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A comparator, as suggested by the name, compares a signal on one input of an op-amp, as shown in figure 3.1, with a known voltage called the reference voltage on the other input. The comparator is nothing other than an open loop op-amp with two analog inputs (differential input) and one digital output (signal ended output). The op-amp has a very large gain when used in an open loop. Hence the output may be in the positive or negative saturation voltage depending upon which input is larger. An op-amp is perfectly suited to comparator applications because of its high input impedance and large open loop gain.

Figure 3.1. An open loop op-amp.

The important characteristics of a comparator are:

1 Speed of operation.

2 Accuracy.

3 Compatibility of output.