Dr. Ahmed G. Abo-Khalil

Electrical Engineering Department

Comparator

In electronics, a comparator is a device that compares two voltages or currents and switches its output to indicate which is larger. They are commonly used in devices such as Analog-to-digital converters (ADCs).

Input voltage range

The input 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 input voltage ranges substantially lower than the power supply voltages (±15 V vs. 36V). Rail-to-rail comparators allow any input voltages within the power supply range. When powered from a bipolar (dual rail) supply,

$V_{S-} le V_+,V_- le V_{S+}$

or, when powered from a unipolar TTL/CMOS power supply:

$0 le V_+,V_- le V_{cc}$

Specific rail-to-rail comparators with p-n-p input transistors, like the LM139 family, allow 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 input signal to swing below the negative rail and above the positive rail, although by a narrow margin of only 0.2V. 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.

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