In electronics, a comparator is a device which compares two voltages or currents, and switches its output to indicate which is larger. More generally, the term is also used to refer to a device that compares two items of data.

A standard op-amp can be used as a comparator as indicated in the following diagram.

When the noninverting input is at a higher voltage than the inverting input, the high gain of the opamp causes it to output the most positive voltage it can. When the noninverting input drops below the inverting, the op-amp outputs the most negative voltage it can. Since the output voltage is limited by the supply voltage, for an op-amp that uses a balanced, split supply, (powered by ± V_S) this action can be written:

V_out = V_S sgn(V₊ − V₋)

where sgn(x) is the signum function.

A dedicated voltage comparator chip, like the LM339, is designed to interface directly to digital logic (such as TTL or CMOS), since the output is a binary state, and is often used to interface real world signals to digital circuitry (see: analog to digital converter). The LM339 accomplishes this with an open-collector output. When the inverting input is higher, the output of the comparator is connected to the negative power supply. When the noninverting input is higher, the output is floating (has a very high impedance to ground). With a pull-up resistor and a 0 to +5 V power supply, for instance, the output takes on the voltages 0 or +5, and can be interfaced to TTL logic.

When comparing a noisy signal to a threshold, the comparator may switch rapidly from state to state as the signal crosses the threshold. If this is unwanted, a Schmitt trigger can be used to provide a cleaner output signal. It uses hysteresis to increase the switching region from a single point to a band.

Source: A previous version of this document was taken from Federal Standard 1037C.