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### Digital electronics

1. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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
16. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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