Transcript Comparators - UniMAP Portal

Op-amp
Comparators
part I of 3
Comparators
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The comparator is an op-amp
circuit that compares two input
voltages and produces an
output indicating the
relationship between them. The
inputs can be two signals (such
as two sine waves) or a signal
and a fixed dc reference
voltage.
Comparators are most
commonly used in digital
applications. Digital circuits
respond to rectangular or
square waves, rather than sine
waves. These waveforms are
made up of alternating (high
and low) dc levels and the
transitions between them (as
shown in Figure 1).
Comparators
• The purpose served by a
comparator is as illustrated.
• Assume that the digital system
is designed to perform a
specific function when a sine
wave input reaches a value of
10 V.
• Unfortunately, digital systems
respond to waveforms like the
one in the top Figure, not sine
waves.
• Using a comparator between
the signal source and the
digital system, as shown in
bottom Figure, solves the
problem.
Comparators
• With the inverting input of the comparator
tied to a reference voltage (Vref ), the
output from the comparator remains low
as long as the value of the input waveform
is less than Vref . As soon as the value of
the input exceeds Vref , the comparator
output goes high. When the value of the
input falls below Vref , the comparator
output goes low again.
Comparators
– The dc output voltage from the comparator
indicates the polarity (or magnitude)
relationship between the two inputs.
– Comparators are not normally used to convert
sine waves into square waves. A Schmitt
trigger usually performs this function.
– When a comparator circuit is used to compare
an input signal to a fixed dc voltage, the circuit
is commonly referred to as a level detector.
Comparators
• The most noticeable circuit recognition feature of
the comparator circuit is the lack of any
feedback path. (See the comparator in the next
slide.) Without a feedback path, the voltage gain
of the circuit is equal to the open loop gain (AOL )
of the op-amp. With such high gain, even the
slightest difference voltage at the inputs results
in the output going to one or the other of the
voltage extremes (depending on the input
polarity).
Comparators
Just as the name implies when an op-amp is used as a
comparator it is comparing one signal to another. In
this particular example of a zero-level detector the opamp is in open loop configuration. The incoming signal
drives the op-amp into saturation producing a squarewave output. Remember that the op-amp is driven into
saturation easily when in open-loop configuration.
Comparators
With nonzero-level detection the voltage divider or zener
diode sets the reference voltage at which the op-amp
turns goes to the maximum voltage level.
Comparators
•Most electronic systems have one
or more supply voltages. The
reference voltage for a comparator is
usually derived from one of them.
Figure illustrates how a voltagedivider circuit is used to establish a
reference voltage using the supply
voltage of the circuit.
Comparators
Remember that the comparator is configured in openloop making the gain really high. this is open-loop
configuration. This makes the comparator very
susceptable to unwanted signals (noise) that could
cause the output to arbitrarily switch states.
Comparators
The effects of the noise can be reduced with feedback for
hysteresis. This sets the trigger upper and lower trigger
points, set by the voltage divider, such that it is unaffected by
the minor variations caused by noise. This type of op-amp
configuration is sometimes called a Schmitt trigger.
Comparators
If the level of the pulse must be less than the output
of a saturated op-amp a zener-diode can be used to
limit the output to a particular voltage. This is called
output bounding. Either positive, negative, or both
halves of the output signal can be bounded by use of
one or two zener diodes respectively.