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About Operational amplifier

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  • 1. Project Report 2011 Table of Contentsi) Amplifiers……….………………………………………………………………02ii) Ideal amplifier……………………….………………………………………….03iii) An introduction to the operational amplifier………………….……………...04 (1) Ideal characteristics.……………………………………………………………..05 (2) Practical characteristics………………………..…………………….………………06iv) Limitations of the op-amp……………………………………..………………06v) Applications of OP-AMPS……………………………………..……………...06 (1) As an Integrator……………………………………………………………………07 (2) As a Differentiator………………………………………………………………..10 (3) As an Inverter………………………………………………………………………13vi) As a Comparator…………………….…………………………………………17vii) Overview of uA741 ……………………………………………………………18viii)Given Task……………………………………………………………………...20ix) Problems regarding Project and their Solutions……………………………20x) Future Direction………………………………………………………………...23xi) References……………………………………………………………………...24 Chapter: Operational amplifiers 1
  • 2. Project Report 2011AmplifiersIn "Electronics", signal amplifiers are widely used devices as they have the ability toamplify a relatively small input signal, for example from a Sensor such as amicrophone, into a much larger output signal to drive a Relay, lamp or loudspeakerThere are many forms of electronic circuits classed as amplifiers. Amplifiers can be thought of as a simple box or block containing the amplifyingdevice, such as a Transistor, Field Effect Transistor or Op-amp, which has two inputterminals and two output terminals (ground being common) with the output signalbeing much greater than that of the input signal as it has been "Amplified”. An idealamplifier has three main properties, Input Resistance or ( Rin ), Output Resistance or( Rout ) and of course amplification known commonly as Gain or ( A ). No matterhow complicated an amplifier circuit is, a general amplifier model can be used toshow the relationship of these three properties.Amplifier GainThen the gain of an amplifier can be said to be the relationship that exists betweenthe signals measured at the output with the signal measured at the input. There arethree different kinds of Amplifier Gain, Voltage Gain, ( Av ), Current Gain ( Ai ) andPower Gain ( Ap ) and examples of these are given below.Amplifier Gain of the Input Signal Chapter: Operational amplifiers 2
  • 3. Project Report 2011Voltage Amplifier GainCurrent Amplifier GainPower Amplifier GainAmplifier EfficiencyIdeal AmplifierWe can know specify the characteristics for an ideal amplifier from our discussionabove with regards to its Gain, meaning voltage gain: 1. The amplifiers gain, ( A ) should remain constant for varying values of input signal. 2. Gain is not be affected by frequency. Signals of all frequencies must be amplified by exactly the same amount. 3. The amplifiers gain must not add noise to the output signal. It should remove any noise that is already exists in the input signal. 4. The amplifiers gain should not be affected by changes in temperature giving good temperature stability. 5. The gain of the amplifier must remain stable over long periods of time. Chapter: Operational amplifiers 3
  • 4. Project Report 2011An Introduction to the Operational AmplifierAn operational amplifier IC is a solid-state integrated circuit that uses externalfeedback to control its functions. It is one of the most versatile devices in all ofelectronics.The term op-amp was originally used to describe a chain of high performance dcamplifiers that was used as a basis for the analog type computers of long ago. Thevery high gain op-amp ICs our days uses external feedback networks to controlresponses. The op-amp without any external devices is called open-loop mode,referring actually to the so-called ideal operational amplifier with infinite open-loopgain, input resistance, bandwidth and a zero output resistance.Operational amplifiers are linear devices that have all the properties required fornearly ideal DC amplification and are therefore used extensively in signalconditioning, filtering or to perform mathematical operations such as add, subtract,integration and differentiation. An ideal Operational Amplifier is basically a three-terminal device which consists of two high impedance inputs, one called the InvertingInput, marked with a negative sign, ("-") and the other one called the Non-invertingInput, marked with a positive plus sign ("+").The third terminal represents the op-amps output port which can both sink andsource either a voltage or a current. In a linear operational amplifier, the outputsignal is the amplification factor, known as the amplifiers gain (A) multiplied by thevalue of the input signal and depending on the nature of these input and outputsignalsEquivalent Circuit for Ideal Operational Amplifiers Chapter: Operational amplifiers 4
  • 5. Project Report 2011Op-amp Idealized CharacteristicsPARAMETER IDEALIZED CHARACTERISTIC I. Open Loop Gain, Infinite - The main function of an (Avo) operational amplifier is to amplify the input signal and the more open loop gain it has the better. Open-loop gain is the gain of the op-amp without positive or negative feedback and for an ideal amplifier the gain will be infinite but typical real values range from about 20,000 to 200,000. II. Input impedance, (Zin) Infinite - Input impedance is the ratio of input voltage to input current and is assumed to be infinite to prevent any current flowing from the source supply into the amplifiers input circuitry (Iin =0). Real op-amps have input leakage currents from a few pico-amps to a few milli-amps. III. Output impedance, Zero - The output impedance of the ideal (Zout) operational amplifier is assumed to be zero acting as a perfect internal voltage source with no internal resistance so that it can supply as much current as necessary to the load. This internal resistance is effectively in series with the load thereby reducing the output voltage available to the load. Real op-amps have output- impedance in the 100-20Ω range. IV. Bandwidth, (BW) Infinite - An ideal operational amplifier has an infinite frequency response and can amplify any frequency signal from DC to the highest AC frequencies so it is therefore assumed to have an infinite bandwidth. With real op-amps, the bandwidth is limited by the Gain-Bandwidth product (GB), which is equal to the frequency where the amplifiers gain becomes unity. V. Offset Voltage, (Vio) Zero - The amplifiers output will be zero Chapter: Operational amplifiers when the voltage difference between the inverting and the non-inverting inputs is zero, or inputs are grounded. 5
  • 6. Project Report 2011Op-amp Practilized CharacteristicsPARAMETER PRACTILIZED CHARACTERISTIC I. Open Loop Gain, (Avo) : High II. Input impedance, (Zin): High III. Output impedance, (Zout): Low IV. Bandwidth, (BW): High V. Offset Voltage, (Vio): LowLimitations of Op-ampSome of the limitations that an operational amplifier has are listed below: 1. Use of two additional batteries 2. Operative on low frequencies 3. Gain is limited 4. The input current isnt exactly zero. 5. The input offset current isnt exactly zero either. 6. The input impedance isnt infinite. 7. There is a limited common mode voltage range. 8. The output impedance isnt zero. 9. There are voltage gain limitations including phase shifts. 10. There is a finite input offset voltage. 11. There is a finite slew rate. 12. There is some temperature dependence. 13. They are not the power amplifier.Applications of the Op-amp Chapter: Operational amplifiersOperational amplifier is used widely in many applications such as: A. Integrator B. Differentiator C. Inverter D. Comparator 6
  • 7. Project Report 2011  As an Integrator:The integrator does just what the name implies. It integrates - in the calculus sense -the input signal to produce the output signal. There is a scaling factor and a minussign again, but thats pretty much what happens.Heres the analysis. We make the usual assumptions:V- = 0We assume that the input voltage at the inverting input is a virtual ground.We assume that no current enters the input terminals of the op-amp.Then, we have - after we write KCL:C(dVout/dt) + V1/R = 0C(dVout/dt) + V1/R = 0Then: Chapter: Operational amplifiersIntegrator Amplifier Circuit 7
  • 8. Project Report 2011As its name implies, the Integrator Amplifier is an operational amplifier circuit thatperforms the mathematical operation of Integration that is we can cause the output torespond to changes in the input voltage over time. The integrator amplifier acts like astorage element that"PRODUCES A VOLTAGE OUTPUT WHICH IS PROPORTIONAL TO THEINTEGRAL OF ITS INPUT VOLTAGE WITH RESPECT TO TIME". In other words the magnitude of the output signal is determined by the length of timea voltage is present at its input as the current through the feedback loop charges ordischarges the capacitor as the required negative feedback occurs through thecapacitor.When a voltage, Vin is firstly applied to the input of an integrating amplifier, theuncharged capacitor C has very little resistance and acts a bit like a short circuit(voltage follower circuit) giving an overall gain of less than one. No current flows intothe amplifiers input and point X is a virtual earth resulting in zero output. As thefeedback capacitor C begins to charge up, its reactance Xc decreases this results inthe ratio of Xc/Rin increasing producing an output voltage that continues to increaseuntil the capacitor is fully charged.At this point the capacitor acts as an open circuit, blocking anymore flow of DCcurrent. The ratio of feedback capacitor to input resistor (Xc/Rin) is now infiniteresulting in infinite gain. The result of this high gain (similar to the op-amps open-loop gain), is that the output of the amplifier goes into saturation as shown below.(Saturation occurs when the output voltage of the amplifier swings heavily to onevoltage supply rail or the other with little or no control in between). Chapter: Operational amplifiersThe rate at which the output voltage increases (the rate of change) is determined bythe value of the resistor and the capacitor, "RC time constant". By changing this RCtime constant value, either by changing the value of the Capacitor, C or the Resistor,R, the time in which it takes the output voltage to reach saturation can also bechanged for example. 8
  • 9. Project Report 2011If we apply a constantly changing input signal such as a square wave to the input ofan Integrator Amplifier then the capacitor will charge and discharge in response tochanges in the input signal. This results in the output signal being that of a saw toothwaveform whose frequency is dependent upon the RC time constant of theresistor/capacitor combination. This type of circuit is also known as a RampGenerator and the transfer function is given below.Ramp GeneratorWe know from first principals that the voltage on the plates of a capacitor is equal tothe charge on the capacitor divided by its capacitance giving Q/C. Then the voltageacross the capacitor is output Vout therefore: -Vout = Q/C. If the capacitor ischarging and discharging, the rate of charge of voltage across the capacitor is givenas: Chapter: Operational amplifiersBut dQ/dt is electric current and since the node voltage of the integrating op-amp atits inverting input terminal is zero, X = 0, the input current I(in) flowing through theinput resistor, Rin is given as: 9
  • 10. Project Report 2011The current flowing through the feedback capacitor C is given as:Assuming that the input impedance of the op-amp is infinite (ideal op-amp), nocurrent flows into the op-amp terminal. Therefore, the nodal equation at the invertinginput terminal is given as:From which we derive an ideal voltage output for the Integrator Amplifier as:To simplify the maths a little, this can also be re-written as:Where jω = 2πƒ and the output voltage Vout is a constant 1/RC times the integral ofthe input voltage Vin with respect to time. The minus sign (-) indicates an 180o phaseshift because the input signal is connected directly to the inverting input terminal ofthe op-amp.  As a Differentiator Chapter: Operational amplifiersDifferentiator is exact opposite to the Integrator, as the position of the capacitor andresistor have been reversed and now the reactance, Xc is connected to the inputterminal of the inverting amplifier while the resistor, Rf forms the negative feedbackelement across the operational amplifier as normal.This circuit performs the mathematical operation of Differentiation that is it"PRODUCES A VOLTAGE OUTPUT WHICH IS DIRECTLY PROPORTIONAL TOTHE INPUT VOLTAGES RATE-OF-CHANGE WITH RESPECT TO TIME". 1 0
  • 11. Project Report 2011In other words the faster or larger the change to the input voltage signal, the greaterthe input current, the greater will be the output voltage change in response,becoming more of a "spike" in shape.As with the integrator circuit, we have a resistor and capacitor forming an RCNetwork across the operational amplifier and the reactance (XC) of the capacitorplays a major role in the performance of a Differentiator Amplifier.Differentiator Amplifier CircuitThe input signal to the differentiator is applied to the capacitor. The capacitor blocksany DC content so there is no current flow to the amplifier summing point, X resultingin zero output voltage. The capacitor only allows AC type input voltage changes topass through and whose frequency is dependent on the rate of change of the inputsignal. At low frequencies the reactance of the capacitor is "High" resulting in a lowgain (Rf/Xc) and low output voltage from the op-amp. At higher frequencies thereactance of the capacitor is much lower resulting in a higher gain and higher outputvoltage from the differentiator amplifier.However, at high frequencies a differentiator circuit becomes unstable and will startto oscillate. This is due mainly to the first-order effect, which determines thefrequency response of the op-amp circuit causing a second-order response which, athigh frequencies gives an output voltage far higher than what would be expected. Toavoid this high frequency gain of the circuit needs to be reduced by adding anadditional small value capacitor across the feedback resistor Rf.Ok, some maths to explain whats going on! Since the node voltage of the Chapter: Operational amplifiersoperational amplifier at its inverting input terminal is zero, the current, i flowingthrough the capacitor will be given as:The charge on the capacitor equals Capacitance x Voltage across the capacitor 1 1
  • 12. Project Report 2011The rate of change of this charge isBut dQ/dt is the capacitor current iFrom which we have an ideal voltage output for the Differentiator Amplifier is givenas:Therefore, the output voltage Vout is a constant -Rf.C times the derivative of theinput voltage Vin with respect to time. The minus sign indicates a 180 o phase shiftbecause the input signal is connected to the inverting input terminal of theoperational amplifier.One final point to mention, the Differentiator Amplifier circuit in its basic form has twomain disadvantages compared to the previous integrator circuit. One is that it suffersfrom instability at high frequencies as mentioned above, and the other is that thecapacitive input makes it very susceptible to random noise signals and any noise orharmonics present in the source circuit will be amplified more than the input signalitself. This is because the output is proportional to the slope of the input voltage sosome means of limiting the bandwidth in order to achieve closed-loop stability isrequired. Chapter: Operational amplifiers 1 2
  • 13. Project Report 2011Differentiator WaveformsIf we apply a constantly changing signal such as a Square-wave, Triangular or Sine-wave type signal to the input of a differentiator amplifier circuit the resultant outputsignal will be changed and whose final shape is dependent upon the RC timeconstant of the Resistor/Capacitor combination.  As an InverterThe triangular gain block symbol is again used to represent an ideal op amp. Theinput terminal, + (Vp), is called the non-inverting input, whereas – (Vn) marks the Chapter: Operational amplifiersinverting input. It is similar to the non-inverting circuit shown in Figure 4 except thatNow the signal is applied to the inverting terminal via R1 and the non-invertingterminal is grounded. 1 3
  • 14. Project Report 2011An op amp can produce a signal which is 180 degrees out of phase (inverted) withrespect to the input signal. To use an op amp as an inverting amplifier, send thesignal into the negative input instead of the positive input. As the op amp will doeverything it possibly can to make the voltage (signal) on the negative input matchthe positive input. In the following diagram, that the positive input is connected toground. Its shown as being connected through a resistor but the resistance toground in unimportant. What is important is that the positive input has no signal (itsconnected to the reference, ground). This means that the op amps negative inputwill have no visible (voltage) signal on it. When youre driving the negative input it willact as a virtual ground. The input is converted from a voltage drive to a current drive.The change in current is what drives the op amp. This is important to know becauseat the negative input with an oscilloscope, no signal (when the circuit is an invertingamplifier). The op amp inputs had very high impedance. While this is true, whenusing the inverting input with feedback (which is necessary for audio reproduction),the input impedance becomes the value of the input resistor.Calculating Voltage Gain (inverting input):By knowing the value of the feedback, inverting input resistor and input voltage, wecan calculate the output voltage. The formula is: Chapter: Operational amplifiers Vout = Vin*(Rf/Ri)*-1Inverting Amplifier Configuration 1 4
  • 15. Project Report 2011In this Inverting Amplifier circuit the operational amplifier is connected with feedbackto produce a closed loop operation. For ideal op-amps there are two very importantrules to remember about inverting amplifiers, these are: "no current flows into theinput terminal" and that "V1 equals V2", (in real op-amps both these rules arebroken). This is because the junction of the input and feedback signal (X) is at thesame potential as the positive (+) input which is at zero volts or ground then, thejunction is a "Virtual Earth". Because of this virtual earth node the input resistance ofthe amplifier is equal to the value of the input resistor, Rin and the closed loop gainof the inverting amplifier can be set by the ratio of the two external resistors.We said above that there are two very important rules to remember about InvertingAmplifiers or any operational amplifier for that matter and these are.1. No Current Flows into the Input Terminals2. The Differential Input Voltage is Zero as V1 = V2 = 0 (Virtual Earth)Then by using these two rules we can derive the equation for calculating the closed-loop gain of an inverting amplifier, using first principles.Current ( i ) flows through the resistor network as shown. Chapter: Operational amplifiers 1 5
  • 16. Project Report 2011Then, the Closed-Loop Voltage Gain of an Inverting Amplifier is given as.And this can be transposed to give Vout as: Chapter: Operational amplifiersLINEAR OUTPUTThe negative sign in the equation indicates an inversion of the output signal withrespect to the input as it is 180o out of phase. This is due to the feedback beingnegative in value. 1 6
  • 17. Project Report 2011The equation for the output voltage Vout also shows that the circuit is linear in naturefor a fixed amplifier gain as Vout = Vin x Gain. This property can be very useful forconverting a smaller sensor signal to a much larger voltage.  As a ComparatorThe operational amplifier was originally developed for analogue computing (our PCsare digital computers) and when introduced were complex and expensivecomponents. Now they are in integrated circuit form and are very cheap, about 50 p(0.7 Euros). They are not very spectacular, but are extremely useful.A comparator compares two input voltages. These are usually a reference voltageand a signal from a sensor. The output switches state when the signal input crossesthe reference voltage.THE COMPARATOR RELIES ON THE VERY HIGH OPEN LOOP GAIN OF THEOP AMP. VERY SMALL CHANGES IN THE INPUT CAUSE THE OP AMP TOSATURATE SO THE OUTPUT IS ALWAYS LOW OR HIGH AND ALMOST NEVERUNDECIDED.For a real-life op amp this gain will be between 105 and 107.This means that a potential difference between V1 and V2 of only a few micro volts issufficient to saturate the op amp. If V1 is greater than V2 then Vout will go high (close to the + supply voltage) If V1 is less than V2 then Vout will go low (close to the - supply voltage). Chapter: Operational amplifiersOperational amplifiers require a dual power supply, which means having a central 0volts rail, and a + 15 V rail and a – 15 V rail. The full circuit diagram is shown below,but generally we will ignore the power supply. 1 7
  • 18. Project Report 2011Notice that the op-amp has two inputs and one output. It amplifies the differencebetween the inverting input and non-inverting input. Be careful not to confuse thesymbol with a non-inverting gate.Overview of the uA741Of the different types of op – amps produced, type 741 has achieved a very widepopularity. It is available in 14- pin dual-in line, 8 – pin dual-in line or in TO- stylepackages. Integrated circuit type 747 accommodates two type 741 operationalamplifiers in a single package.uA 741 various package styles: Chapter: Operational amplifiersDefinition of 741-pin functions: 1. Pin 1 (Offset Null): Since the op-amp is the differential type, input offset voltage must be controlled so as to minimize offset. Offset voltage is nulled by application of a voltage of opposite polarity to the offset. An offset null- adjustment potentiometer may be used to compensate for offset voltage. The null-offset potentiometer also compensates for irregularities in the operational amplifier manufacturing process which may cause an offset. Consequently, the null potentiometer is recommended for critical applications. See Offset Null Adjustment for method. 1 8
  • 19. Project Report 20112. Pin 2 (Inverted Input): All input signals at this pin will be inverted at output pin 6. Pins 2 and 3 are very important (obviously) to get the correct input signals or the op amp cannot do its work.3. Pin 3 (Non-Inverted Input): All input signals at this pin will be processed normally without inversion. The rest is the same as pin 2.4. Pin 4 (-V): The V- pin (also referred to as Vss) is the negative supply voltage terminal. Supply-voltage operating range for the 741 is -4.5 volts (minimum) to -18 volts (max), and it is specified for operation between -5 and -15 Vdc. The device will operate essentially the same over this range of voltages without change in timing period. Sensitivity of time interval to supply voltage change is low, typically 0.1% per volt. (Note: Do not confuse the -V with ground).5. Pin 5 (Offset Null): See pin 16. Pin 6 (Output): Output signals polarity will be the opposite of the inputs when this signal is applied to the op-amps inverting input. For example, a sine-wave at the inverting input will output a square-wave in the case of an inverting comparator circuit.7. Pin 7 (posV): The V+ pin (also referred to as Vcc) is the positive supply voltage terminal of the 741 Op-Amp IC. Supply-voltage operating range for the 741 is +4.5 volts (minimum) to +18 volts (maximum), and it is specified for operation between +5 and +15 Vdc. The device will operate essentially the same over this range of voltages without change in timing period. Actually, the most significant operational difference is the output drive capability, which increases for both current and voltage range as the supply voltage is increased. Sensitivity of time interval to supply voltage change is low, typically 0.1% per volt.8. Pin 8 (N/C): The N/C stands for Not Connected. There is no other explanation. There is nothing connected to this pin, it is just there to make it a standard 8-pin package. Chapter: Operational amplifiers 1 9
  • 20. Project Report 2011Keep this in mind as a rule-of-thumb:An operational amplifier circuit will not work at all unless:1. External feedback limits the gain or desired response to a design value.2. Both inputs have direct-current return path to ground of a similar reference.3. The input frequencies and required gain are well within the performancelimitations of the op-amp used.Given Task:We are given to desing an operational amplifier having following specifications suchas:All the circuits’ i.e. operational amplifier as an integrator, an inverter, a differentiator,and as a comparator should … a) Use single IC 741 for all the 4 circuits b) Use switches to ON just one single at a time c) Single input supply Chapter: Operational amplifiersProblems and their solutionsSome of the problems we faced during our project are listed below: I. Complexity of the practically design circuit II. Compiling of a circuit III. Use of the switches IV. Isolation the input supplyWe have overcome on these problems as: 2 0
  • 21. Project Report 2011  Complexity of the practically design circuits:Ideally the all the circuits are grounded at the non-inverting terminal (03) butpractically we use respective resistances across it to avoid… that makes the circuitcomplexSolution:To make the circuit practically efficient it is recommended to use these resistancesrather than being it grounded  Compiling of the circuit:To achieve the task, all the circuits should be complied up on a single overboardusing IC.Solution:For this we design circuit by our self shown below: Chapter: Operational amplifiers Circuit design by the Group Members BS.(Hons.) 4th Semester 2 1
  • 22. Project Report 2011  Use switches:We are assigned to use switches to ON the one circuit at a timeSolution:We use switches separately for all the circuits at inverting terminal (02) , as the inputis supplied to all circuit through inverting terminal, than by the help of the switcheswe allowed input signal to pass through a selected portion of the circuit Circuit design by the Group Members BS.(Hons.) 4th Semester Chapter: Operational amplifiers  Isolation of the circuit:Input supply can isolatedSolution: For this we have to use some of the extra switches at the input supply to theinverting terminal that isolates the input supply. 2 2
  • 23. Project Report 2011Future Directions:To get more efficient result w can alter the following specifications as: 1. We can use less switches up to just 4 If we does not isolates the input supply 2. For avoid any damage to the LED we can use another resistance at the output terminal of almost 1K. 3. We can use more practically effiecnt circuits design. Chapter: Operational amplifiers 2 3
  • 24. Project Report 2011 References1) www.amplifiercircuits.com ………………………………..26-05-112) www.antonnie-education.co.edu ..………………………15-06-113) www.bcae1.com.……………………….…………………..14-06-114) www.brown.edu …………………………………………...14-06-115) www.bucknell.edu ………..…………………………….....14-06-116) www.circuitstoday.com ……………………………………15-06-117) www.electronicstutorial.com ……………………………..14-06-118) www.forumer.com …………….……….…………………..14-06-119) www.hamradioindia.com …………….……………………15-06-1110) Handbook of Operational amplifier applications by Thomas R .Brown J………………………………..………………...…31-05-1111) www.hyperphyscis.com ……………..……………………14-06-1112) Mixed signal and analogue operational amplifier by Jim Karki(1988)………………………………………………….31-05-1113) www.sentex.net ……………………………………………15-06-1114) www.softwarefrodeucation.com ………………………….14-06-1115) www.swarthmore.edu ……………….…………………….14-06-11 Chapter: Operational amplifiers 2 4

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