Comparator

The comparator circuit has no connection between the output and the inputs. Therefore, changes in the output voltage do not affect the input voltages.

An op-amp comparator

Remember the rules followed by an op-amp. If the non-inverting input is at the same voltage as the inverting input, the output voltage, whatever it is, will not change. However, if the non-inverting input is even a few microvolts higher than the inverting input, what will happen? The output voltage will rise until the input voltages become equal. Since the rising output voltage does not affect the input voltages, it will keep rising until it reaches its upper limit.

For example, assume that the op-amp is operating from dual 10-volt power supplies. If the inverting input is at +1 volt and the non-inverting is at +1.001 volt (very slightly higher than the inverting input), the output voltage will jump to its upper limit, which will be about +9.5 volts, and stay there. If the non-inverting input drops to +0.999 volts (very slightly lower than the inverting input), the output voltage will jump to its lower limit, which will be about -9.5 volts.

The comparator is used to compare one voltage to another. One example would be to indicate the state of a battery. A comparator could be used to light an LED when the voltage of a battery drops below a certain level.