Non Linear Circuits Comparators
Lesson: Non Linear Circuits Comparators
Lesson Developer: Dr.Arun Vir Singh
College/Department: Physics Department, Shivaji
College, University of Delhi
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Table of Contents
Chapter :Applications of Operational Amplifier: Nonlinear Circuits Lesson –II
Comparators
1.1 Introduction
1.1.1Basic comparator
1.2.Noninverting Comparator
1.2.1 (i) Noninverting Comparator with positive reference voltage
1.2.2(ii) Noninverting Comparator with negative reference voltage
1.3 Inverting Comparator
1.3.1(i) Inverting Comparator with positive reference
1.3.2(ii) Inverting Comparator with negative reference
1.4 Applications of Comparators
1.5 Zero Crossing Detectors
1.5.1 Inverting Zero-Crossing Detector
1.5.2 Noninverting Zero-Crossing Detector
1.4 Inverting comparator with positive feed back
Summary
Exercises
Glossary
References
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Comparators
1.1 Introduction
Op-amp in open-loop configuration can be used a comparator. A comparator, as the name
implies, compares the amplitude of one voltage (signal voltage) with another fixed voltage
(reference voltage).These are used as digital interfacing, Schmitt triggers, discriminators,
voltage-level detectors and oscillators.
Op-amp as a comparator is shown in figure.1
Fig.1 Op-amp as a comparator
In this application, these two voltage are compared and output ) may be +
or ,
depending upon which input is the larger.
Depending upon to which terminals, the input is applied; the comparators are classified as,
(i) Noninverting and
(ii) Inverting Comparator
The reference voltage may be positive or negative.
1.2 Comparator in Noninverting Configuration
For this configuration, time-varying signal ( ) is applied at noninverting ‘+’ (pin-3)
terminal and a fixed voltage or reference voltage (
) is applied to inverting ‘-‘(pin-2). If
the positive reference voltage is applied then it is called noninverting Comparator with
positive reference voltage and if the reference voltage is negative then it is called
noninverting Comparator with negative reference voltage.
1.2.1 (i) Noninverting Comparator with positive reference voltage
Figure 2 shows an op-amp used as a comparator in noninverting mode with positive
reference voltage.
A time-varying signal/ sinusoidal input voltage ( ) is applied at
noninverting ‘+’ (pin-3) terminal and a fixed reference voltage (
) is applied to inverting
‘-‘(pin-2) terminal.
From figure
and
Output voltage is given by the expression
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[1.1]
Fig: 2
Noninverting comparator with positive reference voltage.
Where
is open-loop gain =105 very high. So output will be saturated and will depend
upon the relative values of
and
.
For
,
For
,
and
[1.2]
[1.3]
Operation: Reference voltage, input and output waveforms are displayed in figure 3.
When
, ( For points a to b and c to d) the output voltage
is at
,because the voltage at the noninverting terminal is lower than at the inverting terminal.
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Figure 3: Plot of reference voltage, input and output wave forms for a noninverting op-amp.
On the other hand, when
,the inverting (+) input ( points b to c, d to e) the
becomes more positive with respect to the inverting (-) input and output voltage
goes
to +
, because the voltage at the noninverting terminal is higher than at the
inverting terminal. Thus output voltage changes
to
.
1.2.2(ii) Noninverting Comparator with negative reference voltage
Noninverting Comparator with positive reference voltage can be converted into noninverting
comparator with negative reference voltage, if the reference voltage
source shown in
Fig. [1] is replaced by a negative voltage source.i,e Vref is negative with respect to ground.
Figure 4: Reference voltage, input and output waveforms for noninverting comparator with
negative reference voltage.
Working: Plot of input and output waveforms along with the negative reference voltage
(-3V) is displayed in figure 4. When
(between points a to b and c to d ) the output
voltage
is at
,because the voltage at the inverting terminal is higher than
at the noninverting terminal.
When
(between points b and c) the output voltage
goes to is at ,
because the voltage at the noninverting terminal is higher than at the inverting terminal.
Thus output voltage changes
to.
This comparator is a type of analog to digital converter.
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This comparator is sometimes also called a voltage level detector because, for a desired
value of
,the voltage level of
can be detected.
1.3. Inverting Comparator
In this configuration, a fixed voltage (
) or reference voltage is applied to noninverting
‘+’ (pin-3) terminal and the other, time-varying signal ( ) is applied at inverting ‘-‘ (pin-2)
terminal
1.3.1 Inverting Comparator with negative reference voltage
The circuit for the inverting comparator with negative reference voltage is shown in figure 5.
A reference voltage (
) is applied to noninverting ‘+’ (pin-3) terminal and the other, timevarying signal ( ) is applied at inverting ‘-‘ (pin-2) terminal .It can be seen from the
Fig.5:Inverting comparator with negative reference voltage.
figure
and
Output voltage is given by the expression
[1.4]
For
,
[1.5]
For
,
[1.6]
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Fig.6: Reference voltage, input and output waveforms for inverting op-amp for negative
reference voltage.
Working: Reference voltage, input and output waveforms are plotted in figure 6. When
(for points b to c) the output voltage
is at +
.
When
(for points a to b, and c to d) the output voltage
goes to
.
So output voltage changes
to
.
1.3.2 Inverting Comparator with positive reference voltage
Inverting comparator with positive reference voltage can be obtained by replacing the
negative reference voltage
, with a positive reference voltage source
,
shown in figure 5.
So
and
and the output voltage is given by the expression
[1.7]
For
,
[1.8]
For
,
[1.9]
Working: Positive reference voltage, input and output waveforms are shown in figure 7.
For
(between points a and b, c and d ) the output voltage
is at +
.
When
(points b to c, and d to e ) the output voltage
goes to is at . Thus output voltage changes
to
.
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Figure 7 : Wave forms for inverting op-amp for positive reference voltage.
The characteristics of these comparators are summarized in the table.
Comparator
configuration
Ref.
Voltage
Non-inverting
Non-inverting
Inverting
Inverting
positive
Negative
positive
negative
Output for
Vin < Vref
-Vsat
+ Vsat
+ Vsat
-Vsat
Vin > Vref
+ Vsat
-Vsat
-Vsat
+ Vsat
Value addition : FAQ
Limiting Value of differential input voltage
Body Text
For an open-loop configuration (i)
(ii) Supply voltage ±15 V.
For a ±15 V supply ,the saturated voltage are approximately ±13 V.
Max differential voltage +
Max differential voltage Hence input voltage greater than ±65μV produces saturated output voltages. This value of
input is the limiting value.
Source: Op-Amps and Linear Integrated Circuits : Ramakant A. Gayakwad
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1.4 Applications of Comparators
(i) Zero crossing Detectors
(ii)Level detector
(iii)Window detector
(iv) Duty cycle controller
1.5 Zero-Crossing Detectors
An immediate application of the op-amp is the zero-crossing detector or sine to square
wave converter. The basic comparator shown in Fig.[2] and Fig.[5] can be used as the
zero-crossing detector provided that Vref is set to zero (
1.5.1 Inverting Zero-Crossing Detector
Fig. 8 Zero crossing detector
The op-amp in figure 8 operates as a zero-crossing detector in which reference voltage
is applied to noninverting ‘+’ (pin-3) terminal and sinusoidal input voltage ( ) is
applied at inverting ‘-’ (pin-2) terminal of op-amp. The input voltage (
is compared with
a reference voltage of 0V
.Output of zero-crossing detector is given by the equation
[1.10]
In figure
Hence
and
.
[2]
For ideal op-amp Ao is very high, so output
will be saturated and
If
is positive, output will be saturated at
.i.e
is equal to
.
and for negative
The output wave forms along input signal and reference voltage are shown in figure 8 .
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Fig:9 The output wave forms along input signal and reference voltage.
In the regions ‘ a to b’ and ‘c to d’,
input signal passes through zero to positive
direction, the output is driven into –Vsat. Conversely when input signal passes through zero to
negative direction ( b to c), the output switches to +Vsat.This circuit is also called sine to
square wave generator
1.3.1 Noninverting Zero-Crossing Detector
Fig.8- Noninverting Zero-Crossing Detector
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Noninverting zero-crossing detector is illustrated in figure 10. A triangular wave instead of
sinusoidal input voltage ( ) is applied to noninverting ‘+’ (pin-3) terminal and reference
voltage
is applied to inverting ‘-’ (pin-2) terminal of op-amp. The op-amp’s (+)
input compares
with a reference voltage of 0V
.Output of noninverting zerocrossing detector is given by the equation
[1]
In figure
Hence
and
.
[2]
For ideal op-amp Ao is very high, so output
will be saturated and is equal to
.If
is positive
and for negative
. Plot of output wave form, input signal
and reference voltage as a function of time is shown in figure 9.
Fig.9:Plot of output wave form, input signal and reference voltage as a function of time is
shown in
In the regions ‘ a to b’ and ‘c to d’,
input signal passes through zero to negative
direction, the output is driven into +Vsat. Conversely
i.e when input signal passes
through zero to positive direction ( b to c,d to e ), the output switches to –Vsat.
So a triangular wave is also converted into square wave.
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Value Addition Do you know ?
Effect of noise on comparator circuits and how to reduce it ?
Body Test
In many practical situations, noise (unwanted voltage fluctuations) appears on the input line. This noise
voltage becomes superimposed on the input voltage, as shown in figure A, below for the case of a sine
wave.
Figure A: Noise voltage superimposed over sine wave
In order to understand the potential effects of noise voltage, a low-frequency sinusoidal
voltage is applied to the noninverting (+) pin of an op-amp comparator used as a zero-level
detector, as shown in Figure 8. The op-amp’s input signal voltage is drawn both with and
without noise in figure B.
In the figure C the resulting output are shown. When Vin approaches Vref very slowly or
actually hovers close to Vref, =0 V,(a to b, c to d , e to f and f to g ), Vo can either follow all
the noise voltage oscillations or burst into high frequency oscillation or we can say that the
fluctuations due to noise may cause the total input to vary above and below 0 several times,
thus producing an erratic output voltage.
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Fig:C: Effect of noise on zero-crossing `
If no noise is present (g to i) ,the circuit operates as an inverting zero-crossing detector
because Vref=0
This false crossing can be eliminated by positive feed back.
Source: Electronics Devices . Thomas L. Floyd
Value Addition Do you know ?
How a zero crossing detector is used as time marker generator
Body Text:
Time marker generator: It may be used for triggering the SCR, sweep voltage of CRT etc. A
comparator can be used as a time marker generator by differentiating the output of the zero
crossing detectors shown in figure 10. (RC <<T) . The Time marker circuit ,Fig ---input
wave forms (b) Output of comparator (c) differentiated output and
output pulses are
shown
The negative portion is clipped off after passing through the diode, the sin wave gets
converted into a train of positive pulses of spacing T . This T can be adjusted as per
requirement
Source: Linear Integrated circuits By. D.Roy Chudhary & Shail B . Jain
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1.4 Inverting comparator with positive feed back
A comparator with positive feedback is shown in Figure 10. Positive feedback is
accomplished by taking a fraction of the output voltage Vo and applying it to the (+) input.
The output voltage Vo divide between Rf and R1. A fraction of Vo is fed back to the (+) input
and creates a reference voltage that depends on Vo. Now we will study how positive
feedback is used to eliminate the false output changes due to noise.
Fig.10 A comparator with positive feedback
The input voltage
triggers (change of state) the output every time it exceeds certain voltage levels
called the upper threshold voltage
and lower threshold voltage
as shown in figure 11.
and
are calculated using voltage dividing rule.
Upper- threshold voltage
When
, the voltage across
is called upper threshold voltage. This voltage is
divide between
and
and fraction of it is fed back to the (+) input and is given by
.
For
[1.13]
, the voltage at the (+) input is above the voltage at the (-)
is locked at
.
values below
input. Therefore
If
is made slightly more positive than
, the polarity of
reverses and
begins
to decrease . As fraction of
fed back to the positive input is smaller, so
becomes
larger. Vo then is driven to
.
Lower- threshold voltage
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When
, the voltage across
is referred to as lower threshold voltage. This voltage
is divide between
and
and fraction of it is fed back to the (+) input and is given by
[1.14]
Output will stay at
as long as
is above, or positive with respect to ,
.
will
goes more negative than ,or below,
.This circuit is called
switch back to
if
Schmitt trigger. Input and output wave forms are exhibited in figure 11.
Figure 11. Input and output wave form of Schmitt trigger.
The comparator with positive feedback is said to exhibit hysteresis. When the input
of the comparator exceeds hysteresis voltage. ,its output switches from
to and reverts back to its original state,
, when the input goes below
,shown in
figure 12
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Figure 12. Plot of
Vs.
plot of hysteresis voltage.
The hysteresis voltage is, of course, equal to the difference between
and
Therefore
Substituting for
and
[1.15]
[1.16]
and
made larger than input noise voltages, the positive feedback will
eliminate the false output transitions. Also, the positive feedback, because of its
regenerative action, will make Vo switch faster between +Vsat and -Vsat.
Thus, if
Value Addition:
FAQ
How the noise is eliminated using Schmitt Trigger.
Body Text: Output of Schmitt trigger for a sine wave input having noise is illustrated in figure
below.
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It triggers only once when UTP or LTP is reached; thus, there is immunity to noise that is
riding on the input signal.
Source: Electronics Devices. Thomas L. Floyd
Summary:
After completing this section, you should be able to








Describe and analyze the operation of several types of comparator circuits
Discuss the operation of a zero-level detector
Describe the operation of a nonzero-level detector
Calculate the reference voltage
Discuss how input noise affects comparator operation
Calculate the upper and lower trigger points
Explain what a Schmitt trigger is
Mention examples of comparator applications
Exercise
Question Number
Type of question
1
Multiple choice questions
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1. In a zero-level detector, the output changes state when the input
(a) is positive
(b) is negative
(c) crosses zero
(d) has a zero rate
of change
2. The zero-level detector is one application of a
(a) comparator
(b) differentiator
(c) summing amplifier
(d) diode
3. Noise on the input of a comparator can cause the output to
( a) hang up in one state
(b) go to zero
(c) change back and forth erratically
between two states
(d) produce the amplified noise signal
4. The effects of noise can be reduced by
(a) lowering the supply voltage (b) using positive feedback
negative feedback (d) using hysteresis (e) answers (b) and (d)
Correct answers
(1).
(2).
(3).
(4).
(c) using
C
A
C
B and D
Question Number
Type of question
2
Fill in the blanks
1.The difference between the UTP and the LTP is the -------------- voltage.
2. In an op-amp comparator, when the ------- voltage exceeds a ------------ voltage, the
output changes state.
3. Hysteresis gives an op-amp ----- immunity.
4. The output of a comparator has ------- states.
Correct answers
(1).
(2).
(3).
(4).
hysteresis
input, reference
noise
two
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Question Number
Type of question
3
Subjective questions
Non Linear Circuits Comparators
1.What is a comparator?
2. What is voltage limiting, and why is it needed?
3.List important characteristics of the comparator.
4.Sketch a zero-crossing detector and describe its theory of operation
5.What is a Schmitt trigger? How it is different from a zero-crossing detector?
6. Mention the similarities and differences between comparator and Schmitt trigger.
7.To which input would you connect e reference voltage to make an inverting level detector?
Question Number
Type of question
4
Unsolved questions
Comparators
1. A certain op-amp has an open-loop gain of 80,000. The maximum saturated output levels
of this particular device are ± 12V when the dc supply voltages are ± 15V .If a
differential voltage of 0.15 mV rms is applied between the inputs, what is the peak-topeak value of the output?
2. Draw the output voltage waveform for each circuit in shown below with respect to the
input. Show voltage levels.
3.If
,
Correct answers
in figure 10. What are the trip points? What is the hysteresis?
1. 24 V,distorted
2.
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3.=+10.8V,-10.8V ,21.6V
4.
Glossary:
Noise: Unwanted signals in input and output
Feedback: A fraction of output is mixed with input
Schmitt trigger A comparator with built-in hysteresis./or with positive feedback
Hysteresis: Characteristic of a circuit in which two different trigger levels create an offset
or lag in the switching action
References
Electronics Devices and Circuit Theory by Robert. L Boylestad and L. Nashelsky
Op-Amps and Linear Integrated Circuits : Ramakant A. Gayakwad
Electronics Devices . Thomas L. Floyd
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