op-amp experiment

8/3/2019 Op-Amp Experiment

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OP-AMP EXPERIMENTSINTEGRATED CIRCUITS AND OPERATIONAL AMPLIFIERSAn integrated circuit is defined as a combination of interconnected circuitelements inseparably associated on or within a continuous semiconductor (oftencalled a chip).

A number of important electronic devices, such as diodes and transistors, areseparate devices that are individually packaged and interconnected in a circuitwith other devices to form a complete, functional unit. Such devices are referredto as discrete components. In an IC, however, many transistors, diodes,resistors and capacitors are fabricated on a single tiny chip of semiconductormaterial and packaged in a single case to form a functional circuit. An IC is thustreated as a single device.

Operational amplifiers(Op-amps) are integrated electronic devices. In ourlaboratory course, we will be concerned with what the circuit does more from anexternal viewpoint than from an internal, component-level viewpoint.

The operational amplifier is an electronic circuit element designed to be

used with other circuit elements to perform a specified signal processingoperation. It is basically a solid-state device with several circuits within a singlepackage capable of sensing and amplifying dc and ac input signals. (Solid stategets its name from path that electrical signals take through solid pieces ofsemiconductor material. Prior to the use of solid state devices, electricity passedthrough various elements inside of a heated vacuum tube.)Early op-amps were constructed with vacuum tubes and worked with highvoltages. Todays op-amps are linear integrated circuits that use relatively low dcsupply voltage and are reliable and inexpensive.

1. OP-AMP BASICSSYMBOL AND TERMINALSThe schematic diagram for a standard op-amp is represented as a triangle asshown in Figure 1.1.

The inverting input is represented by a minus sign. The voltage at this input willcause the output voltage to be inverted by180. The non-inverting input is


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represented by a plus sign. The voltage at this input will cause the voltage at theoutput to be in phase. The output terminal is at the apex of the triangle. Powersupply leads are shown above and below the triangle. The dual () power supplyconnections enable the output to swing both positive and negative. These dcvoltages must always be connected even though they may not be indicated on a

schematic diagram. Other leads coming out of the op-amp may be used forfrequency compensation or nulling components. These leads are also left off theschematic symbol for simplicity. Thus the simplified standard op-amp symbol is:

CIRCUIT FUNCTION OF THE OP-AMPThe circuit function of the op-amp is that it senses the difference between voltagesignals applied at its two input terminals (vnon-in - vin), multiply this by a number A(or Av, called the differential gain or voltage gain) and cause the resulting voltageA(vnon-in vin) to appear at the output terminal.THE IDEAL AND PRACTICAL OP-AMPTo illustrate what an op-amp is, we consider its ideal characteristics. A practicalop-amp, of course, falls short of these ideal standards, but it is much easier tounderstand and analyze the device from an ideal point of view.Characteristics of an ideal op-amp are:

Infinite voltage gain and infinite bandwidth Infinite input impedance (open) so that it does not load the driving source

Zero output impedanceThese characteristics are illustrated in Figure 1.3.

Although modern IC op-amps approach parameter values that can be treated asideal in many cases, the ideal device can never be made. Any device haslimitations, and the IC op-amp is no exception. Op-amps have both voltage andcurrent limitations. Peak to peak output voltage, for example, is also limited byinternal restrictions such as power dissipation and component ratings.Characteristics of a practical op-amp are:


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Very high input impedance, which produces negligible current at the inputs Very high voltage gain, which is useful for amplifying very small signals

Very low output impedance, so that it is affected very little by other circuitloads

These characteristics are illustrated in Figure 1.4.

INTERNAL BOLCK DIAGRAM OF AN OP-AMPA typical op-amp is made up of three types of amplifier circuits as shown in blockdiagram (Figure 1.5).

THE 741 OP AMPThe 741 operational amplifier is one of the commonly used integrated-circuitop-amps. It has eight pin connections as shown in Figure 1.6.


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The lead identification shown in the Figure 1.6 is usually self-explanatory. Thepositive supply voltage is connected to the +V terminal, and the negative supplyvoltage is connected to theV terminal. Input and output terminals are clearlyindicated. The balance terminals (sometimes designated Offset Null) areconnected to a potentiometer for null adjusting. Terminals marked NC (noconnection) are included for physical ruggedness of the package.OP-AMP INPUT SIGNAL MODESSingle-ended inputWhen an op-amp is operated in the single-ended mode, one input is groundedand the signal voltage is applied only to the other input, as shown in Figure 1.7.In the case where the signal voltage is applied to the inverting input as in Figure1.7a, an inverted, amplified signal voltage appears at the output. In the casewhere the signal is applied to the noninverting input with the inverting inputgrounded, as in Figure 1.7b, a noninverted, amplified signal voltage appears atthe output.

Differential inputIn the differential mode, two opposite-polarity (out-of-phase) signals are appliedto the inputs, as shown in Figure 1.8. This type of operation is also referred to asdouble-ended. The amplified difference between the two inputs appears on theoutput.


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Common-mode inputIn the common-mode, two signal voltages of the same phase, frequency andamplitude are applied to the two inputs, as shown In Figure 1.9. When equalinput signals are applied to both inputs, they cancel, resulting in a zero outputvoltage.

This action is called common-mode rejection. Its importance lies in the situationwhere an unwanted signal appears commonly on both op-amps inputs.Common-mode rejection means that this unwanted signal will not appear on theoutput and distort the desired signal. Common-mode signals (noise) generallyare the result of the pick-up of radiated energy on the input lines, from adjacentlines, the 60 Hz power line, or other sources.INPUT/OUTPUT VOLTAGE POLARITYAn important function to remember about an op-amp is the relationship of inputvoltage polarity to output voltage polarity. Figure 1.10 illustrates this relationship,

where the noninverting input is at 0V or ground. If the inverting input is morepositive than the noninverting input, the output will be at negative voltagepotential. Similarly, if the inverting input is more negati ve than noninverting input,the output voltage will be at a positive potential. This relationship remains even ifboth input voltages are positive or negative.

OP-AMP GAINIdeally, the gain of an op-amp should be infinite, however, practically, the gainmay exceed 200,000 in the open-loop mode. In the open-loop mode, there is no


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feedback from the output to the input and voltage gain (Av) is maximum, asshown in Figure 1.11a.

The open-loop voltage gain, AOL, of an op-amp is the internal voltage gain of thedevice and represents the ratio of output voltage to input voltage when there areno external components. The open-loop voltage gain, also referred to as large-signal voltage gain, is not a well-controlled parameter. In a practical circuit, theslight voltage difference at the inputs will cause the output voltage to attempt toswing to the maximum power-supply level. The maximum voltage at the outputwill be about 90% of the supply voltage because of the internal voltage drops ofthe op-amp. The output is said to be at saturation and can be represented (for

either polarity) by +Vsat andVsat. As an example, an op-amp circuit in the open-loop mode using a 15V supply would have its output swing from +13.5 to -13.5.With this type of circuit the op-amp is very unstable and the output will be 0V fora 0V difference between the inputs, or the output voltage will be at eitherextreme, with a slight voltage difference at the inputs. The open-loop mode isfound primarily in voltage comparators and level-detector circuits.The versatility of the op-amp is demonstrated by the fact that it can be used in somany types of circuits in the closed-loop mode, as shown in Figure 1.11b.

External components are used to feedback a portion of the output voltage to theinverting input. This feedback stabilizes most circuits and can reduce the noiselevel. The voltage gain (Av) will be less than maximum gain in open-loop mode.Closed-loop gain must be controlled to be of any value in a practical. By addingresistor Rin to the inverting input as shown in Figure 1.11c, the gain of theop-amp can be controlled. The resistance ratio of Rf to Rin determines the voltagegain of the circuit and can be found by the formula

fv

in

RAR

The minus sign indicates that the op-amp circuit is in the inverting configuration.


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If both Rin and Rf are the same value, the Av equals 1, or unity gain as shown inFigure 1.11d. In this noninverting configuration, the voltage out equals thevoltage in and Av equals +1.

OP-AMP FREQUENCY RESPONSEThe gain of an op-amp decreases with an increase in frequency. The gain givenby manufactures is generally at zero hertz or dc. At very low frequencies, theopen-loop gain of an op-amp is constant, but starts to taper off at about 6Hz orso at a rate of -6 dB/octave or -20db/decade (an octave is a doubling infrequency, and a decade is ten-fold increase in frequency). This decreasecontinues until the gain is unity, or 0dB. The frequency at which the gain is unityis called the unity gain frequency. The unity gain point occurs at 1MHz. The unitygain frequency establishes the reference point at which many op-amps are

specified by manufacturers.Figure 1.12 shows a voltage-gain versus frequency-response curve. In the open-loop mode, the gain falls off very rapidly as frequency increases. When thefrequency increases tenfold, the gain decreases by 10. The breakover pointoccurs at 70.7% of the maximum gain. The frequency bandwidth is normallyconsidered at the point where the gain falls to the breakover point. Therefore, theopen-loop bandwidth is about 10 Hz for this example. Op-amps usually requiredegenerative feedback in amplifier circuits, and this feedback increasesbandwidth of the circuit. For a closed-loop gain of 100, the bandwidth hasincreased to about 10 kHz. Lowering the gain to 10 increases the bandwidth toabout 100 kHz.


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The gain-bandwidth product is equal to the unity-gain frequency. It not only tellsus the upper useful frequency of a circuit, but allows us to determine thebandwidth for a given gain. For example (referring to Figure 1.12, which shows a

frequency-response curve for a frequency-compensated op-amp, such as the741), if you multiply the gain and bandwidth of a specific circuit, the product willequal the unity-gain frequency:

gain -bandwidth product = gain bandwidth= unity - gain frequenc GBP = 100 10 kHz = 1000000 Hz (1MHz)

or

GBP = 10 100 kHz = 1000000 Hz (1MHz)

Therefore, if we wanted to know the upper frequency limit or bandwidth of acircuit with gain of 100, we would divide the unity-gain frequency by gain:

unity -gain frequencybandwidth =

gain

1000000BW = = 10kHz

100

OFFSET NULLINGIdeally the output voltage of an op-amp should be zero when the voltages at bothinputs are the same or zero. If the two input terminals of the op-amp are tiedtogether and connected to ground, it will be found that a finite dc voltage exists atthe output (Figure 1.13a). This is the output dc offset voltage (VOO). In a critical


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circuit, this offset can cause error voltages at output. If we divide the output dcoffset voltage by the gain AOL, we obtain the input offset voltage V IO. The lattermay be represented by a voltage source connected in series with one of the inputleads of an ideal op-amp, which would cause the output dc voltage to be reducedto zero as shown in Figure 1.13b.

Most integrated circuit op-amps provide a means of compensating for offsetvoltage. An external potentiometer is connected to one of the inputs and then it isadjusted to bring back the output voltage to zero when the voltage difference at

the inputs is zero. This method is called offset nulling or input offset voltagecompensation. Many op-amps have offset nulling pins, as shown in Figure 1.14.The ends of the potentiometer are connected to these pins with the viperattached to theV supply. Often null circuits are used with an op-amp but are notshown on the schematic diagram.

2. OP-AMP PARAMETERSThe following parameters are useful to know when working with op-amps.INPUT PARAMETERS:

Differential input voltageThe difference of voltage between the two inputs is called differential inputvoltage.Input offset voltage (VIO)The ideal op-amp produces zero volts out for zero volts in. In a practical op-amp,however, a small dc voltage, Vout (error), appears at the output when nodifferential input voltage is applied. The input offset voltage, V IO, is the differentialdc voltage required between the inputs to set the output to zero volts. Typical


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values of input offset voltage are in the range of 2mV or less. In the ideal case, itis 0 volts.Input bias current (IB)In order for the (real) op-amp to operate, its two input terminals have to beapplied by finite dc currents, termed the input bias currents. By definition, the

input bias current, IB, is the average of both input currents. Ideally, the two inputbias currents are equal.Input offset current (IIO)Ideally, the two input bias currents are equal, and thus their difference is zero. Ina practical op-amp, however, the bias currents are not exactly equal. The inputoffset current, IIO, is the difference of the input bias currents (expressed as anabsolute value).Common-mode input voltage rangeAll op-amps have limitations on the range of voltages over which they willoperate. The common-mode input voltage range is the range of input voltageswhich, when applied to both inputs, will not cause clipping or other output

distortion. Many op-amps have common-mode input voltage ranges of 10V withdc supply voltages of 15V.Input resistance(ZI)This is the resistance looking in at either input with the remaining inputgrounded.OUTPUT PARAMETERS:Output offset voltage (VOO)Output offset voltage, VOO, is a slight unwanted voltage at the output when thevoltage between inputs is zero. Ideally, VOO should be zero.Output short-circuit current (IOSC)The maximum output current that the op-amp can deliver to a load is calledoutput short-circuit current, I

OSC.

Output voltage maximum swing (VOmax)Depending on the load resistance, output voltage maximum swing, VOmax, is themaximum peak output voltage that the op-amp can supply without saturation orclipping.Output resistance (ZO)This is the resistance looking into the op-amps output. DYNAMIC PARAMETERS:Open-loop voltage gain (AOL)Ratio of the output voltage to the differential input voltage in a differentialamplifier without the external feedback is called open-loop voltage gain, AOL, ordifferential gain.Slew rate (SR)The maximum rate of change of the op-amps output voltage under large signalconditions is called slew rate, SR.

outVSR=t

where out max max V = +V - (-V )


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t is the time interval required for the output voltage to go from its lower limit toits upper limit.The unit of slew rate is volts per microsecond (V/s).Slew rate tells how fast the op-amp can react to changes at input. It reflects theop-amps ability of handling varying signals. If one tries to drive the output at a

rate of voltage change greater than the slew rate, the output would not be able tochange fast enough and would not vary over the full range expected resulting insignal clipping or distortion. In any case, the output would not be an amplifiedduplicate of the input signal if op-amp slew rate is exceeded.Consider the unity gain follower circuit in Figure 2.1 and let the input voltage V be the step voltage of height V (shown in Figure 2.2a). When the op-amp is slewrate limited (or slewing) it is not capable of responding to its input signal withoutdistortion and the output appears as shown in Figure 2.2b.

If sinusoidal waveform is applied at the inputs of the unity gain follower, the op-amp slew rate limiting causes nonlinear distortion as shown in Figure 2.3.


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OTHER PARAMETERS AND DEFINITIONS:Supply currentThis is the current the op-amp will draw from the power supply.

Common-mode voltage (VCM)Common-mode voltage is an unwanted, but unavoidable voltage on both inputs,such as 60-cycle hum.Common-mode gain(ACM)Ideally, an op-amp provides zero gain for common-mode signals but practical op-amps do exhibit a very small common-mode gain, (ACM), which is defined as theratio of the common-mode output voltage to the common-mode input voltage.Common-mode rejection ratio (CMRR)Common-mode rejection ratio, (CMRR), is a measure of the ability of the op-ampto reject signals that are simultaneously present at both inputs. It is the ratio ofthe open-loop voltage gain, AOL, to the common-mode gain, ACM.

OL

CM

ACMRR=A

The higher the CMRR, the better. A high value of CMRR means that the open-loop gain, AOL, is high and the common mode gain ACM, is low and theperformance of the op-amp in terms of rejection of common mode signals isbetter.Power supply voltage rejection ratio (PSRR)The ratio of the change in the power supply voltage to the resulting change ininput offset voltage is called power supply voltage rejection ratio, (PSRR).Variation in power supply voltage will also affect the input offset voltage.Power supply decoupling

Capacitors in the range 0.1 to 1.0 F connected from the power supply voltagesto ground to bypass voltage variations to ground provide the power supplydecoupling.Input protectionDiodes, zener diodes, and/or resistors are used at the inputs to protect the op-amp from excessively large input voltages.Latch-up


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Latch-up is a condition where a large input signal causes the output to remain in+Vsat orVsat. Diodes and resistors used in the output circuit can prevent this.Output protectionA low-value resistor connected in series with the output of an op-amp to limitcurrent during a short-circuit condition provides output protection. Some op-amps

have the protection built in.

3. PRACTICAL OP-AMP CIRCUITS(DESIGN USING OP-AMP)

One of the early applications of operational amplifiers was to build circuits thatperformed mathematical operations. Indeed, the operational amplifier takes itsname from this important application. Many of the op-amp circuits that performmathematical operations are used so often that they have been given names(e.g. summing amplifier, difference amplifier, integrator, differentiator etc).Op-amp can be connected in a large number of circuits to provide various

operating characteristics. Some of the basic applications are discussed below: Open-loop mode circuits Basic linear amplifier circuits Integrator, Differentiator and Square wave generator circuits

OPEN-LOOP MODE CIRCUITS:Comparator is a circuit that compares two input voltages and produces anoutput in either of two states indicating the greater than or less than relationshipof the inputs. In this application, the op-amp is used in the open-loopconfiguration, with the input voltage on one input and a reference voltage on theother.The polarity of the voltage at the output of an op-amp depends on therelationship of the polarity between the voltages at the inputs. The inverting ( -)input is referenced to the noninverting (+) input. When the inverting (-) input ismore positive than the noninverting (+) input, the output will be negative andwhen the inverting (-) input is more negative than the noninverting (+) input, theoutput will be positive. Without a feedback path, the output will either be at +V satorVsat. Figure 3.1 shows a comparator.

A comparator circuit can be used for:Zero-level detectionNonzero-level detectionZero-level detectorFigure 3.2a shows a zero-level detector.


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The inverting (-) input is grounded to produce a zero level and the input signalvoltage is applied to the noninverting (+) input. Because of the high open-loopvoltage gain, a very small voltage difference between the two inputs drives theamplifier into saturation, causing the output voltage to go to its limits.Figure 3.2b shows the result of a sinusoidal input voltage applied to thenoninverting (+) input of the zero-level detector. When the sine wave is positive,the output is at its maximum positive level. When the sine wave crosses zero, theamplifier is driven to its opposite state and the output goes to its maximumnegative level. Thus the zero-level detector can be used as a squaring circuit toproduce a square wave from a sine wave.

Nonzero level detectorAn op-amp comparator can be used to detect a positive voltage level as shown inFigure 3.3a. It is inverting input sensor. The reference voltage at the noninvertinginput is found by the formula

3

ref2 3

R

V = +VR R


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When the voltage at the inverting input is below Vref, the output is at +Vsat. Whenthe voltage at the inverting input increases above Vref, the output swings toVsat.When Vref is at the inverting input as shown in Figure 3.3b, it becomes a

noninverting input sensor. The output will swing to +V sat the instant the voltage atthe noninverting input is greater than V ref.

If point A is moved toV power supply the circuits will detect a negative voltage.Figure 3.4a shows the arrangement with a sinusoidal input voltage applied tononinverting input of the nonzero-level detector.

The resulting output is shown in Figure 3.4b.


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BASIC LINEAR AMPLIFIER CIRCUITS:Linear applications are those in which the output signal is directly proportional tothe input signal.Negative feedback is one of the most useful concepts in electronics, particularlyin op-amp linear applications. Negative feedback is the process whereby aportion of the output voltage of an amplifier is returned to the input with a phaseangle that opposes (or subtracts from) the input signal.The usefulness of an op-amp in an open-loop mode (i.e. without negativefeedback) is severely restricted and is generally limited to comparator and othernonlinear applications. As the inherent open-loop voltage gain of a typical op-amp is very high therefore, an extremely small input voltage drives the op-amp

into its saturated output states and the op-amp becomes nonlinear. With negativefeedback, the closed-loop voltage gain can be reduced and controlled so that theop-amp can function as a linear amplifier. Negative feedback is illustrated inFigure 3.5.


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The inverting input effectively makes the feedback signal 180 out of phase withthe input signal. The negative feedback network closes the loop around the op -amp. The gain of op-amp in such configurations is called the closed loop gain.Inverting AmplifierAn op-amp connected as an inverting amplifier with a controlled amount of

voltage gain is shown in Figure 3.6.

The input signal is applied through a series input resistor Rin to the invertinginput. Also the output is fed back through Rf to the same input. The noninvertinginput is grounded.The gain of the circuit is calculated by the formula Av= -Rf/Rin (the minus signindicates only that the polarity of the output voltage is opposite to the polarity ofthe input voltage) or can be found by A v= -Vout/Vin.The junction of Rf and Rin at the inverting input is about the same voltage as thenoninverting input and is referred to as virtual ground.To reduce the offset bias currents, the noninverting input is not directly groundedbut a resistor Rn is used. Rn is equal to the value of R in and Rf in parallel

(Rn=RinRf/Rin+Rf).When inverting amplifier is used for ac signals, capacitors are used at the inputand output terminals, to block any dc voltage from the circuit which might causedistortion. The frequency response of an op-amp circuit depends on its gain. Thelower the gain, the wider the frequency response.Noninverting AmplifierAn op-amp connected as a noninverting amplifier with a controlled amount ofvoltage gain is shown in Figure 3.7.


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The input signal is applied to the noninverting input. The output is applied back tothe inverting input through the feedback circuit (closed loop) formed by the inputresistor Rin and the feedback resistor Rf.The gain of the circuit is calculated by the formula Av= Rf/Rin+1 or Av= Vout/Vin.When the noninverting amplifier is used for ac signals, capacitors are used at the

input and output terminals, to block any dc voltage form the circuit that mightcause distortion. Even though the input voltage changes, an ampli fiers gainremains the same. A noninverting amplifier is used for high input impedance,where Rin cannot be made larger, because of affecting the gain of the circuit andcreating more noise.Voltage followers (or Source followers)Voltage followers are special cases of the noninverting and inverting amplifiers. Anoninverting amplifier with Rf=0 and Rin=, becomes a noninverting voltagefollower as shown in Figure 3.8a. It has a gain of 1 because of the zeroresistance feedback loop. It is referred to as voltage follower since the outputfollows the input and is in phase with the it. Because of gain of 1, this circuit is

also named as the unity gain amplifier. The impedance to this circuit can bemade very high.

An inverting amplifier with Rf=Rin becomes an inverting voltage follower as shownin Figure 3.8b.

The gain of this circuit is 1 (Av= -Rf/Rin) and the output voltage is 180 out ofphase with the input voltage. The input impedance to this circuit is lower, beinglimited by the value of Rin.Voltage followers are used to match circuit impedances and act as bufferamplifiers, isolating one circuit from another.Summing Amplifier


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If more than one input is used on an inverting amplifier, it becomes a summingcircuit or adder as shown in Figure 3.9.

The output voltage is the algebraic sum of the inputs, but inverted, and can befound by the formula

f f fout 1 2 n

1 2 n

R R RV V V ..... V

R R R

where Rn and Vn are the number of input resistors and input voltages. The outputvoltage is weighted sum of the input signals (V1, V2,.Vn). This circuit istherefore called weighted summer. Each summing coefficient may beindependently adjusted by adjusting the corresponding feed-in resistors (R1 toRn).When all the resistors in the summing amplifier are of the same value, the circuitbecomes unity gain summing amplifier and the formula for V out simplifies to

out 1 2 nV V V ..... V When all input resistors are of the same value with R f a larger value, the circuitbecomes summing amplifier with gain. Vout is given by

fout 1 2 nRV V V ..... VR

where R is the value of each equal-value input resistor.When in the summing amplifier with gain, the ratio Rf/R is set equal to thereciprocal of the number of inputs (n), the circuit becomes averaging amplifier.Hence the summing amplifier produces the mathematical average of the inputvoltages when Rf/R=1/n.When different weights are assigned to each input of a summing amplifier, byadjusting the values of the input resistors, the circuit becomes scaling adder. Inthis circuit, some inputs influence the output voltage more than the others. Theweight of a particular input is set by the ratio of Rf to the resistance Rx for thatinput (Rx = R1, R2,.,Rn). For example, if an input voltage is to have a weight if 1,then. Or, if a weight of 0.5 is required, Rx=2Rf. The smaller the value of inputresistance Rx, the greater the weight, and vice versa.The input currents and current through Rf add up to zero at the inverting input,referred to as the current summing point. The summing amplifier can also beused as an audio signal mixer.Difference Amplifier


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Both inputs are used (or active) for a difference amplifier or subtractor, as shownin Figure 3.10. The output voltage is found by the formula

2 12out 1 2

1 3 4

1 R RRV V V

R 1 R R

If all resistors are equal, the formula simplifies to V out=V2-V1; however, the

polarity of the output voltage depends on the relationship of the inverting andnoninverting inputs polarities, similar to a comparator circuit.A difference amplifier may have gain or use scaling input arrangement where oneinput has more influence on the output.DIFFERENTIATOR, INTEGRATOR AND SQUARE WAVE GENERATORCIRCUITS:Op-amp DifferentiatorAn op-amp differentiator simulates mathematical differentiation, which is aprocess of determining the instantaneous rate of change of a function. The basicop-amp differentiator, shown in Figure 3.11, is similar to the basic invertingamplifier, except that the input element is a capacitor. This circuit produces

output that is proportional to the rate of change of the input voltage and is givenby

inout f

dVV R C

d t

The product RfC is called the time constant and it should be approximately equalto the period of the input signal to be differentiated.

Op-amp IntegratorAn op-amp integrator simulates mathematical integration, which is basically asumming process that determines the total area under the curve of a function.


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The basic op-amp integrator, shown in Figure 3.12, is similar to the basicinverting amplifier, except that the feedback element is a capacitor. This circuit issaid to be inverse of the differentiator circuit, which is consistent with themathematical operation of differentiation and integration.The output voltage of the integrator, as a function of time, is given by

t

out in

0

1V V dtRC

The product RC is the time constant and, as with the differentiator circuit, it ismade approximately equal to the period of the input signal to be integrated.

Op-amp square wave generatorAn op-amp can be constructed to produce a square-wave generator as shown inFigure 3.13. Resistors R2 and R3 form a voltage divider from the output of the op-amp to ground and determine the Vref. Assume, initially, that Vout is at +Vsat.Capacitor C1 begins to charge through R1 to +Vsat. The instant the voltage on thecapacitor is greater than +Vref at the noninverting input, the output switches toVsat. The capacitor now charges towardVsat and the instant it is greater thanVref, the output switches back to +Vsat and the process begins again. The square

wave output at Vout is Vsat in amplitude. The amplitude of VC1 is Vref and can befound by the formula

3ref sat2 3

R+V +V

R R

and 3ref sat

2 3

R-V -V

R R

If R3 is 86% of R2, the approximate output frequency can be found by the formula

out1 1

1f

2R C


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BEFORE STARTING THE EXPERIMENTS Inverter check:Before starting the experiments, check that your IC is

working properly. This can be easily done by connecting the op-amp in aninverting unity gain amplifier (as shown in figure below) and checking theoutput signal on scope for any suitable input signal.

(It is better to do a quick inverter check than to waste timeexperimenting with a damaged IC.)

Power supply range:Op-amps are designed to be powered fromvoltage supply which is typically in the range of 5 to 15 volts. Toavoid damaging the op-amp use 12 volts for voltage supply in theexperiment.

Power supply polarity:Never reverse power supply polarity to the op-amps. Applying a negative voltage to the +V pin and a positive to the-V pin, even momentarily will result in destructive current flow throughthe op-amp!

Power and signal sources:After wiring the circuit, connect or turn onthe power and signal sources to the breadboard last! Planning the experiment:Plan your experiment beforehand. Know

what type of results you are expected to observe. Dont mindlesslytake data unless you have a good idea of what should be observed.You can analyze things before doing the lab or as you go along.


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4. EXPERIMENTS

EXPERIMENT 1To demonstrate the basic operation of an op-amp as a comparator circuit.

Apparatus:dual 12V power supply, digital voltmeter, 741 op-amp,10kpontentimeters, 10kresistors, bread board for constructing circuit

Procedure: Circuit is constructed as shown in Figure 4.1. V1 and V2 are set todefinite values and the corresponding values of Vout are recorded, indicatingpolarity.

EXPERIMENT 2

Observations

V1(V)

V2(V)

Vout(V)

+1 0

-1 0

0 +1

0 -1

+2 +1+1 +2

+1 -1

-1 +1

-1 -2

-2 -1


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To demonstrate the operation of an op-amp inverting amplifier with dc and acvoltages and calculate gain of the circuit.

Apparatus: dual 12V power supply, digital voltmeter, 741 op-amp, oscilloscope,AC signal generator, 10 kpotentiometer, breadboard for constructing circuit,

resistors (4.7kkkkkkF capacitors

Procedure: For dc amplifier, circuit is constructed as shown in Figure 4.2a. Fordifferent values of Rin, Rf and Vin (as shown in data table), Vout is measured. Gainis calculated by the formulae: Av=-Rf/Rin and Av=Vout/Vin.

Observations and Calculations

Rin(k)

Rout=Rf(k)

Vin(V)

Vout(V) Av=-Rf/Rin Av=Vout/Vin

10 47 +1

10 100 +1

10 22 +1

4.7 47 -1

22 47 -1

10 47 -1

For ac amplifier, circuit is constructed as shown in Figure 4.2b. ForRf=100kthe frequency generator is set at Vin=1Vp-p and Vout is measured fordifferent frequencies (f). Gain is calculated by: Av=Vout/Vin. Graph is plotted

between Av and f. Same procedure is repeated for Rf=47k


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Rf=100k Rf=47kf (kHz)(for Vin

at1 Vp-p)

Vout(Vp-p) Av=Vout/Vin

Vout(Vp-p) Av=Vout/Vin

0.1

0.15

0.2

0.51

1.5

2

5

10

15

20

50

100

150200

500

1000

1500

2000


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EXPERIMENT 3To demonstrate the operation of an op-amp non-inverting amplifier with dc and

ac voltages and calculate gain of the circuit.

Apparatus: dual 12V power supply, digital voltmeter, 741 op-amp, oscilloscope,AC signal generator, 10 kpotentiometer, breadboard for constructing circuit,

resistors (4.7kkkkkkF capacitors

Procedure: For dc amplifier, circuit is constructed as shown in Figure 4.3a. Fordifferent values of Rin, Rf and Vin (as shown in data table), Vout is measured. Gainis calculated by the formulae: Av=Rf/Rin+1 and Av=Vout/Vin.


Rin(k)

Rf(k)

Vin(V)

Vout(V) Av=Rf/Rin+1 Av=Vout/Vin

10 47 +1

10 100 +1

10 22 +14.7 47 -1

22 47 -1

10 47 -1

For ac amplifier, circuit is constructed as shown in Figure 4.3b. ForRf=100kgain is calculated by the formula: Av=Rf/Rin+1. The frequency


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generator is set at 1kHzand Vout is measured for different input voltages Vin(Vp-p).Vout is also calculated by the formula: Vout=AvVin.

Observations and calculations

Vin(Vp-p)

Vout

(Vp-p)

(measured)

Vout

=AvV

in

(Vp-p)(calculated)

0.1

0.2

0.5

1.0

1.5

EXPERIMENT 4To demonstrate the operation of op-amp voltage followers, and to show thedifference between the inverting and non-inverting types.

Apparatus: dual 12V power supply, digital voltmeter, 741 op-amp, oscilloscope,AC signal generator, breadboard for constructing circuit, 1F capacitors, resistors(4.7kkk

Procedure: Circuit is constructed as shown in Figures 4.4a and 4.4b. The signalgenerator is set for 1kHz at 2Vp-p for Vin. Vout is measured and the outputwaveform is drawn for both the circuits.


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EXPERIMENT 5To demonstrate how an op-amp can be used to sum algebraically various inputvoltages.

Apparatus: dual 12V power supply, digital voltmeter, 741 op-amp, resistors(4.7kkk 10 kpotentiometers, breadboard for constructing circuit

Procedure: Circuit is constructed as shown in Figure 4.5a, using all 10kresistors. V1 and V2 are set at different voltages and corresponding Vout is

measured. Vout is also calculated by the formula: Vout= -(V1+V2). Same procedureis repeated after changing Rf to 22k but with Vout calculated by:Vout= -Rf(V1/R1+V2/R2).


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Rf=10k Rf=22kInput voltage Vout algebraic sum (inverted) Input voltage Vout algebraic sum (inverte

V1 V2 Calculated Measured V1 V2 Calculated Measured

(V) (V) (V) (V) (V) (V) (V) (V)

+1 +2 +1 +2

+1 -2 +1 -2

+2 +1 +2 +1

+2 +1 +2 +1

-2 -2 -2 -2

Circuit shown in Figure 4.5b is constructed by using the voltage divider circuitsof the first part. V1 and V2 are set at different voltages and corresponding Vout ismeasured. Vout is also calculated by the formula: Vout= -Rf(V1/R1+V2/R2).


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EXPERIMENT 6To demonstrate how an op-amp can be used to find the algebraic differencesbetween two input voltages.

Apparatus: dual 12V power supply, digital voltmeter, 741 op-amp,kresistors 10 kpotentiometers, breadboard for constructing circuit

Procedure: Circuit is constructed as shown in Figure 4.6. V1 and V2 are set atdifferent voltages and corresponding Vout is measured. Vout is also calculated bythe formula: Vout= -(V2 -V1).

Observations and calculationsInput voltage Vout algebraic sum (inverted)

V1(V)

V2(V)

Calculated(V)

Measured(V)

+1 +2+1 -2

+2 +1

+2 -1

-2 -2


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EXPERIMENT 7To demonstrate how an op-amp can sense a specific voltage level and how tocalculate the reference voltage (Vref).

Apparatus: dual 12V power supply, digital voltmeter, 741 op-amp, resistors(kk 10 kpotentiometer, breadboard for constructing circuit

Procedure: Circuit is constructed as shown in Figure 4.7a. Wiper of R1 is placedat gound and Vref is calculated by the formula: Vref= (+V)(R3)/(R2+R3). Using the

voltmeter Vout and Vref are measured. R1 is adjusted until Vout changes and thenew reading is recorded. Wire at point A is removed from the +V supply andconnected to theV supply and the above steps are repeated to detect negativesupply. Same procedure is repeated to detect voltages for circuit shown in Figure4.7b.


Input voltage Vout algebraic difference (inverted)

V1(V)

V2(V)

Calculated(V)

Measured(V)

+2 +4

+4 +2

+4 -2

-2 +4

-4 +2



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To detect positive voltage

Vref= -----V

Vout= -----V when Vin is less than Vref

Vout = -----V when Vin is greater than Vref

To detect negative voltage

Vref = -----V

Vout = -----V when Vin is less than Vref



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EXPERIMENT 8To show how an op-amp can be used as a square-wave generator and how tocalculate its output frequency.

Apparatus: dual 12V power supply, digital voltmeter, 741 op-amp, oscilloscope,breadboard for constructing circuit, capacitors(0.1F, 0.02F, 0.05F), resistors(22k k 4.7kkk

Procedure: Circuit is constructed as shown in Figure 4.8. Vref is calculated usingthe formulae: +Vref=(+Vsat)(R3)/(R2+R3) andVref=(-Vsat)(R3)/(R2+R3). Using theoscilloscope fout,+Vsat, -Vsat, +Vref andVref are measured and waveforms at Vref,Vout and V1 are drawn. Frequency of the generator is calculated by: fout=1/2R1C1.These steps are repeated for different values of R1and C1.


To detect positive voltage

Vref= -----V

Vout= -----V when Vin is less than Vref


To detect negative voltage

Vref = -----V

Vout = -----V when Vin is less than Vref



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R1(k)

C1F)

fout (Hz)Calculated

fout(Hz)Measured

10 0.05

22 0.05

4.7 0.05

10 0.02

10 0.1

REFERENCES: Electronic devices by Thomas L. Floyd Electronic devices and circuit theory by Robert L. Boylestad and Louis

Nashelsky Introduction to electric circuits by Richard C. Dorf and James A. Svoboda Microelectronic circuits by Adel S. Sedra and Kenneth C. Smith Operational Amplifiers (Electronic Technology Series) by Heathkit

educational systems

(Lab Manual Op-Amp Experiments by: Arooj Mukarram)

op-amp experiment

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