n this post we will be discussing about the op-amp as a comparator.
We have already discussed
other applications of the op-amp in rectangular wave form generator circuits like astable (or
free-running) multivibrators, monostable multivibrators (or one-shot) and bistable
multivibrators (or flip-flops).
To get a better understanding of operational amplifiers click here:- Operational Amplifiers (Op-
amp)
Op-amp Comparator
comparator finds its importance in circuits where two voltage signals are to be compared and
to be distinguished on which is stronger. comparator is also an important circuit in the design
of non-sinusoidal waveform generators as rela!ation oscillators.
"n an op-amp with an open loop configuration with a differential or single input signal has a
value greater than #, the high gain which goes to infinity drives the output of the op-amp into
saturation. Thus, an op-amp operating in open loop configuration will have an output that goes to
positive saturation or negative saturation level or switch between positive and negative saturation
levels and thus clips the output above these levels. This principle is used in a comparator circuit
with two inputs and an output. The $ inputs, out of which one is a reference voltage (%ref) is
compared with each other.
Working of 741 IC Op-amp Comparator Circuit
Non-inverting 741 IC Op-amp Comparator Circuit
non-inverting &'( ") op-amp comparator circuit is shown in the figure below. "t is called a
non-inverting comparator circuit as the sinusoidal input signal %in is applied to the non-inverting
terminal. The fi!ed reference voltage %ref is give to the inverting terminal (-) of the op-amp.
When the value of the input voltage %in is greater than the reference voltage %ref the output
voltage %o goes to positive saturation. This is because the voltage at the non-inverting input is
greater than the voltage at the inverting input.
&'( ") *on-"nverting )omparator )ircuit
When the value of the input voltage %in is lesser than the reference voltage %ref, the output
voltage %o goes to negative saturation. This is because the voltage at the non-inverting input is
smaller than the voltage at the inverting input. Thus, output voltage %o changes from positive
saturation point to negative saturation point whenever the difference between %in and %ref
changes. This is shown in the waveform below. The comparator can be called a voltage level
detector, as for a fi!ed value of %ref, the voltage level of %in can be detected.
The circuit diagram shows the diodes +(and +$. These two diodes are used to protect the op-
amp from damage due to increase in input voltage. Thes diodes are called clamp diodes as they
clamp the differential input voltages to either #.&% or -#.&%. ,ost op-amps do not need clamp
diodes as most of them already have built in protection. -esistance -( is connected in series with
input voltage %in and - is connected between the inverting input and reference voltage %ref. -(
limits the current through the clamp diodes and - reduces the offset problem.
&'( ") .p-mp *on-"nverting )omparator Waveform
Inverting 741 IC Op-amp Comparator Circuit
n inverting &'( ") op-amp comparator circuit is shown in the figure below. "t is called a
inverting comparator circuit as the sinusoidal input signal %in is applied to the inverting terminal.
The fi!ed reference voltage %ref is give to the non-inverting terminal (/) of the op-amp.
potentiometer is used as a voltage divider circuit to obtain the reference voltage in the non-
inverting input terminal. 0othe ends of the 1.T are connected to the dc supply voltage /%))
and -%22. The wiper is connected to the non-inverting input terminal. When the wiper is rotated
to a value near /%)), %ref becomes more positive, and when the wiper is rotated towards -%22,
the value of %ref becomes more negative. The waveforms are shown below.
.p-amp &'( ") "nverting )omparator )ircuit
&'( ") .p-mp "nverting )omparator Waveform
Comparator Caracteristics
(. Operation !pee" 3 ccording to change of conditions in the input, a
comparator circuit switches at a good speed beween the saturation levels and the
response is instantaneous.
$. Accurac# 3 ccuracy of the comparator circuit causes the following characteristics:-
(a) $ig %oltage &ain 3 The comparator circuit is said to have a high voltage gain characteristic
that results in the re4uirement of smaller hysteresis voltage. s a result the comparator output
voltage switches between the upper and lower saturation levels.
(b) $ig Common 'o"e (e)ection (atio (C'(() 3 The common mode input voltage
parameters such a noise is re5cted with the help of a high ),--.
(c) %er# !mall Input Offset Current an" Input Offset %oltage 3 negligible amount of "nput
.ffset )urrent and "nput .ffset %oltage causes a lesser amount of offset problems. To reduce
further offset problems, offset voltage compensating networks and offset minimi6ing resistors
can be used.
