In electronics, a comparator is a device that compares two voltages or currents and outputs a digital signal indicating which is larger. It has two analog input terminals ${\displaystyle V_{+}}$ and ${\displaystyle V_{-}}$ and one binary digital output ${\displaystyle V_{\text{o}}}$. The output is ideally

A comparator consists of a specialized high-gain differential amplifier. They are commonly used in devices that measure and digitize analog signals, such as analog-to-digital converters (ADCs), as well as relaxation oscillators.

The differential voltages must stay within the limits specified by the manufacturer. Early integrated comparators, like the LM111 family, and certain high-speed comparators like the LM119 family, require differential voltage ranges substantially lower than the power-supply voltages (±15 V vs. 36 V). Rail-to-rail comparators allow any differential voltages within the power-supply range. When powered from a bipolar (dual rail) supply,

or when powered from an unipolar TTL/CMOS power supply,

Specific rail-to-rail comparators with p–n–p input transistors, like the LM139 family, allow the input potential to drop 0.3 volts below the negative supply rail, but do not allow it to rise above the positive rail. Specific ultra-fast comparators, like the LMH7322, allow the input signal to swing below the negative rail and above the positive rail, although by a narrow margin of only 0.2 V. Differential input voltage (the voltage between two inputs) of a modern rail-to-rail comparator is usually limited only by the full swing of power supply.

An operational amplifier (op-amp) has a well balanced difference input and a very high gain. This parallels the characteristics of comparators and can be substituted in applications with low-performance requirements.

A comparator circuit compares two voltages and outputs either a 1 (the voltage at the plus side) or a 0 (the voltage at the negative side) to indicate which is larger. Comparators are often used, for example, to check whether an input has reached some predetermined value. In most cases a comparator is implemented using a dedicated comparator IC, but op-amps may be used as an alternative. Comparator diagrams and op-amp diagrams use the same symbols.

A simple comparator circuit made using an op-amp without feedback simply heavily amplifies the voltage difference between Vin and VREF and outputs the result as Vout. If Vin is greater than VREF, then voltage at Vout will rise to its positive saturation level; that is, to the voltage at the positive side. If Vin is lower than VREF, then Vout will fall to its negative saturation level, equal to the voltage at the negative side.