable and moveacross the junction in the opposite direction. This is known as the "Reverse Current" and is referenced as IR.This transfer of electrons and holes back and forth across the PN junction is known as diffusion, as shownbelow.Zero Biased Junction DiodeK. Adisesha Page 25
  • 26. Basics of ElectronicsThe potential barrier that now exists discourages the diffusion of any more majority carriers across the junction.However, the potential barrier helps minority carriers (few free electrons in the P-region and few holes in the N-region) to drift across the junction. Then an "Equilibrium" or balance will be established when the majoritycarriers are equal and both moving in opposite directions, so that the net result is zero current flowing in thecircuit. When this occurs the junction is said to be in a state of "Dynamic Equilibrium".The minority carriers are constantly generated due to thermal energy so this state of equilibrium can be brokenby raising the temperature of the PN junction causing an increase in the generation of minority carriers, therebyresulting in an increase in leakage current but an electric current cannot flow since no circuit has beenconnected to the PN junction.Reverse Biased Junction DiodeWhen a diode is connected in a Reverse Bias condition, a positive voltage is applied to the N-type material anda negative voltage is applied to the P-type material. The positive voltage applied to the N-type material attractselectrons towards the positive electrode and away from the junction, while the holes in the P-type end are alsoattracted away from the junction towards the negative electrode. The net result is that the depletion layer growswider due to a lack of electrons and holes and presents a high impedance path, almost an insulator. The result isthat a high potential barrier is created thus preventing current from flowing through the semiconductor material.Reverse Biased Junction Diode showing an Increase in the Depletion LayerThis condition represents a high resistance value to the PN junction and practically zero current flows throughthe junction diode with an increase in bias voltage. However, a very small leakage current does flow throughthe junction which can be measured in microamperes, (μA). One final point, if the reverse bias voltage Vrapplied to the diode is increased to a sufficiently high enough value, it will cause the PN junction to overheatand fail due to the avalanche effect around the junction. This may cause the diode to become shorted and willresult in the flow of maximum circuit current, and this shown as a step downward slope in the reverse staticcharacteristics curve below.Reverse Characteristics Curve for a Junction DiodeSometimes this avalanche effect has practical applications in voltage stabilising circuits where a series limitingresistor is used with the diode to limit this reverse breakdown current to a preset maximum value therebyproducing a fixed voltage output across the diode. These types of diodes are commonly known as Zener Diodesand are discussed in a later tutorial.Forward Biased Junction DiodeWhen a diode is connected in a Forward Bias condition, a negative voltage is applied to the N-type materialand a positive voltage is applied to the P-type material. If this external voltage becomes greater than the value ofK. Adisesha Page 26
  • 27. Basics of Electronicsthe potential barrier, approx. 0.7 volts for silicon and 0.3 volts for germanium, the potential barriers oppositionwill be overcome and current will start to flow. This is because the negative voltage pushes or repels electronstowards the junction giving them the energy to cross over and combine with the holes being pushed in theopposite direction towards the junction by the positive voltage. This results in a characteristics curve of zerocurrent flowing up to this voltage point, called the "knee" on the static curves and then a high current flowthrough the diode with little increase in the external voltage as shown below.Forward Characteristics Curve for a Junction DiodeThe application of a forward biasing voltage on the junction diode results in the depletion layer becoming verythin and narrow which represents a low impedance path through the junction thereby allowing high currents toflow. The point at which this sudden increase in current takes place is represented on the static I-Vcharacteristics curve above as the "knee" point.Forward Biased Junction Diode showing a Reduction in the Depletion LayerThis condition represents the low resistance path through the PN junction allowing very large currents to flowthrough the diode with only a small increase in bias voltage. The actual potential difference across the junctionor diode is kept constant by the action of the depletion layer at approximately 0.3v for germanium andapproximately 0.7v for silicon junction diodes. Since the diode can conduct "infinite" current above this kneepoint as it effectively becomes a short circuit, therefore resistors are used in series with the diode to limit itscurrent flow. Exceeding its maximum forward current specification causes the device to dissipate more powerin the form of heat than it was designed for resulting in a very quick failure of the device.Junction Diode SummaryThe PN junction region of a Junction Diode has the following important characteristics: 1). Semiconductors contain two types of mobile charge carriers, Holes and Electrons. 2). The holes are positively charged while the electrons negatively charged. 3). A semiconductor may be doped with donor impurities such as Antimony (N-type doping), so that it contains mobile charges which are primarily electrons. 4). A semiconductor may be doped with acceptor impurities such as Boron (P-type doping), so that it contains mobile charges which are mainly holes. 5). The junction region itself has no charge carriers and is known as the depletion region. 6). The junction (depletion) region has a physical thickness that varies with the applied voltage. 7).When a diode is Zero Biased no external energy source is applied and a natural Potential Barrier is developed across a depletion layer which is approximately 0.5 to 0.7v for silicon diodes and approximately 0.3 of a volt for germanium diodes. 8). When a junction diode is Forward Biased the thickness of the depletion region reduces and the diode acts like a short circuit allowing full current to flow. 9). When a junction diode is Reverse Biased the thickness of the depletion region increases and the diode acts like an open circuit blocking any current flow, (only a very small leakage current).K. Adisesha Page 27
  • 28. Basics of ElectronicsThe Signal DiodeThe semiconductor Signal Diode is a small non-linear semiconductor devices generally used in electroniccircuits, where small currents or high frequencies are involved such as in radio, television and digital logiccircuits. The signal diode which is also sometimes known by its older name of the Point Contact Diode or theGlass Passivated Diode, are physically very small in size compared to their larger Power Diode cousins.Generally, the PN junction of a small signal diode is encapsulated in glass to protect the PN junction, andusually have a red or black band at one end of their body to help identify which end is the cathode terminal. Themost widely used of all the glass encapsulated signal diodes is the very common 1N4148 and its equivalent1N914 signal diode. Small signal and switching diodes have much lower power and current ratings, around150mA, 500mW maximum compared to rectifier diodes, but they can function better in high frequencyapplications or in clipping and switching applications that deal with short-duration pulse waveforms.The characteristics of a signal point contact diode are different for both germanium and silicon types and aregiven as: Germanium Signal Diodes - These have a low reverse resistance value giving a lower forward volt drop across the junction, typically only about 0.2-0.3v, but have a higher forward resistance value because of their small junction area. Silicon Signal Diodes - These have a very high value of reverse resistance and give a forward volt drop of about 0.6-0.7v across the junction. They have fairly low values of forward resistance giving them high peak values of forward current and reverse voltage.The electronic symbol given for any type of diode is that of an arrow with a bar or line at its end and this isillustrated below along with the Steady State V-I Characteristics Curve.Silicon Diode V-I Characteristic CurveThe arrow points in the direction of conventional current flow through the diode meaning that the diode willonly conduct if a positive supply is connected to the Anode (a) terminal and a negative supply is connected tothe Cathode (k) terminal thus only allowing current to flow through it in one direction only, acting more like aone way electrical valve, (Forward Biased Condition). However, we know from the previous tutorial that if weconnect the external energy source in the other direction the diode will block any current flowing through it andinstead will act like an open switch, (Reversed Biased Condition) as shown below.Forward and Reversed Biased DiodeThen we can say that an ideal small signal diode conducts current in one direction (forward-conducting) andblocks current in the other direction (reverse-blocking). Signal Diodes are used in a wide variety of applicationssuch as a switch in rectifiers, limiters, snubbers or in wave-shaping circuits.K. Adisesha Page 28
  • 29. Basics of ElectronicsSignal Diode ParametersSignal Diodes are manufactured in a range of voltage and current ratings and care must be taken when choosinga diode for a certain application. There are a bewildering array of static characteristics associated with thehumble signal diode but the more important ones are.1. Maximum Forward CurrentThe Maximum Forward Current (IF(max)) is as its name implies the maximum forward current allowed to flowthrough the device. When the diode is conducting in the forward bias condition, it has a very small "ON"resistance across the PN junction and therefore, power is dissipated across this junction (Ohm´s Law) in theform of heat. Then, exceeding its (IF(max)) value will cause more heat to be generated across the junction and thediode will fail due to thermal overload, usually with destructive consequences. When operating diodes aroundtheir maximum current ratings it is always best to provide additional cooling to dissipate the heat produced bythe diode.For example, our small 1N4148 signal diode has a maximum current rating of about 150mA with a powerdissipation of 500mW at 25oC. Then a resistor must be used in series with the diode to limit the forward current,(IF(max)) through it to below this value.2. Peak Inverse VoltageThe Peak Inverse Voltage (PIV) or Maximum Reverse Voltage (VR(max)), is the maximum allowable Reverseoperating voltage that can be applied across the diode without reverse breakdown and damage occurring to thedevice. This rating therefore, is usually less than the "avalanche breakdown" level on the reverse biascharacteristic curve. Typical values of VR(max) range from a few volts to thousands of volts and must beconsidered when replacing a diode.The peak inverse voltage is an important parameter and is mainly used for rectifying diodes in AC rectifiercircuits with reference to the amplitude of the voltage were the sinusoidal waveform changes from a positive toa negative value on each and every cycle.3. Total Power DissipationSignal diodes have a Total Power Dissipation, (PD(max)) rating. This rating is the maximum possible powerdissipation of the diode when it is forward biased (conducting). When current flows through the signal diode thebiasing of the PN junction is not perfect and offers some resistance to the flow of current resulting in powerbeing dissipated (lost) in the diode in the form of heat. As small signal diodes are nonlinear devices theresistance of the PN junction is not constant, it is a dynamic property then we cannot use Ohms Law to definethe power in terms of current and resistance or voltage and resistance as we can for resistors. Then to find thepower that will be dissipated by the diode we must multiply the voltage drop across it times the current flowingthrough it: PD = VxI4. Maximum Operating TemperatureThe Maximum Operating Temperature actually relates to the Junction Temperature (TJ) of the diode and isrelated to maximum power dissipation. It is the maximum temperature allowable before the structure of thediode deteriorates and is expressed in units of degrees centigrade per Watt, ( oC/W ). This value is linkedclosely to the maximum forward current of the device so that at this value the temperature of the junction is notexceeded. However, the maximum forward current will also depend upon the ambient temperature in which thedevice is operating so the maximum forward current is usually quoted for two or more ambient temperaturevalues such as 25oC or 70oC.Then there are three main parameters that must be considered when either selecting or replacing a signal diodeand these are: The Reverse Voltage Rating The Forward Current Rating The Forward Power Dissipation RatingSignal Diode ArraysWhen space is limited, or matching pairs of switching signal diodes are required, diode arrays can be veryuseful. They generally consist of low capacitance high speed silicon diodes such as the 1N4148 connectedK. Adisesha Page 29
  • 30. Basics of Electronicstogether in multiple diode packages called an array for use in switching and clamping in digital circuits. Theyare encased in single inline packages (SIP) containing 4 or more diodes connected internally to give either anindividual isolated array, common cathode, (CC), or a common anode, (CA) configuration as shown.Signal Diode ArraysSignal diode arrays can also be used in digital and computer circuits to protect high speed data lines or otherinput/output parallel ports against electrostatic discharge, (ESD) and voltage transients. By connecting twodiodes in series across the supply rails with the data line connected to their junction as shown, any unwantedtransients are quickly dissipated and as the signal diodes are available in 8-fold arrays they can protect eightdata lines in a single package.CPU Data Line ProtectionSignal diode arrays can also be used to connect together diodes in either series or parallel combinations to formvoltage regulator or voltage reducing type circuits or to produce a known fixed voltage. We know that theforward volt drop across a silicon diode is about 0.7v and by connecting together a number of diodes in seriesthe total voltage drop will be the sum of the individual voltage drops of each diode. However, when signaldiodes are connected together in series, the current will be the same for each diode so the maximum forwardcurrent must not be exceeded.Connecting Signal Diodes in SeriesAnother application for the small signal diode is to create a regulated voltage supply. Diodes are connectedtogether in series to provide a constant DC voltage across the diode combination. The output voltage across thediodes remains constant in spite of changes in the load current drawn from the series combination or changes inthe DC power supply voltage that feeds them. Consider the circuit below.Signal Diodes in SeriesK. Adisesha Page 30
  • 31. Basics of ElectronicsAs the forward voltage drop across a silicon diode is almost constant at about 0.7v, while the current through itvaries by relatively large amounts, a forward-biased signal diode can make a simple voltage regulating circuit.The individual voltage drops across each diode are subtracted from the supply voltage to leave a certain voltagepotential across the load resistor, and in our simple example above this is given as 10v - (3 x 0.7v) = 7.9v. Thisis because each diode has a junction resistance relating to the small signal current flowing through it and thethree signal diodes in series will have three times the value of this resistance, along with the load resistance R,forms a voltage divider across the supply.By adding more diodes in series a greater voltage reduction will occur. Also series connected diodes can beplaced in parallel with the load resistor to act as a voltage regulating circuit. Here the voltage applied to the loadresistor will be 3 x 0.7v = 2.1v. We can of course produce the same constant voltage source using a singleZener Diode. Resistor, RD is used to prevent excessive current flowing through the diodes if the load isremoved.Freewheel DiodesSignal diodes can also be used in a variety of clamping, protection and wave shaping circuits with the mostcommon form of clamping diode circuit being one which uses a diode connected in parallel with a coil orinductive load to prevent damage to the delicate switching circuit by suppressing the voltage spikes and/ortransients that are generated when the load is suddenly turned "OFF". This type of diode is generally known as a"Free-wheeling Diode" or Freewheel diode as it is more commonly called.The Freewheel diode is used to protect solid state switches such as power transistors and MOSFETs fromdamage by reverse battery protection as well as protection from highly inductive loads such as relay coils ormotors, and an example of its connection is shown below.Use of the Freewheel DiodeModern fast switching, power semiconductor devices require fast switching diodes such as free wheeling diodesto protect them form inductive loads such as motor coils or relay windings. Every time the switching deviceabove is turned "ON", the freewheel diode changes from a conducting state to a blocking state as it becomesreversed biased. However, when the device rapidly turns "OFF", the diode becomes forward biased and thecollapse of the energy stored in the coil causes a current to flow through the freewheel diode. Without theprotection of the freewheel diode high di/dt currents would occur causing a high voltage spike or transient toflow around the circuit possibly damaging the switching device.Previously, the operating speed of the semiconductor switching device, either transistor, MOSFET, IGBT ordigital has been impaired by the addition of a freewheel diode across the inductive load with Schottky andZener diodes being used instead in some applications. But during the past few years however, freewheel diodeshad regained importance due mainly to their improved reverse-recovery characteristics and the use of super fastsemiconductor materials capable at operating at high switching frequencies.Other types of specialized diodes not included here are Photo-Diodes, PIN Diodes, Tunnel Diodes and SchottkyBarrier Diodes. By adding more PN junctions to the basic two layer diode structure other types ofsemiconductor devices can be made. For example a three layer semiconductor device becomes a Transistor, afour layer semiconductor device becomes a Thyristor or Silicon Controlled Rectifier and five layer devicesknown as Triacs are also available.Half Wave RectificationA rectifier is a circuit which converts the Alternating Current (AC) input power into a Direct Current (DC)output power. The input power supply may be either a single-phase or a multi-phase supply with the simplest ofall the rectifier circuits being that of the Half Wave Rectifier. The power diode in a half wave rectifier circuitpasses just one half of each complete sine wave of the AC supply in order to convert it into a DC supply. ThenK. Adisesha Page 31
  • 32. Basics of Electronicsthis type of circuit is called a "half-wave" rectifier because it passes only half of the incoming AC power supplyas shown below.Half Wave Rectifier CircuitDuring each "positive" half cycle of the AC sine wave, the diode is forward biased as the anode is positive withrespect to the cathode resulting in current flowing through the diode. Since the DC load is resistive (resistor, R),the current flowing in the load resistor is therefore proportional to the voltage (Ohm´s Law), and the voltageacross the load resistor will therefore be the same as the supply voltage, Vs (minus Vf), that is the "DC" voltageacross the load is sinusoidal for the first half cycle only so Vout = Vs.During each "negative" half cycle of the AC sine wave, the diode is reverse biased as the anode is negative withrespect to the cathode therefore, No current flows through the diode or circuit. Then in the negative half cycle ofthe supply, no current flows in the load resistor as no voltage appears across it so Vout = 0.The current on the DC side of the circuit flows in one direction only making the circuit Unidirectional and thevalue of the DC voltage VDC across the load resistor is calculated as follows.Where Vmax is the maximum voltage value of the AC supply, and VS is the r.m.s. value of the supply.Example No1.Calculate the current, ( IDC ) flowing through a 100Ω resistor connected to a 240v single phase half-waverectifier as shown above. Also calculate the power consumed by the load.During the rectification process the resultant output DC voltage and current are therefore both "ON" and "OFF"during every cycle. As the voltage across the load resistor is only present during the positive half of the cycle(50% of the input waveform), this results in a low average DC value being supplied to the load. The variation ofthe rectified output waveform between this ON and OFF condition produces a waveform which has largeamounts of "ripple" which is an undesirable feature. The resultant DC ripple has a frequency that is equal to thatof the AC supply frequency.Very often when rectifying an alternating voltage we wish to produce a "steady" and continuous DC voltagefree from any voltage variations or ripple. One way of doing this is to connect a large value Capacitor acrossthe output voltage terminals in parallel with the load resistor as shown below. This type of capacitor is knowncommonly as a "Reservoir" or Smoothing Capacitor.Half-wave Rectifier with Smoothing CapacitorK. Adisesha Page 32
  • 33. Basics of ElectronicsWhen rectification is used to provide a direct voltage power supply from an alternating source, the amount ofripple can be further reduced by using larger value capacitors but there are limits both on cost and size. For agiven capacitor value, a greater load current (smaller load resistor) will discharge the capacitor more quickly( RC Time Constant ) and so increases the ripple obtained. Then for single phase, half-wave rectifier circuits itis not very practical to try and reduce the ripple voltage by capacitor smoothing alone, it is more practical to use"Full-wave Rectification" instead.The Full Wave RectifierIn the previous Power Diodes tutorial we discussed ways of reducing the ripple or voltage variations on a directDC voltage by connecting capacitors across the load resistance. While this method may be suitable for lowpower applications it is unsuitable to applications which need a "steady and smooth" DC supply voltage. Onemethod to improve on this is to use every half-cycle of the input voltage instead of every other half-cycle. Thecircuit which allows us to do this is called a Full Wave Rectifier.Like the half wave circuit, a full wave rectifier circuit produces an output voltage or current which is purely DCor has some specified DC component. Full wave rectifiers have some fundamental advantages over their halfwave rectifier counterparts. The average (DC) output voltage is higher than for half wave, the output of the fullwave rectifier has much less ripple than that of the half wave rectifier producing a smoother output waveform.In a Full Wave Rectifier circuit two diodes are now used, one for each half of the cycle. A transformer is usedwhose secondary winding is split equally into two halves with a common centre tapped connection, (C). Thisconfiguration results in each diode conducting in turn when its anode terminal is positive with respect to thetransformer centre point C producing an output during both half-cycles, twice that for the half wave rectifier soit is 100% efficient as shown below.Full Wave Rectifier CircuitThe full wave rectifier circuit consists of two power diodes connected to a single load resistance (RL) with eachdiode taking it in turn to supply current to the load. When point A of the transformer is positive with respect topoint C, diode D1 conducts in the forward direction as indicated by the arrows. When point B is positive (in thenegative half of the cycle) with respect to point C, diode D2 conducts in the forward direction and the currentflowing through resistor R is in the same direction for both half-cycles. As the output voltage across the resistorR is the phasor sum of the two waveforms combined, this type of full wave rectifier circuit is also known as a"bi-phase" circuit.K. Adisesha Page 33
  • 34. Basics of ElectronicsAs the spaces between each half-wave developed by each diode is now being filled in by the other diode theaverage DC output voltage across the load resistor is now double that of the single half-wave rectifier circuitand is about 0.637Vmax of the peak voltage, assuming no losses.Where: VMAX is the maximum peak value in one half of the secondary winding and VRMS is the rms value.The peak voltage of the output waveform is the same as before for the half-wave rectifier provided each half ofthe transformer windings have the same rms voltage value. To obtain a different DC voltage output differenttransformer ratios can be used. The main disadvantage of this type of full wave rectifier circuit is that a largertransformer for a given power output is required with two separate but identical secondary windings making thistype of full wave rectifying circuit costly compared to the "Full Wave Bridge Rectifier" circuit equivalent.The Full Wave Bridge RectifierAnother type of circuit that produces the same output waveform as the full wave rectifier circuit above, is thatof the Full Wave Bridge Rectifier. This type of single phase rectifier uses four individual rectifying diodesconnected in a closed loop "bridge" configuration to produce the desired output. The main advantage of thisbridge circuit is that it does not require a special centre tapped transformer, thereby reducing its size and cost.The single secondary winding is connected to one side of the diode bridge network and the load to the other sideas shown below.The Diode Bridge RectifierThe four diodes labelled D1 to D4 are arranged in "series pairs" with only two diodes conducting current duringeach half cycle. During the positive half cycle of the supply, diodes D1 and D2 conduct in series while diodesD3 and D4 are reverse biased and the current flows through the load as shown below.The Positive Half-cycleDuring the negative half cycle of the supply, diodes D3 and D4 conduct in series, but diodes D1 and D2 switch"OFF" as they are now reverse biased. The current flowing through the load is the same direction as before.The Negative Half-cycleK. Adisesha Page 34
  • 35. Basics of ElectronicsAs the current flowing through the load is unidirectional, so the voltage developed across the load is alsounidirectional the same as for the previous two diode full-wave rectifier, therefore the average DC voltageacross the load is 0.637Vmax. However in reality, during each half cycle the current flows through two diodesinstead of just one so the amplitude of the output voltage is two voltage drops ( 2 x 0.7 = 1.4V ) less than theinput VMAX amplitude. The ripple frequency is now twice the supply frequency (e.g. 100Hz for a 50Hz supply)Typical Bridge RectifierAlthough we can use four individual power diodes to make a full wave bridge rectifier, pre-made bridgerectifier components are available "off-the-shelf" in a range of different voltage and current sizes that can besoldered directly into a PCB circuit board or be connected by spade connectors. The image to the right shows atypical single phase bridge rectifier with one corner cut off. This cut-off corner indicates that the terminalnearest to the corner is the positive or +ve output terminal or lead with the opposite (diagonal) lead being thenegative or -ve output lead. The other two connecting leads are for the input alternating voltage from atransformer secondary winding.The Smoothing CapacitorWe saw in the previous section that the single phase half-wave rectifier produces an output wave every halfcycle and that it was not practical to use this type of circuit to produce a steady DC supply. The full-wavebridge rectifier however, gives us a greater mean DC value (0.637 Vmax) with less superimposed ripple whilethe output waveform is twice that of the frequency of the input supply frequency. We can therefore increase itsaverage DC output level even higher by connecting a suitable smoothing capacitor across the output of thebridge circuit as shown below.Full-wave Rectifier with Smoothing CapacitorThe smoothing capacitor converts the full-wave rippled output of the rectifier into a smooth DC output voltage.Generally for DC power supply circuits the smoothing capacitor is an Aluminium Electrolytic type that has acapacitance value of 100uF or more with repeated DC voltage pulses from the rectifier charging up thecapacitor to peak voltage. However, their are two important parameters to consider when choosing a suitablesmoothing capacitor and these are its Working Voltage, which must be higher than the no-load output value ofthe rectifier and its Capacitance Value, which determines the amount of ripple that will appear superimposed ontop of the DC voltage. Too low a value and the capacitor has little effect but if the smoothing capacitor is largeenough (parallel capacitors can be used) and the load current is not too large, the output voltage will be almostas smooth as pure DC. As a general rule of thumb, we are looking to have a ripple voltage of less than 100mVpeak to peak.K. Adisesha Page 35
  • 36. Basics of ElectronicsThe maximum ripple voltage present for a Full Wave Rectifier circuit is not only determined by the value ofthe smoothing capacitor but by the frequency and load current, and is calculated as:Bridge Rectifier Ripple VoltageWhere: I is the DC load current in amps, ƒ is the frequency of the ripple or twice the input frequency in Hertz,and C is the capacitance in Farads.The main advantages of a full-wave bridge rectifier is that it has a smaller AC ripple value for a given load anda smaller reservoir or smoothing capacitor than an equivalent half-wave rectifier. Therefore, the fundamentalfrequency of the ripple voltage is twice that of the AC supply frequency (100Hz) where for the half-waverectifier it is exactly equal to the supply frequency (50Hz).The amount of ripple voltage that is superimposed on top of the DC supply voltage by the diodes can bevirtually eliminated by adding a much improved π-filter (pi-filter) to the output terminals of the bridge rectifier.This type of low-pass filter consists of two smoothing capacitors, usually of the same value and a choke orinductance across them to introduce a high impedance path to the alternating ripple component. Another morepractical and cheaper alternative is to use a 3-terminal voltage regulator IC, such as a LM78xx for a positiveoutput voltage or the LM79xx for a negative output voltage which can reduce the ripple by more than 70dB(Datasheet) while delivering a constant output current of over 1 amp.The Zener DiodeIn the previous Signal Diode tutorial, we saw that a "reverse biased" diode blocks current in the reversedirection, but will suffer from premature breakdown or damage if the reverse voltage applied across it is toohigh. However, the Zener Diode or "Breakdown Diode" as they are sometimes called, are basically the same asthe standard PN junction diode but are specially designed to have a low pre-determined Reverse BreakdownVoltage that takes advantage of this high reverse voltage. The zener diode is the simplest types of voltageregulator and the point at which a zener diode breaks down or conducts is called the "Zener Voltage" (Vz).The Zener diode is like a general-purpose signal diode consisting of a silicon PN junction. When biased in theforward direction it behaves just like a normal signal diode passing the rated current, but as soon as a reversevoltage applied across the zener diode exceeds the rated voltage of the device, the diodes breakdown voltage V Bis reached at which point a process called Avalanche Breakdown occurs in the semiconductor depletion layerand a current starts to flow through the diode to limit this increase in voltage.The current now flowing through the zener diode increases dramatically to the maximum circuit value (which isusually limited by a series resistor) and once achived this reverse saturation current remains fairly constant overa wide range of applied voltages. This breakdown voltage point, VB is called the "zener voltage" for zenerdiodes and can range from less than one volt to hundreds of volts.The point at which the zener voltage triggers the current to flow through the diode can be very accuratelycontrolled (to less than 1% tolerance) in the doping stage of the diodes semiconductor construction giving thediode a specific zener breakdown voltage, (Vz) for example, 4.3V or 7.5V. This zener breakdown voltage onthe I-V curve is almost a vertical straight line.Zener Diode I-V CharacteristicsK. Adisesha Page 36
  • 37. Basics of ElectronicsThe Zener Diode is used in its "reverse bias" or reverse breakdown mode, i.e. the diodes anode connects to thenegative supply. From the I-V characteristics curve above, we can see that the zener diode has a region in itsreverse bias characteristics of almost a constant negative voltage regardless of the value of the current flowingthrough the diode and remains nearly constant even with large changes in current as long as the zener diodescurrent remains between the breakdown current IZ(min) and the maximum current rating IZ(max).This ability to control itself can be used to great effect to regulate or stabilise a voltage source against supply orload variations. The fact that the voltage across the diode in the breakdown region is almost constant turns outto be an important application of the zener diode as a voltage regulator. The function of a regulator is to providea constant output voltage to a load connected in parallel with it in spite of the ripples in the supply voltage or thevariation in the load current and the zener diode will continue to regulate the voltage until the diodes currentfalls below the minimum IZ(min) value in the reverse breakdown region.The Zener Diode RegulatorZener Diodes can be used to produce a stabilised voltage output with low ripple under varying load currentconditions. By passing a small current through the diode from a voltage source, via a suitable current limitingresistor (RS), the zener diode will conduct sufficient current to maintain a voltage drop of Vout. We rememberfrom the previous tutorials that the DC output voltage from the half or full-wave rectifiers contains ripplesuperimposed onto the DC voltage and that as the load value changes so to does the average output voltage. Byconnecting a simple zener stabiliser circuit as shown below across the output of the rectifier, a more stableoutput voltage can be produced.Zener Diode RegulatorThe resistor, RS is connected in series with the zener diode to limit the current flow through the diode with thevoltage source, VS being connected across the combination. The stabilised output voltage Vout is taken fromacross the zener diode. The zener diode is connected with its cathode terminal connected to the positive rail ofthe DC supply so it is reverse biased and will be operating in its breakdown condition. Resistor RS is selected soto limit the maximum current flowing in the circuit.With no load connected to the circuit, the load current will be zero, ( IL = 0 ), and all the circuit current passesthrough the zener diode which inturn dissipates its maximum power. Also a small value of the series resistor R Swill result in a greater diode current when the load resistance RL is connected and large as this will increase thepower dissipation requirement of the diode so care must be taken when selecting the appropriate value of seriesresistance so that the zeners maximum power rating is not exceeded under this no-load or high-impedancecondition.The load is connected in parallel with the zener diode, so the voltage across RL is always the same as the zenervoltage, ( VR = VZ ). There is a minimum zener current for which the stabilization of the voltage is effective andthe zener current must stay above this value operating under load within its breakdown region at all times. Theupper limit of current is of course dependant upon the power rating of the device. The supply voltage V S mustbe greater than VZ.One small problem with zener diode stabiliser circuits is that the diode can sometimes generate electrical noiseon top of the DC supply as it tries to stabilise the voltage. Normally this is not a problem for most applicationsbut the addition of a large value decoupling capacitor across the zeners output may be required to giveadditional smoothing.Then to summarise a little. A zener diode is always operated in its reverse biased condition. A voltage regulatorcircuit can be designed using a zener diode to maintain a constant DC output voltage across the load in spite ofK. Adisesha Page 37
  • 38. Basics of Electronicsvariations in the input voltage or changes in the load current. The zener voltage regulator consists of a currentlimiting resistor RS connected in series with the input voltage VS with the zener diode connected in parallel withthe load RL in this reverse biased condition. The stabilized output voltage is always selected to be the same asthe breakdown voltage VZ of the diode.Example No1A 5.0V stabilised power supply is required to be produced from a 12V DC power supply input source. Themaximum power rating PZ of the zener diode is 2W. Using the zener regulator circuit above calculate:a) The maximum current flowing through the zener diode.b) The minimum value of the series resistor, RSc) The load current IL if a load resistor of 1kΩ is connected across the Zener diode.d) The total supply current IS at full load.Zener Diode VoltagesAs well as producing a single stabilised voltage output, zener diodes can also be connected together in seriesalong with normal silicon signal diodes to produce a variety of different reference voltage output values asshown below.Zener Diodes Connected in SeriesThe values of the individual Zener diodes can be chosen to suit the application while the silicon diode willalways drop about 0.6 - 0.7V in the forward bias condition. The supply voltage, Vin must of course be higherthan the largest output reference voltage and in our example above this is 19v.A typical zener diode for general electronic circuits is the 500mW, BZX55 series or the larger 1.3W, BZX85series were the zener voltage is given as, for example, C7V5 for a 7.5V diode giving a diode reference numberof BZX55C7V5. The 500mW series of zener diodes are available from about 2.4 up to about 100 volts andtypically have the same sequence of values as used for the 5% (E24) resistor series with the individual voltageratings for these small but very useful diodes are given in the table below.Zener Diode Clipping CircuitsDiode clipping and clamping circuits are circuits that are used to shape or modify an input AC waveform (orany sinusoid) producing a differently shape output waveform depending on the circuit arrangement. DiodeK. Adisesha Page 38
  • 39. Basics of Electronicsclipper circuits are also called limiters because they limit or clip-off the positive (or negative) part of an inputAC signal. As zener clipper circuits limit or cut-off part of the waveform across them, they are mainly used forcircuit protection or in waveform shaping circuits. For example, if we wanted to clip an output waveform at+7.5V, we would use a 7.5V zener diode. If the output waveform tries to exceed the 7.5V limit, the zener diodewill "clip-off" the excess voltage from the input producing a waveform with a flat top still keeping the outputconstant at +7.5V. Note that in the forward bias condition a zener diode is still a diode and when the ACwaveform output goes negative below -0.7V, the zener diode turns "ON" like any normal silicon diode wouldand clips the output at -0.7V as shown below.Square Wave SignalThe back to back connected zener diodes can be used as an AC regulator producing what is jokingly called a"poor mans square wave generator". Using this arrangement we can clip the waveform between a positive valueof +8.2V and a negative value of -8.2V for a 7.5V zener diode. If we wanted to clip an output waveformbetween different minimum and maximum values for example, +8V and -6V, use would simply use twodifferently rated zener diodes.Note that the output will actually clip the AC waveform between +8.7V and -6.7V due to the addition of theforward biasing diode voltage, which adds another 0.7V voltage drop to it. This type of clipper configuration isfairly common for protecting an electronic circuit from over voltage. The two zeners are generally placed acrossthe power supply input terminals and during normal operation, one of the zener diodes is "OFF" and the diodeshave little or no affect. However, if the input voltage waveform exceeds its limit, then the zeners turn "ON" andclip the input to protect the circuit.Introduction to Digital Logic GatesA Digital Logic Gate is an electronic device that makes logical decisions based on the different combinationsof digital signals present on its inputs. A digital logic gate may have more than one input but only has onedigital output. Standard commercially available digital logic gates are available in two basic families or forms,TTL which stands for Transistor-Transistor Logic such as the 7400 series, and CMOS which stands forComplementary Metal-Oxide-Silicon which is the 4000 series of chips. This notation of TTL or CMOS refers tothe logic technology used to manufacture the integrated circuit, (IC) or a "chip" as it is more commonly called.Digital Logic GateGenerally speaking, TTL ICs use NPN (or PNP) type Bipolar Junction Transistors while CMOS ICs useField Effect Transistors or FETs for both their input and output circuitry. As well as TTL and CMOStechnology, simple digital logic gates can also be made by connecting together diodes, transistors and resistorsto produce RTL, Resistor-Transistor logic gates, DTL, Diode-Transistor logic gates or ECL, Emitter-Coupledlogic gates but these are less common now compared to the popular CMOS family.K. Adisesha Page 39
  • 40. Basics of ElectronicsIntegrated Circuits or ICs as they are more commonly called, can be grouped together into families accordingto the number of transistors or "gates" that they contain. For example, a simple AND gate my contain only afew individual transistors, were as a more complex microprocessor may contain many thousands of individualtransistor gates. Integrated circuits are categorised according to the number of logic gates or the complexity ofthe circuits within a single chip with the general classification for the number of individual gates given as:Classification of Integrated Circuits Small Scale Integration or (SSI) - Contain up to 10 transistors or a few gates within a single package such as AND, OR, NOT gates. Medium Scale Integration or (MSI) - between 10 and 100 transistors or tens of gates within a single package and perform digital operations such as adders, decoders, counters, flip-flops and multiplexers. Large Scale Integration or (LSI) - between 100 and 1,000 transistors or hundreds of gates and perform specific digital operations such as I/O chips, memory, arithmetic and logic units. Very-Large Scale Integration or (VLSI) - between 1,000 and 10,000 transistors or thousands of gates and perform computational operations such as processors, large memory arrays and programmable logic devices. Super-Large Scale Integration or (SLSI) - between 10,000 and 100,000 transistors within a single package and perform computational operations such as microprocessor chips, micro-controllers, basic PICs and calculators. Ultra-Large Scale Integration or (ULSI) - more than 1 million transistors - the big boys that are used in computers CPUs, GPUs, video processors, micro-controllers, FPGAs and complex PICs.While the "ultra large scale" ULSI classification is less well used, another level of integration which representsthe complexity of the Integrated Circuit is known as the System-on-Chip or (SOC) for short. Here theindividual components such as the microprocessor, memory, peripherals, I/O logic etc, are all produced on asingle piece of silicon and which represents a whole electronic system within one single chip, literally puttingthe word "integrated" into integrated circuit.Moores LawIn 1965, Gordon Moore co-founder of the Intel corporation predicted that "The number of transistors andresistors on a single chip will double every 18 months" regarding the development of semiconductor gatetechnology. When Moore made his famous comment way back in 1965 there were approximately only 60individual transistor gates on a single silicon chip or die. Today, the Intel Corporation have placed around 2.0Billion individual transistor gates onto its new Quad-core Itanium 64-bit microprocessor chip and the count isstill rising!.Digital Logic StatesThe Digital Logic Gate is the basic building block from which all digital electronic circuits and microprocessorbased systems are constructed from. Basic digital logic gates perform logical operations of AND, OR and NOTon binary numbers. In digital logic design only two voltage levels or states are allowed and these states aregenerally referred to as Logic "1" and Logic "0", High and Low, True and False and which are represented inBoolean Algebra and Truth Tables by the binary digits of "1" and "0" respectively. A good example of adigital signal is a simple light as it is either "ON" or "OFF" but not both at the same time.Most digital logic gates and logic systems use "Positive logic", in which a logic level "0" or "LOW" isrepresented by a zero voltage, 0v or ground and a logic level "1" or "HIGH" is represented by a higher voltagesuch as +5 volts, with the switching from one voltage level to the other, from either a logic level "0" to a "1" ora "1" to a "0" being made as quickly as possible to prevent any faulty operation of the logic circuit. There alsoexists a complementary "Negative Logic" system in which the values and the rules of a logic "0" and a logic "1"are reversed but in this tutorial section about digital logic gates we shall only refer to the positive logicconvention as it is the most commonly used.In standard TTL (transistor-transistor logic) ICs there is a pre-defined voltage range for the input and outputvoltage levels which define exactly what is a logic "1" level and what is a logic "0" level and these are shownbelow.TTL Input & Output Voltage LevelsK. Adisesha Page 40
  • 41. Basics of ElectronicsThere are a large variety of logic gate types in both the bipolar 7400 and the CMOS 4000 families of digitallogic gates such as 74Lxx, 74LSxx, 74ALSxx, 74HCxx, 74HCTxx, 74ACTxx etc, with each one having its owndistinct advantages and disadvantages compared to the other. The exact switching voltage required to produceeither a logic "0" or a logic "1" depends upon the specific logic group or family. However, when using astandard +5 volt supply any TTL voltage input between 2.0v and 5v is considered to be a logic "1" or "HIGH"while any voltage input below 0.8v is recognised as a logic "0" or "LOW". The voltage region in between thesetwo voltage levels either as an input or as an output is called the Indeterminate Region and operating within thisregion may cause the logic gate to produce a false output. The CMOS 4000 logic family uses a different level ofvoltages compared to the TTL types with a logic "1" level operating between 3.0 and 18 volts and a logic "0"level below 1.5 volts.Then from the above observations, we can define the ideal Digital Logic Gate as one that has a "LOW" levellogic "0" of 0 volts (ground) and a "HIGH" level logic "1" of +5 volts and this can be demonstrated as:Ideal Digital Logic Voltage LevelsWhere the opening or closing of the switch produces either a logic level "1" or a logic level "0" with the resistorR being known as a "pull-up" resistor.Simple Basic Digital Logic GatesSimple digital logic gates can be made by combining transistors, diodes and resistors with a simple example ofa Diode-Resistor Logic (DRL) AND gate and a Diode-Transistor Logic (DTL) NAND gate given below. Diode-Resistor circuit Diode-Transistor circuit 2-input AND gate 2-input NAND gateThe simple 2-input Diode-Resistor AND gate can be converted into a NAND gate by the addition of a singletransistor inverting (NOT) stage. Using discrete components such as diodes, resistors and transistors to makedigital logic gate circuits are not used in practical commercially available logic ICs as these circuits suffer frompropagation delay or gate delay and power loss due to the pull-up resistors, also there is no "Fan-out" facilitywhich is the ability of a single output to drive many inputs of the next stages. Also this type of design does notK. Adisesha Page 41
  • 42. Basics of Electronicsturn fully "OFF" as a Logic "0" produces an output voltage of 0.6v (diode voltage drop), so the following TTLand CMOS circuit designs are used instead.Basic TTL Logic GatesThe simple Diode-Resistor AND gate above uses separate diodes for its inputs, one for each input. As atransistor is made up off two diode circuits connected together representing an NPN or a PNP device, the inputdiodes of the DTL circuit can be replaced by one single NPN transistor with multiple emitter inputs as shown.2-input NAND gateAs the gate contains a single stage inverting NPN transistor circuit (TR2) an output logic level "1" at Q is onlypresent when both the emitters of TR1 are connected to logic level "0" or ground allowing base current to passthrough the PN junctions of the emitter and not the collector. The multiple emitters of TR1 are connected asinputs thus producing a NAND gate function.In standard TTL logic gates, the transistors operate either completely in the "cut off" region, or else completelyin the saturated region, Transistor as a Switch type operation.Emitter-Coupled Digital Logic GateEmitter Coupled Logic or ECL is another type of digital logic gate that uses bipolar transistor logic where thetransistors are not operated in the saturation region, as they are with the standard TTL digital logic gate. Insteadthe input and output circuits are push-pull connected transistors with the supply voltage negative with respect toground. This has the effect of increasing the speed of operation of the ECL gates up to the Gigahertz rangecompared with the standard TTL types, but noise has a greater effect in ECL logic, because the unsaturatedtransistors operate within their active region and amplify as well as switch signals.The "74" Sub-families of Integrated CircuitsWith improvements in the circuit design to take account of propagation delays, current consumption, fan-in andfan-out requirements etc, this type of TTL bipolar transistor technology forms the basis of the prefixed "74"family of digital logic ICs, such as the "7400" Quad 2-input AND gate, or the "7402" Quad 2-input OR gate.Sub-families of the 74xx series ICs are available relating to the different technologies used to fabricate thegates and they are denoted by the letters in between the 74 designation and the device number. There are anumber of TTL sub-families available that provide a wide range of switching speeds and power consumptionsuch as the 74L00 or 74ALS00 AND gate, were the "L" stands for "Low-power TTL" and the "ALS" stands for"Advanced Low-power Schottky TTL" and these are listed below. 74xx or 74Nxx: Standard TTL - These devices are the original TTL family of logic gates introduced in the early 70s. They have a propagation delay of about 10ns and a power consumption of about 10mW. 74Lxx: Low Power TTL - Power consumption was improved over standard types by increasing the number of internal resistances but at the cost of a reduction in switching speed. 74Hxx: High Speed TTL - Switching speed was improved by reducing the number of internal resistances. This also increased the power consumption. 74Sxx: Schottky TTL - Schottky technology is used to improve input impedance, switching speed and power consumption (2mW) compared to the 74Lxx and 74Hxx types. 74LSxx: Low Power Schottky TTL - Same as 74Sxx types but with increased internal resistances to improve power consumption. 74ASxx: Advanced Schottky TTL - Improved design over 74Sxx Schottky types optimised to increase switching speed at the expense of power consumption of about 22mW. 74ALSxx: Advanced Low Power Schottky TTL - Lower power consumption of about 1mW and higher switching speed of about 4nS compared to 74LSxx types.K. Adisesha Page 42
  • 43. Basics of Electronics 74HCxx: High Speed CMOS - CMOS technology and transistors to reduce power consumption of less than 1uA with CMOS compatible inputs. 74HCTxx: High Speed CMOS - CMOS technology and transistors to reduce power consumption of less than 1uA but has increased propagation delay of about 16nS due to the TTL compatible inputs.Basic CMOS Digital Logic GateOne of the main disadvantages of the TTL logic series is that the gates are based on bipolar transistor logictechnology and as transistors are current operated devices, they consume large amounts of power from a fixed+5 volt power supply. Also, TTL bipolar transistor gates have a limited operating speed when switching froman "OFF" state to an "ON" state and vice-versa called the "gate" or "propagation delay". To overcome theselimitations complementary MOS called "CMOS" logic gates using "Field Effect Transistors" or FETs weredeveloped.As these gates use both P-channel and N-channel MOSFETs as their input device, at quiescent conditions withno switching, the power consumption of CMOS gates is almost zero, (1 to 2uA) making them ideal for use inlow-power battery circuits and with switching speeds upwards of 100MHz for use in high frequency timing andcomputer circuits.2-input NAND gateThis CMOS gate example contains 3 N-channel MOSFETs, one for each input FET1 and FET2 and one for theoutput FET3. When both the inputs A and B are at logic level "0", FET1 and FET2 are both switched "OFF"giving an output logic "1" from the source of FET3. When one or both of the inputs are at logic level "1" currentflows through the corresponding FET giving an output state at Q equivalent to logic "0", thus producing aNAND gate function.Improvements in the circuit design with regards to switching speed, low power consumption and improvedpropagation delays has resulted in the standard CMOS 4000 "CD" family of logic ICs being developed thatcomplement the TTL range. As with the standard TTL digital logic gates, all the major digital logic gates anddevices are available in the CMOS package such as the CD4011, a Quad 2-input NAND gate, or the CD4001, aQuad 2-input NOR gate along with all their sub-families.Like TTL logic, complementary MOS (CMOS) circuits take advantage of the fact that both N-channel and P-channel devices can be fabricated on the same substrate and connected together to form logic functions. Onemain disadvantage with the CMOS range of ICs compared to their equivalent TTL types is that they are easilydamaged by static electricity so extra care must be taken when handling these devices. Also unlike TTL logicgates that operate on single +5V voltages for both their input and output levels, CMOS digital logic gatesoperate on a single supply voltage of between +3 and +18 volts.The Logic "AND" GateDefinitionA Logic AND Gate is a type of digital logic gate that has an output which is normally at logic level "0" andonly goes "HIGH" to a logic level "1" when ALL of its inputs are at logic level "1". The output of a Logic ANDGate only returns "LOW" again when ANY of its inputs are at a logic level "0". The logic or Booleanexpression given for a logic AND gate is that for Logical Multiplication which is denoted by a single dot or fullstop symbol, (.) giving us the Boolean expression of: A.B = Q.Then we can define the operation of a 2-input logic AND gate as being:"If both A and B are true, then Q is true"2-input Transistor AND GateA simple 2-input logic AND gate can be constructed using RTL Resistor-transistor switches connected togetheras shown below with the inputs connected directly to the transistor bases. Both transistors must be saturated"ON" for an output at Q.K. Adisesha Page 43
  • 44. Basics of ElectronicsLogic AND Gates are available using digital circuits to produce the desired logical function and is given asymbol whose shape represents the logical operation of the AND gate.The Digital Logic "AND" Gate2-input AND Gate Symbol Truth Table B A Q 0 0 0 0 1 0 2-input AND Gate 1 0 0 1 1 1 Boolean Expression Q = A.B Read as A AND B gives QCommonly available digital logic AND gate ICs include: TTL Logic Types CMOS Logic Types 74LS08 Quad 2-input CD4081 Quad 2-input 74LS11 Triple 3-input CD4073 Triple 3-input 74LS21 Dual 4-input CD4082 Dual 4-inputQuad 2-input AND Gate 7408The Logic "OR" GateDefinitionA Logic OR Gate or Inclusive-OR gate is a type of digital logic gate that has an output which is normally atlogic level "0" and only goes "HIGH" to a logic level "1" when ANY of its inputs are at logic level "1". Theoutput of a Logic OR Gate only returns "LOW" again when ALL of its inputs are at a logic level "0". The logicor Boolean expression given for a logic OR gate is that for Logical Addition which is denoted by a plus sign, (+)giving us the Boolean expression of: A+B = Q.Then we can define the operation of a 2-input logic OR gate as being:"If either A or B is true, then Q is true"2-input Transistor OR GateA simple 2-input logic OR gate can be constructed using RTL Resistor-transistor switches connected togetheras shown below with the inputs connected directly to the transistor bases. Either transistor must be saturated"ON" for an output at Q.K. Adisesha Page 44
  • 45. Basics of ElectronicsLogic OR Gates are available using digital circuits to produce the desired logical function and is given asymbol whose shape represents the logical operation of the OR gate.The Digital Logic "OR" Gate2-input OR Gate Symbol Truth Table B A Q 0 0 0 0 1 1 2-input OR Gate 1 0 1 1 1 1 Boolean Expression Q = A+B Read as A OR B gives QCommonly available OR gate ICs include: TTL Logic Types CMOS Logic Types 74LS32 Quad 2-input CD4071 Quad 2-input CD4075 Triple 3-input CD4072 Dual 4-inputQuad 2-input OR Gate 7432In the next tutorial about Digital Logic Gates, we will look at the digital logic NOT Gate function as used inboth TTL and CMOS logic circuits as well as its Boolean Algebra definition and truth table.The Digital Logic "NOT" GateDefinitionThe digital Logic NOT Gate is the most basic of all the logical gates and is sometimes referred to as anInverting Buffer or simply a Digital Inverter. It is a single input device which has an output level that isnormally at logic level "1" and goes "LOW" to a logic level "0" when its single input is at logic level "1", inother words it "inverts" (complements) its input signal. The output from a NOT gate only returns "HIGH" againwhen its input is at logic level "0" giving us the Boolean expression of: A = Q.Then we can define the operation of a single input logic NOT gate as being:"If A is NOT true, then Q is true"Transistor NOT GateK. Adisesha Page 45
  • 46. Basics of ElectronicsA simple 2-input logic NOT gate can be constructed using a RTL Resistor-transistor switches as shown belowwith the input connected directly to the transistor base. The transistor must be saturated "ON" for an inversedoutput "OFF" at Q.Logic NOT Gates are available using digital circuits to produce the desired logical function. The standard NOTgate is given a symbol whose shape is of a triangle pointing to the right with a circle at its end. This circle isknown as an "inversion bubble" and is used in NOT, NAND and NOR symbols at their output to represent thelogical operation of the NOT function. This bubble denotes a signal inversion (complementation) of the signaland can be present on either or both the output and/or the input terminals.The Digital Inverter or NOT gate Symbol Truth Table A Q 0 1 Inverter or NOT Gate 1 0 Boolean Expression Q = not A or A Read as inverse of A gives QLogic NOT gates provide the complement of their input signal and are so called because when their input signalis "HIGH" their output state will NOT be "HIGH". Likewise, when their input signal is "LOW" their outputstate will NOT be "LOW". As they are single input devices, logic NOT gates are not normally classed as"decision" making devices or even as a gate, such as the AND or OR gates which have two or more logicinputs. Commercial available NOT gates ICs are available in either 4 or 6 individual gates within a single i.c.package.The "bubble" (o) present at the end of the NOT gate symbol above denotes a signal inversion (complimentation)of the output signal. But this bubble can also be present at the gates input to indicate an active-LOW input. Thisinversion of the input signal is not restricted to the NOT gate only but can be used on any digital circuit or gateas shown with the operation of inversion being exactly the same whether on the input or output terminal. Theeasiest way is to think of the bubble as simply an inverter.Signal Inversion using Active-low input Bubble Bubble Notation for Input InversionNAND and NOR Gate EquivalentsAn Inverter or logic NOT gate can also be made using standard NAND and NOR gates by connecting togetherALL their inputs to a common input signal for example.K. Adisesha Page 46
  • 47. Basics of Electronics Also a very simple inverter can also be made using just a single stage transistor switching circuit as shown. When the transistors base input at "A" is high, the transistor conducts and collector current flows producing a voltage drop across the resistor R thereby connecting the output point at "Q" to ground thus resulting in a zero voltage output at "Q". When the transistors base input at "A" is low, the transistor now switches "OFF" and no collector current flows through the resistor resulting in an output voltage at "Q" high at a value near to +Vcc.Then, with an input voltage at "A" HIGH, the output at "Q" will be LOW and an input voltage at "A" LOW theresulting output voltage at "Q" is HIGH producing the complement of the input signal.Hex Schmitt InvertersA standard Inverter or Logic NOT Gate, is usually made up from transistor switching circuits that do notswitch from one state to the next instantly, there is some delay. Also as a transistor is a basic current amplifier,it can also operate in a linear mode and any small variation to its input level will cause a variation to its outputlevel or may even switch "ON" and "OFF" several times if there is any noise present in the circuit. One way toovercome these problems is to use a Schmitt Inverter or Hex Inverter.We know from the previous pages that all digital gates use only two logic voltage states and that these aregenerally referred to as Logic "1" and Logic "0" any TTL voltage input between 2.0v and 5v is recognised asa logic "1" and any voltage input below 0.8v is recognised as a logic "0" respectively. A Schmitt Inverter isdesigned to operate or switch state when its input signal goes above an "Upper Threshold Voltage" limit inwhich case the output changes and goes "LOW", and will remain in that state until the input signal falls belowthe "Lower Threshold Voltage" level in which case the output signal goes "HIGH". In other words a SchmittInverter has some form of Hysteresis built into its switching circuit. This switching action between an upperand lower threshold limit provides a much cleaner and faster "ON/OFF" switching output signal and makes theSchmitt inverter ideal for switching any slow-rising or slow-falling input signal either an analogue or digitalsignal.Schmitt InverterA very useful application of Schmitt inverters is when they are used as oscillators or sine-to-square waveconverters for use as square wave clock signals.Schmitt Inverter Oscillator & ConverterThe first circuit shows a very simple low power RC type oscillator using a Schmitt inverter to generate squarewaves. Initially the capacitor C is fully discharged so the input to the inverter is "LOW" resulting in an invertedoutput which is "HIGH". As the output from the inverter is fed back to its input and the capacitor via the resistorR the capacitor begins to charge up. When the capacitors charging voltage reaches the upper threshold limit ofthe inverter, the inverter changes state, the output becomes "LOW" and the capacitor begins to dischargethrough the resistor until it reaches the lower threshold level were the inverter changes state again. Thisswitching back and forth by the inverter produces a square wave output signal with a 33% duty cycle and whosefrequency is given as: ƒ = 680/RC.K. Adisesha Page 47
  • 48. Basics of ElectronicsThe second circuit converts a sine wave input (or any oscillating input for that matter) into a square waveoutput. The input to the inverter is connected to the junction of the potential divider network which is used toset the quiescent point of the circuit. The input capacitor blocks any DC component present in the input signalonly allowing the sine wave signal to pass. As this signal passes the upper and lower threshold points of theinverter the output also changes from "HIGH" to "LOW" and so on producing a square wave output waveform.This circuit produces an output pulse on the positive rising edge of the input waveform, but by connecting asecond Schmitt inverter to the output of the first, the basic circuit can be modified to produce an output pulse onthe negative falling edge of the input signal.Commonly available logic NOT gate and Inverter ICs include TTL Logic Types CMOS Logic Types 74LS04 Hex Inverting NOT Gate CD4009 Hex Inverting NOT Gate 74LS04 Hex Inverting NOT Gate CD4069 Hex Inverting NOT Gate 74LS14 Hex Schmitt Inverting NOT Gate 74LS1004 Hex Inverting DriversInverter or NOT Gate 7404The Logic "NAND" GateDefinitionThe Logic NAND Gate is a combination of the digital logic AND gate with that of an inverter or NOT gateconnected together in series. The NAND (Not - AND) gate has an output that is normally at logic level "1" andonly goes "LOW" to logic level "0" when ALL of its inputs are at logic level "1". The Logic NAND Gate is thereverse or "Complementary" form of the AND gate we have seen previously.Logic NAND Gate EquivalenceThe logic or Boolean expression given for a logic NAND gate is that for Logical Addition, which is the oppositeto the AND gate, and which it performs on the complements of the inputs. The Boolean expression for a logicNAND gate is denoted by a single dot or full stop symbol, (.) with a line or Overline, ( ‾‾ ) over the expressionto signify the NOT or logical negation of the NAND gate giving us the Boolean expression of: A.B = Q.Then we can define the operation of a 2-input logic NAND gate as being:"If either A or B are NOT true, then Q is true"Transistor NAND GateA simple 2-input logic NAND gate can be constructed using RTL Resistor-transistor switches connectedtogether as shown below with the inputs connected directly to the transistor bases. Either transistor must be cut-off "OFF" for an output at Q.K. Adisesha Page 48
  • 49. Basics of ElectronicsLogic NAND Gates are available using digital circuits to produce the desired logical function and is given asymbol whose shape is that of a standard AND gate with a circle, sometimes called an "inversion bubble" at itsoutput to represent the NOT gate symbol with the logical operation of the NAND gate given as.The Digital Logic "NAND" Gate2-input NAND Gate Symbol Truth Table B A Q 0 0 1 0 1 1 2-input NAND Gate 1 0 1 1 1 0 Boolean Expression Q = A.B Read as A AND B gives NOT QThe "Universal" NAND GateThe Logic NAND Gate is generally classed as a "Universal" gate because it is one of the most commonly usedlogic gate types. NAND gates can also be used to produce any other type of logic gate function, and in practicethe NAND gate forms the basis of most practical logic circuits. By connecting them together in variouscombinations the three basic gate types of AND, OR and NOT function can be formed using only NANDs, forexample.Various Logic Gates using only NAND GatesAs well as the three common types above, Ex-Or, Ex-Nor and standard NOR gates can be formed using justindividual NAND gates.Commonly available logic NAND gate ICs include: TTL Logic Types CMOS Logic Types 74LS00 Quad 2-input CD4011 Quad 2-input 74LS10 Triple 3-input CD4023 Triple 3-input 74LS20 Dual 4-input CD4012 Dual 4-input 74LS30 Single 8-inputQuad 2-input NAND Gate 7400The Logic "NOR" GateDefinitionThe Logic NOR Gate or Inclusive-NOR gate is a combination of the digital logic OR gate with that of aninverter or NOT gate connected together in series. The NOR (Not - OR) gate has an output that is normally atlogic level "1" and only goes "LOW" to logic level "0" when ANY of its inputs are at logic level "1". The LogicNOR Gate is the reverse or "Complementary" form of the OR gate we have seen previously.K. Adisesha Page 49
  • 50. Basics of ElectronicsNOR Gate EquivalentThe logic or Boolean expression given for a logic NOR gate is that for Logical Multiplication which it performson the complements of the inputs. The Boolean expression for a logic NOR gate is denoted by a plus sign, (+)with a line or Overline, ( ‾‾ ) over the expression to signify the NOT or logical negation of the NOR gate givingus the Boolean expression of: A+B = Q.Then we can define the operation of a 2-input logic NOR gate as being:"If both A and B are NOT true, then Q is true"Transistor NOR GateA simple 2-input logic NOR gate can be constructed using RTL Resistor-transistor switches connected togetheras shown below with the inputs connected directly to the transistor bases. Both transistors must be cut-off"OFF" for an output at Q.Logic NOR Gates are available using digital circuits to produce the desired logical function and is given asymbol whose shape is that of a standard OR gate with a circle, sometimes called an "inversion bubble" at itsoutput to represent the NOT gate symbol with the logical operation of the NOR gate given as.The Digital Logic "NOR" Gate2-input NOR Gate Symbol Truth Table B A Q 0 0 1 0 1 0 2-input NOR Gate 1 0 0 1 1 0 Boolean Expression Q = A+B Read as A OR B gives NOT QAs with the OR function, the NOR function can also have any number of individual inputs and commercialavailable NOR Gate ICs are available in standard 2, 3, or 4 input types. If additional inputs are required, thenthe standard NOR gates can be cascaded together to provide more inputs for example.Various Logic Gates using only NOR GatesK. Adisesha Page 50
  • 51. Basics of ElectronicsAs well as the three common types above, Ex-Or, Ex-Nor and standard NOR gates can also be formed usingjust individual NOR gates.Commonly available NOR gate ICs include: TTL Logic Types CMOS Logic Types 74LS02 Quad 2-input CD4001 Quad 2-input 74LS27 Triple 3-input CD4025 Triple 3-input 74LS260 Dual 4-input CD4002 Dual 4-inputQuad 2-input NOR Gate 7402The Exclusive-OR GateDefinitionPreviously, we have seen that for a 2-input OR gate, if A = "1", OR B = "1", OR BOTH A + B = "1" then theoutput from the gate is also at logic level "1" and this is known as an Inclusive-OR function because it includesthe case of Q = "1" when both A and B = "1". If however, an output "1" is obtained ONLY when A = "1" orwhen B = "1" but NOT both together at the same time, then this type of gate is known as an Exclusive-ORfunction or an Ex-Or function for short because it excludes the "OR BOTH" case of Q = "1" when both A andB = "1".In other words the output of an Exclusive-OR gate ONLY goes "HIGH" when its two input terminals are at"DIFFERENT" logic levels with respect to each other and they can both be at logic level "1" or both at logiclevel "0" giving us the Boolean expression of: Q = (A B) = A.B + A.BThe Exclusive-OR Gate function is achieved is achieved by combining standard gates together to form morecomplex gate functions. An example of a 2-input Exclusive-OR gate is given below.The Digital Logic "Ex-OR" Gate2-input Ex-OR Gate Symbol Truth Table B A Q 0 0 0 0 1 1 2-input Ex-OR Gate 1 0 1 1 1 0 Boolean Expression Q = A B Read as A OR B but NOT BOTH gives QThen, the logic function implemented by a 2-input Ex-OR is given as "either A OR B but NOT both" will givean output at Q. In general, an Ex-OR gate will give an output value of logic "1" ONLY when there are an ODDnumber of 1s on the inputs to the gate. Then an Ex-OR function with more than two inputs is called an "oddfunction" or modulo-2-sum (Mod-2-SUM), not an Ex-OR. This description can be expanded to apply to anynumber of individual inputs as shown below for a 3-input Ex-OR gate.Ex-OR Function Realisation using NAND gatesK. Adisesha Page 51
  • 52. Basics of ElectronicsExclusive-OR Gates are used mainly to build circuits that perform arithmetic operations and calculationsespecially Adders and Half-Adders as they can provide a "carry-bit" function or as a controlled inverter, whereone input passes the binary data and the other input is supplied with a control signal.Commonly available Exclusive-OR gate ICs include: TTL Logic Types CMOS Logic Types 74LS86 Quad 2-input CD4030 Quad 2-inputQuad 2-input Ex-OR Gate 7486The Exclusive-NOR GateDefinitionThe Exclusive-NOR Gate function or Ex-NOR for short, is a digital logic gate that is the reverse orcomplementary form of the Exclusive-OR function we look at in the previous section. It is a combination of theExclusive-OR gate and the NOT gate but has a truth table similar to the standard NOR gate in that it has anoutput that is normally at logic level "1" and goes "LOW" to logic level "0" when ANY of its inputs are at logiclevel "1". However, an output "1" is also obtained if BOTH of its inputs are at logic level "1". For example, A ="1" and B = "1" at the same time giving us the Boolean expression of: Q = (A B) = A.B + A.BIn other words, the output of an Exclusive-NOR gate ONLY goes "HIGH" when its two input terminals, A andB are at the "SAME" logic level which can be either at a logic level "1" or at a logic level "0". Then this type ofgate gives and output "1" when its inputs are "logically equal" or "equivalent" to each other, which is why anExclusive-NOR gate is sometimes called an Equivalence Gate. The logic symbol for an Exclusive-NOR gateis simply an Exclusive-OR gate with a circle or "inversion bubble", ( ο ) at its output to represent the NOTfunction. Then the Logic Exclusive-NOR Gate is the reverse or "Complementary" form of the Exclusive-ORgate, ( ) we have seen previously.Ex-NOR Gate EquivalentThe Exclusive-NOR Gate function is achieved by combining standard gates together to form more complexgate functions and an example of a 2-input Exclusive-NOR gate is given below.The Digital Logic "Ex-NOR" Gate2-input Ex-NOR Gate Symbol Truth Table B A Q 0 0 1 0 1 0 2-input Ex-NOR Gate 1 0 0 1 1 1 Boolean Expression Q = A B Read if A AND B the SAME gives QThen, the logic function implemented by a 2-input Ex-NOR gate is given as "when both A AND B are theSAME" will give an output at Q. In general, an Exclusive-NOR gate will give an output value of logic "1"K. Adisesha Page 52
  • 53. Basics of ElectronicsONLY when there are an EVEN number of 1s on the inputs to the gate (the inverse of the Ex-OR gate) exceptwhen all its inputs are "LOW". Then an Ex-NOR function with more than two inputs is called an "evenfunction" or modulo-2-sum (Mod-2-SUM), not an Ex-NOR. This description can be expanded to apply to anynumber of individual inputs as shown below for a 3-input Exclusive-NOR gate.Ex-NOR Gate Equivalent CircuitOne of the main disadvantages of implementing the Ex-NOR function above is that it contains three differenttypes logic gates the AND, NOT and finally an OR gate within its basic design. One easier way of producingthe Ex-NOR function from a single gate type is to use NAND gates as shown below.Ex-NOR Function Realisation using NAND gatesEx-NOR gates are used mainly in electronic circuits that perform arithmetic operations and data checking suchas Adders, Subtractors or Parity Checkers, etc. As the Ex-NOR gate gives an output of logic level "1"whenever its two inputs are equal it can be used to compare the magnitude of two binary digits or numbers andso Ex-NOR gates are used in Digital Comparator circuits.Commonly available Exclusive-NOR gate ICs include: TTL Logic Types CMOS Logic Types 74LS266 Quad 2-input CD4077 Quad 2-inputQuad 2-input Ex-NOR Gate 74266The Digital Tri-state BufferDefinitionIn a previous tutorial we look at the digital Not Gate or Inverter, and we saw that the NOT gates output is the"complement" or inverse of its input signal. For example, when its input signal is "HIGH" its output state willNOT be "HIGH" and when its input signal is "LOW" its output state will NOT be "LOW", in other words itinverts the signal. Another single input logical device used a lot in electronic circuits and which is the reverse ofthe NOT gate inverter is called a Buffer, Digital Buffer or Non-inverting Buffer.A Digital Buffer is another single input device that does no invert or perform any type of logical operation onits input signal as its output exactly matches that of its input signal. In other words, its Output equals its Input. Itis a "Non-inverting" device and so will give us the Boolean expression of: Q = A.Then we can define the operation of a single input Digital Buffer as being:"If A is true, then Q is true"K. Adisesha Page 53
  • 54. Basics of ElectronicsThe Tri-state Buffer Symbol Truth Table A Q 0 0 A Tri-state Buffer 1 1 Boolean Expression Q = A Read as A gives QThe Digital Tri-state Buffer can also be made by connecting together two NOT gates as shown below. Thefirst will "invert" the input signal A and the second will "re-invert" it back to its original level.Double Inversion using NOT GatesYou may think "what is the point of a Digital Buffer", if it does not alter its input signal in any way or make anylogical operations like the AND or OR gates, then why not use a piece of wire instead and thats a good point.But a non-inverting digital Buffer has many uses in digital electronic circuits, as they can be used to isolateother gates or circuits from each other or they can be used to drive high current loads such as transistor switchesbecause their output drive capability is much higher than their input signal requirements, in other words buffersare uses for power amplification giving them a high fan-out capability.Buffer Fan-out ExampleFan-out is the output driving capability or output current capability of a logic gate giving greater poweramplification of the signal. It may be necessary to connect more than just one logic gate to the output of anotheror to switch a high current load such as an LED, then a Buffer will allow us to do just that by having a high fan-out rating of up to 50.The "Tri-state Buffer"As well as the standard Digital Buffer seen above, there is another type of digital Buffer circuit whose outputcan be "electronically" disconnected from its output circuitry when required. This type of Buffer is known as a3-State Buffer or commonly Tri-state Buffer.A Tri-state Buffer can be thought of as an input controlled switch which has an output that can be electronicallyturned "ON" or "OFF" by means of an external "Control" or "Enable" signal input. This control signal can beeither a logic "0" or a logic "1" type signal resulting in the Tri-state Buffer being in one state allowing its outputto operate normally giving either a logic "0" or logic "1" output. But when activated in the other state it disablesor turns "OFF" its output producing an open circuit condition that is neither "High" or "low", but instead givesan output state of very high impedance, high-Z, or more commonly Hi-Z. Then this type of device has twologic state inputs, "0" or a "1" but can produce three different output states, "0", "1" or "Hi-Z" which is why it iscalled a "3-state" device.There are two different types of Tri-state Buffer, one whose output is controlled by an "Active-HIGH" controlsignal and the other which is controlled by an "Active-LOW" control signal, as shown below.Active "HIGH" Tri-state Buffer Symbol Truth Table Enable A Q 1 0 0 1 1 1 0 0 Hi-Z Tri-state Buffer 0 1 Hi-Z Read as Output = Input if Enable is equal to "1"K. Adisesha Page 54
  • 55. Basics of ElectronicsAn Active-high Tri-state Buffer is activated when a logic level "1" is applied to its "enable" control line and thedata passes through from its input to its output. When the enable control line is at logic level "0", the bufferoutput is disabled and a high impedance condition, Hi-Z is present on the output.Active "LOW" Tri-state Buffer Symbol Truth Table Enable A Q 0 0 0 0 1 1 1 0 Hi-Z Tri-state Buffer 1 1 Hi-Z Read as Output = Input if Enable is NOT equal to "1"An Active-low Tri-state Buffer is the opposite to the above, and is activated when a logic level "0" is applied toits "enable" control line. The data passes through from its input to its output. When the enable control line is atlogic level "1", the buffer output is disabled and a high impedance condition, Hi-Z is present on the output.Tri-state Buffer ControlThe Tri-state Buffer is used in many electronic and microprocessor circuits as they allow multiple logic devicesto be connected to the same wire or bus without damage or loss of data. For example, suppose we have a dataline or data bus with some memory, peripherals, I/O or a CPU connected to it. Each of these devices is capableof sending or receiving data onto this data bus. If these devices start to send or receive data at the same time ashort circuit may occur when one device outputs to the bus a logic "1" the supply voltage, while another is set atlogic level "0" or ground, resulting in a short circuit condition and possibly damage to the devices.Then, the Tri-state Buffer can be used to isolate devices and circuits from the data bus and one another. If theoutputs of several Tri-state Buffers are electrically connected together Decoders are used to allow only one Tri-state Buffer to be active at any one time while the other devices are in their high impedance state. An exampleof Tri-state Buffers connected to a single wire or bus is shown below.Tri-state Buffer ControlIt is also possible to connect Tri-state Buffer "back-to-back" to produce a Bi-directional Buffer circuit with one"active-high buffer" connected in parallel but in reverse with one "active-low buffer". Here, the "enable" controlinput acts more like a directional control signal causing the data to be both read "from" and transmitted "to" thesame data bus wire.Commonly available Digital Buffer and Tri-state Buffer ICs include: TTL Logic Types CMOS Logic Types 74LS07 Hex Non-inverting Buffer CD4050 Hex Non-inverting Buffer 74LS17 Hex Buffer/Driver CD4503 Hex Tri-state Buffer 74LS244 Octal Buffer/Line Driver HEF40244 Octal Buffer with 3-state Output 74LS245 Octal Bi-directional BufferDigital Non-inverting Buffer 7407K. Adisesha Page 55
  • 56. Basics of ElectronicsOctal Tri-state Buffer 74244Combinational Logic CircuitsCombinational Logic Circuit, the output is dependent at all times on the combination of its inputs and if oneof its inputs condition changes state so does the output as combinational circuits have "no memory", "timing" or"feedback loops".Combinational LogicCombinational Logic Circuits are made up from basic logic NAND, NOR or NOT gates that are "combined"or connected together to produce more complicated switching circuits. These logic gates are the building blocksof combinational logic circuits. An example of a combinational circuit is a decoder, which converts the binarycode data present at its input into a number of different output lines, one at a time producing an equivalentdecimal code at its output.Combinational logic circuits can be very simple or very complicated and any combinational circuit can beimplemented with only NAND and NOR gates as these are classed as "universal" gates. The three main ways ofspecifying the function of a combinational logic circuit are: Truth Table Truth tables provide a concise list that shows the output values in tabular form for each possible combination of input variables. Boolean Algebra Forms an output expression for each input variable that represents a logic "1" Logic Diagram Shows the wiring and connections of each individual logic gate that implements the circuit.and all three are shown below.As combinational logic circuits are made up from individual logic gates only, they can also be considered as"decision making circuits" and combinational logic is about combining logic gates together to process two ormore signals in order to produce at least one output signal according to the logical function of each logic gate.Common combinational circuits made up from individual logic gates that carry out a desired application includeMultiplexers, De-multiplexers, Encoders, Decoders, Full and Half Adders etc.K. Adisesha Page 56
  • 57. Basics of ElectronicsClassification of Combinational LogicOne of the most common uses of combinational logic is in Multiplexer and De-multiplexer type circuits. Here,multiple inputs or outputs are connected to a common signal line and logic gates are used to decode an addressto select a single data input or output switch. A multiplexer consist of two separate components, a logic decoderand some solid state switches, but before we can discuss multiplexers, decoders and de-multiplexers in moredetail we first need to understand how these devices use these "solid state switches" in their design.The Binary AdderAnother common and very useful combinational logic circuit which can be constructed using just a few basiclogic gates and adds together binary numbers is the Binary Adder circuit. The Binary Adder is made up fromstandard AND and Ex-OR gates and allow us to "add" together single bit binary numbers, a and b to producetwo outputs, the SUM of the addition and a CARRY called the Carry-out, ( Cout ) bit. One of the main uses forthe Binary Adder is in arithmetic and counting circuits.Consider the addition of two denary (base 10) numbers below. 123 A (Augend) + 789 B (Addend) 912 SUMEach column is added together starting from the right hand side and each digit has a weighted value dependingupon its position in the columns. As each column is added together a carry is generated if the result is greater orequal to ten, the base number. This carry is then added to the result of the addition of the next column to the leftand so on, simple school maths addition. The adding of binary numbers is basically the same as that of addingdecimal numbers but this time a carry is only generated when the result in any column is greater or equal to "2",the base number of binary.Binary AdditionBinary Addition follows the same basic rules as for the denary addition above except in binary there are onlytwo digits and the largest digit is "1", so any "SUM" greater than 1 will result in a "CARRY". This carry 1 ispassed over to the next column for addition and so on. Consider the single bit addition below. 0 0 1 1 +0 +1 +0 +1 0 1 1 10The single bits are added together and "0 + 0", "0 + 1", or "1 + 0" results in a sum of "0" or "1" until you get to"1 + 1" then the sum is equal to "2". For a simple 1-bit addition problem like this, the resulting carry bit couldbe ignored which would result in an output truth table resembling that of an Ex-OR Gate as seen in the LogicGates section and whose result is the sum of the two bits but without the carry. An Ex-OR gate only producesan output "1" when either input is at logic "1", but not both. However, all microprocessors and electroniccalculators require the carry bit to correctly calculate the equations so we need to rewrite them to include 2 bitsof output data as shown below. 00 00 01 01 + 00 + 01 + 00 + 01 00 01 01 10From the above equations we know that an Ex-OR gate will only produce an output "1" when "EITHER" inputis at logic "1", so we need an additional output to produce a carry output, "1" when "BOTH" inputs "A" and "B"K. Adisesha Page 57
  • 58. Basics of Electronicsare at logic "1" and a standard AND Gate fits the bill nicely. By combining the Ex-OR gate with the AND gateresults in a simple digital binary adder circuit known commonly as the "Half Adder" circuit.The Half Adder Circuit1-bit Adder with Carry-Out Symbol Truth Table A B SUM CARRY 0 0 0 0 0 1 1 0 1 0 1 0 1 1 0 1 Boolean Expression: Sum = A ⊕ B Carry = A . BFrom the truth table we can see that the SUM (S) output is the result of the Ex-OR gate and the Carry-out(Cout) is the result of the AND gate. One major disadvantage of the Half Adder circuit when used as a binaryadder, is that there is no provision for a "Carry-in" from the previous circuit when adding together multiple databits. For example, suppose we want to add together two 8-bit bytes of data, any resulting carry bit would needto be able to "ripple" or move across the bit patterns starting from the least significant bit (LSB). The mostcomplicated operation the half adder can do is "1 + 1" but as the half adder has no carry input the resultantadded value would be incorrect. One simple way to overcome this problem is to use a Full Adder type binaryadder circuit.The Full Adder CircuitThe main difference between the Full Adder and the previous seen Half Adder is that a full adder has threeinputs, the same two single bit binary inputs A and B as before plus an additional Carry-In (C-in) input asshown below.Full Adder with Carry-In Symbol Truth Table A B C-in Sum C-out 0 0 0 0 0 0 1 0 1 0 1 0 0 1 0 1 1 0 0 1 0 0 1 1 0 0 1 1 0 1 1 0 1 0 1 1 1 1 1 1 Boolean Expression: Sum = A ⊕ B ⊕ C-inThe 1-bit Full Adder circuit above is basically two half adders connected together and consists of three Ex-ORgates, two AND gates and an OR gate, six logic gates in total. The truth table for the full adder includes anadditional column to take into account the Carry-in input as well as the summed output and carry-output. 4-bitfull adder circuits are available as standard IC packages in the form of the TTL 74LS83 or the 74LS283 whichcan add together two 4-bit binary numbers and generate a SUM and a CARRY output. But what if we wanted toadd together two n-bit numbers, then n 1-bit full adders need to be connected together to produce what is knownas the Ripple Carry Adder.The 4-bit Binary AdderThe Ripple Carry Binary Adder is simply n, full adders cascaded together with each full adder represents asingle weighted column in the long addition with the carry signals producing a "ripple" effect through the binaryadder from right to left. For example, suppose we want to "add" together two 4-bit numbers, the two outputs ofthe first full adder will provide the first place digit sum of the addition plus a carry-out bit that acts as the carry-in digit of the next binary adder. The second binary adder in the chain also produces a summed output (the 2ndK. Adisesha Page 58
  • 59. Basics of Electronicsbit) plus another carry-out bit and we can keep adding more full adders to the combination to add largernumbers, linking the carry bit output from the first full binary adder to the next full adder, and so forth. Anexample of a 4-bit adder is given below.A 4-bit Binary AdderOne main disadvantage of "cascading" together 1-bit binary adders to add large binary numbers is that ifinputs A and B change, the sum at its output will not be valid until any carry-input has "rippled" through everyfull adder in the chain. Consequently, there will be a finite delay before the output of a adder responds to achange in its inputs resulting in the accumulated delay especially in large multi-bit binary adders becomingprohibitively large. This delay is called Propagation delay. Also "overflow" occurs when an n-bit adder addstwo numbers together whose sum is greater than or equal to 2nOne solution is to generate the carry-input signals directly from the A and B inputs rather than using the ripplearrangement above. This then produces another type of binary adder circuit called a Carry Look AheadBinary Adder were the speed of the parallel adder can be greatly improved using carry-look ahead logic.The 4-bit Binary SubtractorNow that we know how to "ADD" together two 4-bit binary numbershow would we subtract two 4-bit binary numbers, for example, A - Busing the circuit above. The answer is to use 2’s-complement notationon all the bits in B must be complemented (inverted) and an extra oneadded using the carry-input. This can be achieved by inverting each Binput bit using an inverter or NOT-gate.Also, in the above circuit for the 4-bit binary adder, the first carry-ininput is held LOW at logic "0", for the circuit to perform subtractionthis input needs to be held HIGH at "1". With this in mind a ripplecarry adder can with a small modification be used to perform halfsubtraction, full subtraction and/or comparison.There are a number of 4-bit full-adder ICs available such as the 74LS283 and CD4008. which will add two 4-bitbinary number and provide an additional input carry bit, as well as an output carry bit, so you can cascade themtogether to produce 8-bit, 12-bit, 16-bit, etc. adders.Sequential Logic BasicsUnlike Combinational Logic circuits that change state depending upon the actual signals being applied to theirinputs at that time, Sequential Logic circuits have some form of inherent "Memory" built in to them as they areable to take into account their previous input state as well as those actually present, a sort of "before" and"after" is involved with sequential circuits.In other words, the output state of a sequential logic circuit is a function of the following three states, the"present input", the "past input" and/or the "past output". Sequential Logic circuits remember these conditionsand stay fixed in their current state until the next clock signal changes one of the states, giving sequential logiccircuits "Memory".Sequential logic circuits are generally termed as two state or Bistable devices which can have their output oroutputs set in one of two basic states, a logic level "1" or a logic level "0" and will remain "latched" (hence thename latch) indefinitely in this current state or condition until some other input trigger pulse or signal is appliedwhich will cause the bistable to change its state once again.Sequential Logic RepresentationK. Adisesha Page 59
  • 60. Basics of ElectronicsThe word "Sequential" means that things happen in a "sequence", one after another and in Sequential Logiccircuits, the actual clock signal determines when things will happen next. Simple sequential logic circuits can beconstructed from standard Bistable circuits such as Flip-flops, Latches and Counters and which themselves canbe made by simply connecting together universal NAND Gates and/or NOR Gates in a particularcombinational way to produce the required sequential circuit.Classification of Sequential LogicAs standard logic gates are the building blocks of combinational circuits, bistable latches and flip-flops are thebuilding blocks of Sequential Logic Circuits. Sequential logic circuits can be constructed to produce eithersimple edge-triggered flip-flops or more complex sequential circuits such as storage registers, shift registers,memory devices or counters. Either way sequential logic circuits can be divided into the following three maincategories: 1. Event Driven - asynchronous circuits that change state immediately when enabled. 2. Clock Driven - synchronous circuits that are synchronised to a specific clock signal. 3. Pulse Driven - which is a combination of the two that responds to triggering pulses.As well as the two logic states mentioned above logic level "1" and logic level "0", a third element is introducedthat separates sequential logic circuits from their combinational logic counterparts, namely TIME. Sequentiallogic circuits that return back to their original state once reset, i.e. circuits with loops or feedback paths are saidto be "cyclic" in nature.We now know that in sequential circuits changes occur only on the application of a clock signal making itsynchronous, otherwise the circuit is asynchronous and depends upon an external input. To retain their currentstate, sequential circuits rely on feedback and this occurs when a fraction of the output is fed back to the inputand this is demonstrated as:Sequential Feedback LoopThe two inverters or NOT gates are connected in series with the output at Q fed back to the input.Unfortunately, this configuration never changes state because the output will always be the same, either a "1" ora "0", it is permanently set. However, we can see how feedback works by examining the most basic sequentiallogic components, called the SR flip-flop.SR Flip-FlopThe SR flip-flop, also known as a SR Latch, can be considered as one of the most basic sequential logic circuitpossible. This simple flip-flop is basically a one-bit memory bistable device that has two inputs, one which willK. Adisesha Page 60
  • 61. Basics of Electronics"SET" the device (meaning the output = "1"), and is labelled S and another which will "RESET" the device(meaning the output = "0"), labelled R. Then the SR description stands for "Set-Reset". The reset input resetsthe flip-flop back to its original state with an output Q that will be either at a logic level "1" or logic "0"depending upon this set/reset condition.A basic NAND gate SR flip-flop circuit provides feedback from both of its outputs back to its opposing inputsand is commonly used in memory circuits to store a single data bit. Then the SR flip-flop actually has threeinputs, Set, Reset and its current output Q relating to its current state or history. The term "Flip-flop" relates tothe actual operation of the device, as it can be "flipped" into one logic Set state or "flopped" back into theopposing logic Reset state.The NAND Gate SR Flip-FlopThe simplest way to make any basic single bit set-reset SR flip-flop is to connect together a pair of cross-coupled 2-input NAND gates as shown, to form a Set-Reset Bistable also known as an active LOW SR NANDGate Latch, so that there is feedback from each output to one of the other NAND gate inputs. This deviceconsists of two inputs, one called the Set, S and the other called the Reset, R with two corresponding outputs Qand its inverse or complement Q (not-Q) as shown below.The Basic SR Flip-flopThe Set StateConsider the circuit shown above. If the input R is at logic level "0" (R = 0) and input S is at logic level "1" (S =1), the NAND gate Y has at least one of its inputs at logic "0" therefore, its output Q must be at a logic level "1"(NAND Gate principles). Output Q is also fed back to input "A" and so both inputs to NAND gate X are at logiclevel "1", and therefore its output Q must be at logic level "0". Again NAND gate principals. If the reset input Rchanges state, and goes HIGH to logic "1" with S remaining HIGH also at logic level "1", NAND gate Y inputsare now R = "1" and B = "0". Since one of its inputs is still at logic level "0" the output at Q still remains HIGHat logic level "1" and there is no change of state. Therefore, the flip-flop circuit is said to be "Latched" or "Set"with Q = "1" and Q = "0".Reset StateIn this second stable state, Q is at logic level "0", (not Q = "0") its inverse output at Q is at logic level "1", (Q ="1"), and is given by R = "1" and S = "0". As gate X has one of its inputs at logic "0" its output Q must equallogic level "1" (again NAND gate principles). Output Q is fed back to input "B", so both inputs to NAND gate Yare at logic "1", therefore, Q = "0". If the set input, S now changes state to logic "1" with input R remaining atlogic "1", output Q still remains LOW at logic level "0" and there is no change of state. Therefore, the flip-flopcircuits "Reset" state has also been latched and we can define this "set/reset" action in the following truth table.Truth Table for this Set-Reset Function State S R Q Q Description 1 0 1 0 Set Q » 1 Set 1 1 1 0 no change 0 1 0 1 Reset Q » 0 Reset 1 1 0 1 no change 0 0 0 1 memory with Q = 0 Invalid 0 0 1 0 memory with Q = 1It can be seen that when both inputs S = "1" and R = "1" the outputs Q and Q can be at either logic level "1" orK. Adisesha Page 61
  • 62. Basics of Electronics"0", depending upon the state of inputs S or R BEFORE this input condition existed. However, input state R ="0" and S = "0" is an undesirable or invalid condition and must be avoided because this will give both outputs Qand Q to be at logic level "1" at the same time and we would normally want Q to be the inverse of Q. However,if the two inputs are now switched HIGH again after this condition to logic "1", both the outputs will go LOWresulting in the flip-flop becoming unstable and switch to an unknown data state based upon the unbalance. Thisunbalance can cause one of the outputs to switch faster than the other resulting in the flip-flop switching to onestate or the other which may not be the required state and data corruption will exist. This unstable condition isknown as its Meta-stable state.Then, a bistable SR flip-flop or SR latch is activated or set by a logic "1" applied to its S input and deactivatedor reset by a logic "1" applied to its R. The SR flip-flop is said to be in an "invalid" condition (Meta-stable) ifboth the set and reset inputs are activated simultaneously.As well as using NAND gates, it is also possible to construct simple one-bit SR Flip-flops using two cross-coupled NOR gates connected in the same configuration. The circuit will work in a similar way to the NANDgate circuit above, except that the inputs are active HIGH and the invalid condition exists when both its inputsare at logic level "1", and this is shown below.The NOR Gate SR Flip-flopSwitch Debounce CircuitsEdge-triggered flip-flops require a nice clean signal transition, and one practical use of this type of set-resetcircuit is as a latch used to help eliminate mechanical switch "bounce". As its name implies, switch bounceoccurs when the contacts of any mechanically operated switch, push-button or keypad are operated and theinternal switch contacts do not fully close cleanly, but bounce together first before closing (or opening) whenthe switch is pressed. This gives rise to a series of individual pulses which can be as long as tens of millisecondsthat an electronic system or circuit such as a digital counter may see as a series of logic pulses instead of onelong single pulse and behave incorrectly. For example, during this bounce period the output voltage canfluctuate wildly and may register multiple input counts instead of one single count. Then set-reset SR Flip-flopsor Bistable Latch circuits can be used to eliminate this kind of problem and this is demonstrated below.SR Bistable Switch Debounce CircuitDepending upon the current state of the output, if the set or reset buttons are depressed the output will changeover in the manner described above and any additional unwanted inputs (bounces) from the mechanical actionK. Adisesha Page 62
  • 63. Basics of Electronicsof the switch will have no effect on the output at Q. When the other button is pressed, the very first contact willcause the latch to change state, but any additional mechanical switch bounces will also have no effect. The SRflip-flop can then be RESET automatically after a short period of time, for example 0.5 seconds, so as toregister any additional and intentional repeat inputs from the same switch contacts, for example multiple inputsfrom a keyboards "RETURN" key.Commonly available ICs specifically made to overcome the problem of switch bounce are the MAX6816,single input, MAX6817, dual input and the MAX6818 octal input switch debouncer ICs. These chips containthe necessary flip-flop circuitry to provide clean interfacing of mechanical switches to digital systems.Set-Reset bistable latches can also be used as Monostable (one-shot) pulse generators to generate a single outputpulse, either high or low, of some specified width or time period for timing or control purposes. The 74LS279 isa Quad SR Bistable Latch IC, which contains four individual NAND type bistables within a single chipenabling switch debounce or monostable/astable clock circuits to be easily constructed.Quad SR Bistable Latch 74LS279Gated or Clocked SR Flip-FlopIt is sometimes desirable in sequential logic circuits to have a bistable SR flip-flop that only changes state whencertain conditions are met regardless of the condition of either the Set or the Reset inputs. By connecting a 2-input AND gate in series with each input terminal of the SR Flip-flop a Gated SR Flip-flop can be created. Thisextra conditional input is called an "Enable" input and is given the prefix of "EN". The addition of this inputmeans that the output at Q only changes state when it is HIGH and can therefore be used as a clock (CLK) inputmaking it level-sensitive as shown below.Gated SR Flip-flopWhen the Enable input "EN" is at logic level "0", the outputs of the two AND gates are also at logic level "0",(AND Gate principles) regardless of the condition of the two inputs S and R, latching the two outputs Q and Qinto their last known state. When the enable input "EN" changes to logic level "1" the circuit responds as anormal SR bistable flip-flop with the two AND gates becoming transparent to the Set and Reset signals. Thisenable input can also be connected to a clock timing signal adding clock synchronisation to the flip-flopcreating what is sometimes called a "Clocked SR Flip-flop". So a Gated Bistable SR Flip-flop operates as astandard bistable latch but the outputs are only activated when a logic "1" is applied to its EN input anddeactivated by a logic "0".The JK Flip-flopFrom the previous tutorial we now know that the basic gated SR NAND flip-flop suffers from two basicproblems: number one, the S = 0 and R = 0 condition or S = R = 0 must always be avoided, and number two, ifS or R change state while the enable input is high the correct latching action may not occur. Then to overcomethese two fundamental design problems with the SR flip-flop, the JK flip-Flop was developed.This simple JK flip-Flop is the most widely used of all the flip-flop designs and is considered to be a universalflip-flop circuit. The sequential operation of the JK flip-flop is exactly the same as for the previous SR flip-flopwith the same "set" and "reset" inputs. The difference this time is that the JK flip-flop has no invalid orforbidden input states of the SR Latch (when S and R are both 1).K. Adisesha Page 63
  • 64. Basics of ElectronicsThe JK flip-flop is basically a gated SR flip-flop with the addition of a clock input circuitry that prevents theillegal or invalid output condition that can occur when both inputs S and R are equal to logic level "1". Due tothis additional clocked input, a JK flip-flop has four possible input combinations, "logic 1", "logic 0", "nochange" and "toggle". The symbol for a JK flip-flop is similar to that of an SR Bistable Latch as seen in theprevious tutorial except for the addition of a clock input.The Basic JK Flip-flopBoth the S and the R inputs of the previous SR bistable have now been replaced by two inputs called the J andK inputs, respectively after its inventor Jack Kilby. Then this equates to: J = S and K = R.The two 2-input AND gates of the gated SR bistable have now been replaced by two 3-input NAND gates withthe third input of each gate connected to the outputs at Q and Q. This cross coupling of the SR flip-flop allowsthe previously invalid condition of S = "1" and R = "1" state to be used to produce a "toggle action" as the twoinputs are now interlocked. If the circuit is "SET" the J input is inhibited by the "0" status of Q through thelower NAND gate. If the circuit is "RESET" the K input is inhibited by the "0" status of Q through the upperNAND gate. As Q and Q are always different we can use them to control the input. When both inputs J and Kare equal to logic "1", the JK flip-flop toggles as shown in the following truth table.The Truth Table for the JK Function Input Output Description J K Q Q 0 0 0 0 Memory same as 0 0 0 1 no change for the SR Latch 0 1 1 0 Reset Q » 0 0 1 0 1 1 0 0 1 Set Q » 1 1 0 1 0 toggle 1 1 0 1 Toggle action 1 1 1 0Then the JK flip-flop is basically an SR flip-flop with feedback which enables only one of its two inputterminals, either SET or RESET to be active at any one time thereby eliminating the invalid condition seenpreviously in the SR flip-flop circuit. Also when both the J and the K inputs are at logic level "1" at the sametime, and the clock input is pulsed either "HIGH", the circuit will "toggle" from its SET state to a RESET state,or visa-versa. This results in the JK flip-flop acting more like a T-type toggle flip-flop when both terminals are"HIGH".Although this circuit is an improvement on the clocked SR flip-flop it still suffers from timing problems called"race" if the output Q changes state before the timing pulse of the clock input has time to go "OFF". To avoidthis the timing pulse period (T) must be kept as short as possible (high frequency). As this is sometimes notpossible with modern TTL ICs the much improved Master-Slave JK Flip-flop was developed. This eliminatesall the timing problems by using two SR flip-flops connected together in series, one for the "Master" circuit,which triggers on the leading edge of the clock pulse and the other, the "Slave" circuit, which triggers on thefalling edge of the clock pulse. This results in the two sections, the master section and the slave section beingenabled during opposite half-cycles of the clock signal.K. Adisesha Page 64
  • 65. Basics of ElectronicsThe 74LS73 is a Dual JK flip-flop IC, which contains two individual JK type bistables within a single chipenabling single or master-slave toggle flip-flops to be made. Other JK flip-flop ICs include the 74LS107 DualJK flip-flop with clear, the 74LS109 Dual positive-edge triggered JK flip-flop and the 74LS112 Dual negative-edge triggered flip-flop with both preset and clear inputs.Dual JK Flip-flop 74LS73The Master-Slave JK Flip-flopThe Master-Slave Flip-Flop is basically two gated SR flip-flops connected together in a series configurationwith the slave having an inverted clock pulse. The outputs from Q and Q from the "Slave" flip-flop are fed backto the inputs of the "Master" with the outputs of the "Master" flip-flop being connected to the two inputs of the"Slave" flip-flop. This feedback configuration from the slaves output to the masters input gives thecharacteristic toggle of the JK flip-flop as shown below.The Master-Slave JK Flip-FlopThe input signals J and K are connected to the gated "master" SR flip-flop which "locks" the input conditionwhile the clock (Clk) input is "HIGH" at logic level "1". As the clock input of the "slave" flip-flop is the inverse(complement) of the "master" clock input, the "slave" SR flip-flop does not toggle. The outputs from the"master" flip-flop are only "seen" by the gated "slave" flip-flop when the clock input goes "LOW" to logic level"0". When the clock is "LOW", the outputs from the "master" flip-flop are latched and any additional changes toits inputs are ignored. The gated "slave" flip-flop now responds to the state of its inputs passed over by the"master" section. Then on the "Low-to-High" transition of the clock pulse the inputs of the "master" flip-flopare fed through to the gated inputs of the "slave" flip-flop and on the "High-to-Low" transition the same inputsare reflected on the output of the "slave" making this type of flip-flop edge or pulse-triggered.Then, the circuit accepts input data when the clock signal is "HIGH", and passes the data to the output on thefalling-edge of the clock signal. In other words, the Master-Slave JK Flip-flop is a "Synchronous" device as itonly passes data with the timing of the clock signal.The D flip-flopOne of the main disadvantages of the basic SR NAND Gate bistable circuit is that the indeterminate inputcondition of "SET" = logic "0" and "RESET" = logic "0" is forbidden. This state will force both outputs to be atlogic "1", over-riding the feedback latching action and whichever input goes to logic level "1" first will losecontrol, while the other input still at logic "0" controls the resulting state of the latch. In order to prevent thisfrom happening an inverter can be connected between the "SET" and the "RESET" inputs to produce anothertype of flip-flop circuit called a Data Latch, Delay flip-flop, D-type Bistable or simply a D-type flip-flop as itis more generally called.The D flip-flop is by far the most important of the clocked flip-flops as it ensures that ensures that inputs S andR are never equal to one at the same time. D-type flip-flops are constructed from a gated SR flip-flop with aninverter added between the S and the R inputs to allow for a single D (data) input. This single data input D isused in place of the "set" signal, and the inverter is used to generate the complementary "reset" input therebymaking a level-sensitive D-type flip-flop from a level-sensitive RS-latch as now S = D and R = not D as shown.K. Adisesha Page 65
  • 66. Basics of ElectronicsD flip-flop CircuitWe remember that a simple SR flip-flop requires two inputs, one to "SET" the output and one to "RESET" theoutput. By connecting an inverter (NOT gate) to the SR flip-flop we can "SET" and "RESET" the flip-flopusing just one input as now the two input signals are complements of each other. This complement avoids theambiguity inherent in the SR latch when both inputs are LOW, since that state is no longer possible.Thus the single input is called the "DATA" input. If this data input is HIGH the flip-flop would be "SET" andwhen it is LOW the flip-flop would be "RESET". However, this would be rather pointless since the flip-flopsoutput would always change on every data input. To avoid this an additional input called the "CLOCK" or"ENABLE" input is used to isolate the data input from the flip-flop after the desired data has been stored. Theeffect is that D is only copied to the output Q when the clock is active. This then forms the basis of a D flip-flop.The D flip-flop will store and output whatever logic level is applied to its data terminal so long as the clockinput is HIGH. Once the clock input goes LOW the "set" and "reset" inputs of the flip-flop are both held at logiclevel "1" so it will not change state and store whatever data was present on its output before the clock transitionoccurred. In other words the output is "latched" at either logic "0" or logic "1".Truth Table for the D Flip-flop Clk D Q Q Description Memory ↓»0 X Q Q no change ↑»1 0 0 1 Reset Q » 0 ↑»1 1 1 0 Set Q » 1Note: ↓ and ↑ indicates direction of clock pulse as it is assumed D flip-flops are edge triggeredThe Shift RegisterThe Shift Register is another type of sequential logic circuit that is used for the storage or transfer of data in theform of binary numbers and then "shifts" the data out once every clock cycle, hence the name "shift register". Itbasically consists of several single bit "D-Type Data Latches", one for each bit (0 or 1) connected together in aserial or daisy-chain arrangement so that the output from one data latch becomes the input of the next latch andso on. The data bits may be fed in or out of the register serially, i.e. one after the other from either the left or theright direction, or in parallel, i.e. all together. The number of individual data latches required to make up asingle Shift Register is determined by the number of bits to be stored with the most common being 8-bits wide,i.e. eight individual data latches.The Shift Register is used for data storage or data movement and are used in calculators or computers to storedata such as two binary numbers before they are added together, or to convert the data from either a serial toparallel or parallel to serial format. The individual data latches that make up a single shift register are all drivenby a common clock (Clk) signal making them synchronous devices. Shift register ICs are generally providedwith a clear or reset connection so that they can be "SET" or "RESET" as required.Generally, shift registers operate in one of four different modes with the basic movement of data through a shiftregister being: Serial-in to Parallel-out (SIPO) - the register is loaded with serial data, one bit at a time, with the stored data being available in parallel form. Serial-in to Serial-out (SISO) - the data is shifted serially "IN" and "OUT" of the register, one bit at a time in either a left or right direction under clock control. Parallel-in to Serial-out (PISO) - the parallel data is loaded into the register simultaneously and is shifted out of the register serially one bit at a time under clock control. Parallel-in to Parallel-out (PIPO) - the parallel data is loaded simultaneously into the register, and transferred together to their respective outputs by the same clock pulse.K. Adisesha Page 66
  • 67. Basics of ElectronicsThe effect of data movement from left to right through a shift register can be presented graphically as:Also, the directional movement of the data through a shift register can be either to the left, (left shifting) to theright, (right shifting) left-in but right-out, (rotation) or both left and right shifting within the same registerthereby making it bidirectional. In this tutorial it is assumed that all the data shifts to the right, (right shifting).Serial-in to Parallel-out (SIPO)4-bit Serial-in to Parallel-out Shift RegisterThe operation is as follows. Lets assume that all the flip-flops (FFA to FFD) have just been RESET (CLEARinput) and that all the outputs QA to QD are at logic level "0" i.e, no parallel data output. If a logic "1" isconnected to the DATA input pin of FFA then on the first clock pulse the output of FFA and therefore theresulting QA will be set HIGH to logic "1" with all the other outputs still remaining LOW at logic "0". Assumenow that the DATA input pin of FFA has returned LOW again to logic "0" giving us one data pulse or 0-1-0.The second clock pulse will change the output of FFA to logic "0" and the output of FFB and QB HIGH to logic"1" as its input D has the logic "1" level on it from QA. The logic "1" has now moved or been "shifted" oneplace along the register to the right as it is now at QA. When the third clock pulse arrives this logic "1" valuemoves to the output of FFC (QC) and so on until the arrival of the fifth clock pulse which sets all the outputs Q Ato QD back again to logic level "0" because the input to FFA has remained constant at logic level "0".The effect of each clock pulse is to shift the data contents of each stage one place to the right, and this is shownin the following table until the complete data value of 0-0-0-1 is stored in the register. This data value can nowbe read directly from the outputs of QA to QD. Then the data has been converted from a serial data input signalto a parallel data output. The truth table and following waveforms show the propagation of the logic "1" throughthe register from left to right as follows.Basic Movement of Data through a Shift Register Clock Pulse No QA QB QC QD 0 0 0 0 0 1 1 0 0 0 2 0 1 0 0 3 0 0 1 0 4 0 0 0 1 5 0 0 0 0K. Adisesha Page 67
  • 68. Basics of ElectronicsNote that after the fourth clock pulse has ended the 4-bits of data (0-0-0-1) are stored in the register and willremain there provided clocking of the register has stopped. In practice the input data to the register may consistof various combinations of logic "1" and "0". Commonly available SIPO ICs include the standard 8-bit74LS164 or the 74LS594.Serial-in to Serial-out (SISO)This shift register is very similar to the SIPO above, except were before the data was read directly in a parallelform from the outputs QA to QD, this time the data is allowed to flow straight through the register and out of theother end. Since there is only one output, the DATA leaves the shift register one bit at a time in a serial pattern,hence the name Serial-in to Serial-Out Shift Register or SISO.The SISO shift register is one of the simplest of the four configurations as it has only three connections, theserial input (SI) which determines what enters the left hand flip-flop, the serial output (SO) which is taken fromthe output of the right hand flip-flop and the sequencing clock signal (Clk). The logic circuit diagram belowshows a generalized serial-in serial-out shift register.4-bit Serial-in to Serial-out Shift RegisterYou may think whats the point of a SISO shift register if the output data is exactly the same as the input data.Well this type of Shift Register also acts as a temporary storage device or as a time delay device for the data,with the amount of time delay being controlled by the number of stages in the register, 4, 8, 16 etc or by varyingthe application of the clock pulses. Commonly available ICs include the 74HC595 8-bit Serial-in/Serial-outShift Register all with 3-state outputs.Parallel-in to Serial-out (PISO)The Parallel-in to Serial-out shift register acts in the opposite way to the serial-in to parallel-out one above. Thedata is loaded into the register in a parallel format i.e. all the data bits enter their inputs simultaneously, to theparallel input pins PA to PD of the register. The data is then read out sequentially in the normal shift-right modefrom the register at Q representing the data present at PA to PD. This data is outputted one bit at a time on eachclock cycle in a serial format. It is important to note that with this system a clock pulse is not required toparallel load the register as it is already present, but four clock pulses are required to unload the data.4-bit Parallel-in to Serial-out Shift RegisterAs this type of shift register converts parallel data, such as an 8-bit data word into serial format, it can be usedto multiplex many different input lines into a single serial DATA stream which can be sent directly to acomputer or transmitted over a communications line. Commonly available ICs include the 74HC166 8-bitParallel-in/Serial-out Shift Registers.Parallel-in to Parallel-out (PIPO)The final mode of operation is the Parallel-in to Parallel-out Shift Register. This type of register also acts as atemporary storage device or as a time delay device similar to the SISO configuration above. The data ispresented in a parallel format to the parallel input pins PA to PD and then transferred together directly to theirrespective output pins QA to QA by the same clock pulse. Then one clock pulse loads and unloads the register.This arrangement for parallel loading and unloading is shown below.K. Adisesha Page 68
  • 69. Basics of Electronics4-bit Parallel-in to Parallel-out Shift RegisterThe PIPO shift register is the simplest of the four configurations as it has only three connections, the parallelinput (PI) which determines what enters the flip-flop, the parallel output (PO) and the sequencing clock signal(Clk).Similar to the Serial-in to Serial-out shift register, this type of register also acts as a temporary storage device oras a time delay device, with the amount of time delay being varied by the frequency of the clock pulses. Also, inthis type of register there are no interconnections between the individual flip-flops since no serial shifting of thedata is required.Universal Shift RegisterToday, high speed bi-directional "universal" type Shift Registers such as the TTL 74LS194, 74LS195 or theCMOS 4035 are available as a 4-bit multi-function devices that can be used in either serial-to-serial, leftshifting, right shifting, serial-to-parallel, parallel-to-serial, and as a parallel-to-parallel multifunction dataregister, hence the name "Universal". These devices can perform any combination of parallel and serial input tooutput operations but require additional inputs to specify desired function and to pre-load and reset the device.4-bit Universal Shift Register 74LS194Universal shift registers are very useful digital devices. They can be configured to respond to operations thatrequire some form of temporary memory, delay information such as the SISO or PIPO configuration modes ortransfer data from one point to another in either a serial or parallel format. Universal shift registers arefrequently used in arithmetic operations to shift data to the left or right for multiplication or division.Summary of Shift Registers Then to summarise. A simple Shift Register can be made using only D-type flip-Flops, one flip-Flop for each data bit. The output from each flip-Flop is connected to the D input of the flip-flop at its right. Shift registers hold the data in their memory which is moved or "shifted" to their required positions on each clock pulse. Each clock pulse shifts the contents of the register one bit position to either the left or the right. The data bits can be loaded one bit at a time in a series input (SI) configuration or be loaded simultaneously in a parallel configuration (PI). Data may be removed from the register one bit at a time for a series output (SO) or removed all at the same time from a parallel output (PO). One application of shift registers is converting between serial and parallel data. Shift registers are identified as SIPO, SISO, PISO, PIPO, and universal shift registers.K. Adisesha Page 69