1. EASWARI ENGINEERING COLLEGE
(AN AUTONOMOUS INSTITUTION)
( A Unit of SRM Group of Educational Institutions,
Affiliated to Anna University, Chennai and ISO certified )
Accredited by NAAC with A grade and NBA
191EEC303T- LINEAR INTEGRATED CIRCUITS
UNIT – 2 CHARACTERISTICS OF OPAMP
Ms.B.PONKARTHIKA
ASSISTANT PROFESSOR
DEPARTMENT OF EEE
EASWARI ENGINEERING COLLEGE
CHENNAI
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2. Block Diagram of OP-AMP
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3. ⮚ The Op-AMP consists of:
1. Input stage
2. Intermediate stage
3. Level shifting stage
4. Output stage
1. Input stage:
The input stage must satisfy the following requirements:
⮚ It must have two inputs (inverting and non-inverting)
⮚ It must provide a very high input impedance and low output
impedance.
⮚ It must be directly coupled amplifier and must have a very
high CMRR.
2. Intermediate stage (gain stage):
The requirements of the intermediate stage are:
⮚ Moderately high input and output resistance.
⮚ High voltage gain.
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4. 3. Level shifting stage:
Due to the direct coupling between the first two
stages, the input of level shifting stage is an amplified
signal with some nonzero dc level.
Level shifting stage is used to bring this dc level to
zero volts with respect to ground.
4. Output stage:
Requirements of output stage:
⮚ Low output resistance
⮚ Large current sourcing capacity.
⮚ Large output voltage swing.
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5. Introduction
⮚ Various branches of electronics are: Industrial
electronics, Instrumentation, Communication, Power
electronics etc.
⮚ Various electronic circuits used in the applications of
these branches are amplifiers, waveform generators,
timers, various arithmetic circuits such as adder,
subtractor, multipliers, Log-antilog amplifier etc.
⮚ One electronic device which can be used to construct all
the circuits mentioned above is called an operational
amplifier or OP-AMP.
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6. OP-AMP
⮚ Voltage amplifier using a transistor uses many other
components such as coupling and bypass capacitors,
biasing resistors etc.
⮚ In order to get higher voltage gain, we have to use more
than one such amplifiers together. This will make the
circuit bulky.
⮚ So people started using the amplifier in the integrated
circuit (IC) form.
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7. Why it is called Operational Amplifier?
⮚ An OP-AMP is basically an amplifier which could be
configured to perform a variety of operations such as
amplification, addition, subtraction, differentiation and
integration. Hence it is called operational amplifier.
⮚ OP-AMP is basically a multistage amplifier which uses a
number of amplifier stages interconnected to each other
in a complicated manner.
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8. Advantages of OP-AMP over
conventional amplifier:
1. It has smaller size.
2. Its reliability is higher than conventional amplifier.
3. Reduced cost.
4. Less power consumption.
5. Easy to replace.
6. Same OP-AMP can be used for various applications.
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9. OP-AMP Symbol and
Terminals
⮚ The OP-AMP is basically a voltage amplifier with high
voltage gain of 2 x 105 or 106 db.
2
3
7
4
6
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10. OP-AMP IC 741
⮚ This is one of the oldest and most popular OP-AMP IC.
Features of IC 741:
1. No frequency compensation required.
2. Short circuit protection.
3. Offset voltage null capability.
4. Large common mode and differential voltage ranges.
5. No latch ups.
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12. Pin configuration:
IC 741 is 8 pin IC.
⮚ Pin 1 and 5:
These pins can be used to nullify the offset voltage.
⮚ Pin 2 and 3:
Pin number 2 and 3 are inverting and non-inverting inputs
respectively.
⮚ Pin 4 and 7:
Pin number 7 is for connecting the positive supply
voltage +VCC while pin number 4 is to be connected to a
negative supply voltage. Thus IC 741 needs a dual polarity
power supply.
⮚ Pin 6:
We get the output voltage at pin number 6.
⮚ Pin 8:
Pin number 8 is a dummy pin which is not connected
anywhere and hence should be left open or unconnected.
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13. Applications:
1. Inverting and non inverting amplifiers.
2. Adder, subtractor.
3. Integrator, differentiator circuits.
4. Other applications such as V to I and I to V converter,
precision rectifier, log, antilog amplifier etc.
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14. Ideal op-amp characteristics:
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1. Infinite voltage gain (AV = ∞)
2. Infinite input resistance (Ri = ∞)
3. Zero output resistance (Ro = 0)
4. Zero offset voltage:
In practical OP-AMPs a small output voltage is
present even though both the inputs V1 and V2 are
having zero value. This voltage is called as the offset
voltage.
5. Infinite bandwidth:
Bandwidth of an amplifier is the range of frequencies
over which all the signal frequencies are amplified
almost equally.
15. 6. Infinite CMRR
7. Infinite slew rate (S = ∞):
The slew rate is defined change in output voltage
occur simultaneously with the input voltage changes.
8. Zero power supply rejection ratio (PSRR = 0):
PSRR is specifies the degree of dependence of OP-
AMP output, on the changes in power supply voltage.
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17. Characteristics of an OP-AMP
• The OP-AMP characteristics(parameters) are important in
practice because, we can use them to compare the
performance of various op amp ICs and select the best
suitable from them for the required application.
• OP-AMP characteristics are classified into two categories
namely AC characteristic and DC characteristic.
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18. • Open loop voltage gain-It is the differential gain of an
OP-AMP in the open loop mode of operation.
• Input resistance-It is defined as the equivalent resistance
which can be measured at either at inverting or non-
inverting terminal with the other terminal connected to
ground.
• Output resistance-It is the resistance measured by
looking into the output terminal of OP-AMP, with the
input source short circuited.
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19. • Bandwidth-It is the range over which all signal frequencies
are amplified almost equally.
• Common mode rejection ratio-It is defined as the ratio of
differential gain to common mode gain.
• Slew rate-It is defined as the maximum rate of change of
output voltage per unit time.
• Power supply rejection ratio-It is the change in an OP-AMPs
input offset voltage caused by variation in the supply voltage.
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20. • Input offset voltage-Ideally, for a zero input voltage
output should be zero. But practically it is not so.
This is due to unavoidable unbalances inside the OP-
AMP.
• Input bias current-It is the average of the currents
flowing into the two input terminal of the OP-AMP.
• Input offset current- It is the algebraic difference
between the currents flowing into the inverting and
non-inverting terminal of OP-AMP.
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21. Comparison of ideal and practical
OP- AMP
characteristics Practical value Ideal value
Voltage gain 2 ×105 ∞
Input resistance 2MΩ ∞
Output resistance 75Ω 0
Bandwidth 1 MHz ∞
CMRR 90 dB ∞
Slew rates 0.5V/µs ∞
PSRR 150µV/V 0
Input offset voltage 2mV 0
Input bias current 50 nA 0
Input offset current 6 nA 0
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22. CONFIGURATION OF OP-AMP
• Open loop configuration
In open loop configuration , there is no feedback
from output to input.
The differential signal present between the inputs will
be amplified by it’s open loop gain.(Av=2×105)
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23. • Therefore even for very
differential voltage output will reach positive
small magnitude of
or
negative saturation
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24. Why is OP-AMP not used as an amplifier in
the open loop configuration?
• Due to very open loop gain, distortion is introduced in
the amplified output signal.
• The open loop gain does not remain constant but
varies with temperature and power supply as well
due to mass production technique.
• The bandwidth of an OP-AMP is very small almost
equal to zero. For this reason the open loop OP-
AMP is not used in practice as an amplifier.
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25. • Close loop configuration
In close loop configuration , a feedback is introduced
i.e. a part of output is fed back to the input.
The feedback can be of the following two types:
1. Positive feedback/regenerative feedback
2.Negative feedback/degenerative feedback
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26. Positive feedback
If the feedback signal and the input signal are in
phase with each other then it is called as the positive
feedback.
It is used in application such as oscillators and
schmitt trigger or regenerative comparators.
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27. Negative feedback
If the signal fed back to the input and the original input
signal are 180º out of phase, then it is called as the
negative feedback.
In application of op amp as an amplifier, the negative
feedback is used.
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28. Advantages of negative
feedback
• It stabilizes gain
• Reduces the distortion
• Increases the bandwidth
• Reduces the effect of variations in temperature and
supply voltage on the output of op amp
⮚ The only disadvantage of negative feedback is low
gain
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29. Concept of virtual short
• According to virtual short concept, the potential
difference between the two input terminals of an op
amp is almost zero.
• In other words both the terminals are approximately
at the same potential.
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30. infinite. Hence current flowing from one
• The input impedance of an OP-AMP is ideally
input
terminal to the other will be zero.
• Thus the voltage drop across Ri will be zero and both
the terminals will be at the same potential.
• Means they are virtually shorted to each other
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31. Virtual Ground
If one of the terminal of OP-AMP is connected to
ground then due to the virtual short existing between
the other input terminal, the other terminal is said to
be at ground potential.
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32. DC Characteristics
⮚ The non ideal dc characteristics that add error
components to the dc output voltage are,
A. Input Bias Current
B. Input Offset Current
C. Input offset Voltage
D. Total Output offset Voltage
E. Thermal drift
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33. A. Input Bias Current
⮚ In an ideal op-amp, we assumed that no current is drawn
from the input terminals.
⮚ The base currents entering into the inverting and non-
inverting terminals (IB
+ & IB
- respectively).
⮚ Input bias current IB is the average value of the base
currents entering into terminal of an op-amp
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34. Input Bias Current
⮚ The effect can be compensated with compensation resistor Rcomp. By
KVL,Vo = V2 – V1
⮚ By selecting proper value of
Rcomp, V2 can be cancelled with
V1 and the Vo = 0.
⮚ The value of Rcomp is derived as,
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36. B.Input offset current
The difference between the bias currents at the input
terminals of the op- amp is called as input offset current.
The input terminals conduct a small value of dc current
to bias the input transistors.
Since the input transistors cannot be made identical,
there exists a difference in bias currents
⮚ Even with bias current compensation, offset current will
produce an output voltage when Vi = 0.
V1 = IB
+ Rcomp
I1 = V1/R1
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37. Input offset current
⮚ So even with bias current compensation and with
feedback resistor of 1M, a BJT op-amp has an output
offset voltage
Vo = 1M Ω X 200nA
Vo = 200mV with Vi = 0
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38. Input offset current
⮚ T-feed back network is a good solution
⮚ Provides a feedback
signal as if the network
were a single feedback
resistor
⮚ By T to π conversion,
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39. C.Input offset voltage
A small voltage applied to the input terminals to make
the output voltage as zero when the two input terminals
are grounded is called input offset voltage, Vos
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40. Input offset voltage
⮚ Let us determine the Vos on the output of inverting and
non-inverting amplifier. If Vi = 0 (Fig (b) and (c)) become
the same as in figure (d).
⮚ V2 at the –ve input terminal is given by,
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41. Total Output Offset Voltage
⮚ The maximum offset voltage at the output of an inverting
and non-inverting amplifier without any compensation
technique used is given by
⮚ With Rcomp in the circuit, total output offset voltage will
be given by
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43. 43
D.THERMAL DRIFT
⮚ Bias current, offset current and offset voltage change
with temperature.
⮚ A circuit carefully nulled at 25oc may not remain so
when the temperature rises to 35oc.
⮚ This is called drift.
⮚ Offset current drift is expressed in nA/ºC.
⮚ Offset voltage drift is expressed in mV/ºC.
⮚ Indicates the change in offset for each degree Celsius
change in temperature
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44. Applications
⮚ Audio and radio communication
⮚ Medical Electronics
⮚ Instrumentation control
⮚ Voltage comparator
⮚ Filter
⮚ Waveform generator
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45. AC Characteristics:
⮚ For small signal sinusoidal (AC) application one has to
know the ac characteristics such as frequency response
and slew-rate.
1. Frequency response
2. Circuit stability
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62. Differential Amplifier
⮚ Operational amplifier is expected to amplify the
differential signal present between its two input
terminals.
= V1 – V2
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63. ⮚ Differential input signal (Vd):
The different between the input signals V1 and V2 is
called as the differential signal Vd.
∴ Vd = V1 – V2
⮚ Common mode signal:
A common signal to both the input terminal i.e. V1 = V2 =
V is called as common mode signal.
⮚ Differential gain Ad:
Ad = Vo/ Vd
Ad (dB) = 20 log 10 [Vo/Vd]
⮚ Common mode gain Ac:
Ac = Vo/ Vc
where Vc = (V1 + V2)/ 2
⮚ Total output voltage Vo:
Vo = Ad Vd + Ac Vc
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64. ⮚Common Mode Rejection ratio (CMRR):
CMRR is the ability of a differential amplifier
to reject the common mode signal successfully.
It is also called figure of merit.
∴ CMRR = ρ = |Ad/ Ac|
IB1 = 0
IB2 = 0
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66. Inverting Amplifier
⮚ In inverting amplifier, the signal which is to be amplified
is applied at the inverting terminal of OP-AMP.
⮚ The amplified output signal will be 1800 out of phase or
“inverted” with the input signal.
⮚ As shown in fig., input signal Vin is connected to inverting
terminal via resistor R1.
⮚ Feedback resistor Rf connected between output and
inverting terminal and non inverting terminal connected
to ground.
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68. ⮚ As we know, output voltage is given by,
VO = AV x Vd
but Vd = V1 – V2
⮚ V1 = 0 since the NI (+) terminal is connected to ground.
Hence as per virtual ground concept the INV (-) terminal
is also at 0 V potential.
∴ V2 = 0
⮚ As Ri = ∞, the current IB2 goint into (-) terminal is zero.
⮚ Hence current through R1 and Rf is same equal to I.
∴ Vin = I x R1 and VO = -I x Rf
⮚ Output voltage VO = AV x Vin
∴ - IRf = AV x IR1
∴ AV = -Rf /R1
⮚ The negative sign indicates that there is a phase shift of
1800 between the input and output voltages
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69. Non- Inverting Amplifier
⮚ In non-inverting amplifier, input signal is applied to the
non-inverting (+) terminal.
⮚ Feedback resistor Rf is connected between output and
inverting terminal.
⮚ Input and output voltages are in phase with each other.
⮚ For ideal Op-AMP, Ri = ∞, therefore the currents
entering into both the input terminals of opamp will be
zero. (I1 = I2 = 0)
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70. ⮚ Therefore voltage across R1 is given by,
V2 = [R1 / (Rf + R1)] x VO
⮚ As per the virtual short concept,
V2 = V1 = Vin
∴ Vin = [R1 / (Rf + R1)] x VO
⮚ Therefore closed loop voltage gain AV is given by,
AV = VO / Vin = (R1 + Rf) / R1
∴ AV = 1 + (Rf / R1)
⮚ The positive sign indicates that the input and output are
in phase with each other and closed loop gain is always
greater than unity (1).
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73. Summing amplifier or Adder
⮚ If more than one input signal is applied to an any one
input terminal of OP-AMP, the circuit will add all these
input signals to produce their addition at the output.
⮚ Adder circuit can be classified into two categories as:
1. Inverting adder
2. Non-inverting adder
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74. Inverting Adder
⮚ Fig. shows the inverting summing amplifier with three
inputs Vin1, Vin2 and Vin3 are applied to the inverting
terminal through resistors Rin1, Rin2 and Rin3 respectively
and Rf is the feedback resistor.
⮚ Expression for output voltage:
⮚ Let current through resistors Rin1, Rin2 and Rin3 be I1, I2
and I3 respectively.
⮚ apply KCL at node A to write,
I1 + I2 + I3 = IB2 + If ------(1)
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75. ⮚ But for ideal op-amp, Ri = ∞, therefore IB2 = 0 and VA = VB = 0
due to virtual ground concept.
Hence, I1 + I2 + I3 = If
From input side, I1 = (V1 - VA) / Rin1 = V1 / Rin1
Similarly, I2 = (V2 - VA) / Rin2 = V2 / Rin2
and I3 = (V3 - VA) / Rin3 = V3 / Rin3
⮚ And from output side,
If = (VA – VO) / Rf = (- VO) / Rf
⮚ Substituting these values in eq. 1, we get,
(V1/ Rin1) + (V2/ Rin2) + (V3 / Rin3) = (-VO / Rf)
∴ VO = - [(Rf/Rin1)V1 + (Rf/Rin2)V2 + (Rf/Rin3)V3]
⮚ If we substitute Rf = Rin1 = Rin2 = Rin3 = R, then we get
VO = - (V1 + V2 + V3)
⮚ Thus output voltage is the negative sum of the input voltage.
Therefore this circuit is called as “inverting adder”.
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77. Non - Inverting Adder
⮚ An adder circuit which can produce the addition of its
input signals without inversion is called as the non-
inverting adder.
⮚ Fig. shows the non-inverting adder with two inputs V1
and V2.
⮚ The output voltage is given by,
VO = V1 + V2
⮚ The positive sign indicates that the inversion does not
take place in this adder and therefore it is called as the
non-inverting adder.
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79. Difference Amplifier
⮚ The difference amplifier and subtractor circuits are used to
obtain the subtraction of two input voltages.
⮚ Fig. shows the difference amplifier.
⮚ Assume V2 = 0, then circuit become non-inverting amplifier
with output voltage VO1 and it is given by,
VO1 = AV x VA
VO1 = [1 + (Rf / R1)] x [Rf / (Rf + R1)] V1
= [(R1 + Rf) / R1] x [Rf /(Rf + R1)] V1
∴ VO1 = (Rf / R1) V1
⮚ The output voltage of the difference amplifier is given as,
VO = (Rf / R1) x (V1 – V2)
⮚ (Rf / R1) is called as the “gain of the difference amplifier”.
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81. Subtractor
⮚ The output voltage of difference amplifier is given by,
VO = (Rf / R1) x (V1 – V2)
⮚ If we substitute Rf = R1 = R in above equation, then we
get,
VO = V1 – V2
⮚ And difference amplifier gets transformed into a
subtractor.
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83. Integrator
⮚ Ideal integrator circuit:
I1
If
IB
V2
V1
IB
1
f
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If feedback component used is a capacitor ,the resulting connection is called
integrator.
84. • The output voltage is negative of input voltage and
inversely proportional to time constant R and C .
Vo(s)= -Vin(s) %SRC
• The gain A, A=Vo(s)%Vin(s) = - 1/(jwCR)
• Taking magnitude of A A=
1/(wCR) = W/Wa Where
Wa=1/CR
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85. • The integrator work as a low pass filter circuit
when time constant is very large .
• At w=0, the gain A is infinite for an ideal op-
amp .
• At dc , the capacitor C behaves as an open
circuits and there is no negative feedback.
• But in practice output never becomes infinite .
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86. Practical Integrator:
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• The gain of an integrator at low frequency (dc) can be
limited to avoid saturation by introducing a feedback
resistance(Rf) in shunt with feedback capacitance(Cf) .
• The resistor Rf limits the low frequency gain to –Rf/R
( generally Rf=10R) .
88. Applications of an integrator:
1. In the triangular wave or ramp generator
2. In the analog to digital converter
3. In analog computers to solve differential equations.
4. As a low pass filter.
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89. Differentiator
⮚ Ideal Differentiator Circuit:
C
=
Rf
f
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The op-amp circuits that contain capacitor is the differentiating amplifier .
90. ⮚ The differentiator can be constructed from the basic
inverting amplifier by interchanging resistance Rf and C1.
⮚ The expression for the output voltage of differentiator is
given by,
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91. • The output voltage Vo is a constant (-RC)
times the derivative of the input voltage V1 .
• In Laplace form ,s=jw
Vo(s)= -sCRVin(s)
• The gain is ,A=Vo(s)/Vin(s)
A=-sCR = -jwCR
• The magnitude of A=wCR
•A=W/Wa = f/fa
where Wa=1/CR
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92. • At high frequency a differentiator may become
unstable and break into oscillation .
• The input impedence (Xc=1/wc) decrease with
increase in frequency ,therewise making the
circuitsensitive to high frequency noise .
• To overcome through the problem of
unstability and high frequency noise we use
the practical differentiator .
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94. Output Equation
• Vo(s)/Vin(s) = - sRfC1/{(1+sCfRf)(1+sC1R1)}
• If CfRf=C1R1 and s=jw
• Vo(s)/Vin(s) = jwRfC1/{(1+jwCfRf)^2}
• The magnitude of A,
A= wRfC1/{(1+jf/fb)^2}
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95. Applications of differentiator:
1. In the P-I-D controller.
2. As a high pass filter
3. In the wave shaping circuits to generate narrow pulses
corresponding to any sharp change in the input signal.
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Voltage to Current Converter with floating
loads (V/I):
1.Voltage to current converter in which load resistor RL is floating (not
connected to ground).
2.Vin is applied to the non inverting input terminal, and the feedback voltage
across R1 devices the inverting input terminal.
3.This circuit is also called as a current – series negative feedback amplifier.
4.Because the feedback voltage across R1 (applied Non-inverting terminal)
depends on the output current i0 and is in series with the input difference
voltage Vid .
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From the fig input voltage Vin is converted into
output current of Vin/R1 [Vin -> i0 ] . In other words,
input volt appears across R1. If R1 is a precision
resistor, the output current (i0 = Vin/R1 ) will be
precisely fixed.
Applications:
1.Low voltage ac and dc voltmeters
2.Diode match finders
3.LED
4.Zener diode testers.
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Writing the voltage equation for the input loop.
vin = vd+vf
But vd » since A is very large, therefore
vin=vf
vin=Riin
iin = v in / R.
and since input current is zero.
iL = iin = vin / R
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Voltage to Current converter with Grounded
load:
This is the other type V – I converter, in which one terminal of the load is
connected to ground.
Grounded Load:
If the load has to be grounded, then the above circuit cannot be
used. The modified circuit is shown in figure.
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The collector and emitter currents are equal to a close approximation and the
input impedance of OPAMP is very high, the load current also flows through the feedback
resistor R. On account of this, there is still current feedback, which means that the load
current is stabilized.
Since
vd=0
v2=v1=vin
iout = (vCC – vin ) / R
Thus the load current becomes nearly equal to iout. There is a limit to the output
current that the circuit can supply. The base current in the transistor equals iout / bdc. Since
the op-amp has to supply this base current iout / bdc must be less than Iout (max) of the op-
amp, typically 10 to 15mA.
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Current to Voltage Converter (I –V):
1. The output voltage V0 = -RF Iin.
2. Hence the gain of this converter is equal to -RF. The magnitude of the gain
(i.e) is also called as sensitivity of I to V converter.
3. The amount of change in output volt ∆V0 for a given change in the input
current ∆Iin is decide by the sensitivity of I-V converter.
4. By keeping RF variable, it is possible to vary the sensitivity as per the
requirements
