Editing Comparator (section)

==Design==

{{Unreferenced section|date=February 2022}}

A comparator consists of a [[gain (electronics)|high gain]] [[differential amplifier]] whose output is compatible with the [[logic gate]]s used in the digital circuit. The gain is high enough that a very small difference between the input voltages will saturate the output, the output voltage will be in either the low logic voltage band or the high logic voltage band of the gate input.   Analogue [[op amp]]s have been used as comparators, however a dedicated comparator chip will generally be faster than a general-purpose operational amplifier used as a comparator, and may also contain additional features such as an accurate, internal reference voltage, adjustable [[hysteresis]], and a clock gated input.

A dedicated voltage comparator chip such as LM339 is designed to interface with a digital logic interface (to a [[Transistor-transistor logic|TTL]] or a [[CMOS]]). The output is a binary state often used to interface real world signals to digital circuitry (see [[analog-to-digital converter]]). If there is a fixed voltage source from, for example, a DC adjustable device in the signal path, a comparator is just the equivalent of a cascade of amplifiers. When the voltages are nearly equal, the output voltage will not fall into one of the logic levels, thus analog signals will enter the digital domain with unpredictable results. To make this range as small as possible, the amplifier cascade is high gain. The circuit consists of mainly [[bipolar transistor]]s. For very high frequencies, the input [[Electrical impedance|impedance]] of the stages is low. This reduces the saturation of the slow, large [[p–n junction]] bipolar transistors that would otherwise lead to long recovery times. Fast small [[Schottky diode]]s, like those found in binary logic designs, improve the performance significantly though the performance still lags that of circuits with amplifiers using analog signals. Slew rate has no meaning for these devices. For applications in [[flash ADC]]s the distributed signal across eight ports matches the voltage and current gain after each amplifier, and resistors then behave as level-shifters.

=== Open collector output ===
{{Main article|Open collector}}
Some comparators (e.g. LM339) use [[open collector]] output to help interface to different logic families. When the inverting input is at a higher voltage than the non inverting input, the output of the comparator connects to the negative power supply. When the non inverting input is higher than the inverting input, the output is [[high impedance]], so the output voltage in this state can be set by an external [[pull-up resistor]] to a different voltage supply.