In the previous section, we saw how the use of a feedback resistor
can provide stability to an op-amp allowing it to operate as a linear amplifier.
In this section, we shall look at an op-amp configuration which does not use
feedback: the comparator. The purpose of the comparator is to compare two
voltages and produce a signal that indicates which voltage is greater. Figure
shows an op-amp as a comparator.
The principle of the comparator is based in Equation . Since the
gain, A, of an
op-amp is
very large, any difference will be magnified to the power supply
rails 
, as was
discussed in Section . If 
is greater than 
then the difference
will be positive and the result will be amplified to 
. If, however,
is greater, then the difference is negative and the result will be amplified
to 
. Finally, if the two voltages are exactly equal, then the difference
will be zero and the output will also be zero.