A circuit that compares two voltages, and produces a gate signal telling which one is higher. A comparator circuit is an essential component of a conventional sawtooth core VCO; it compares the voltage being produced by the integrator with a constant reference voltage. When the reference voltage is reached, the comparator triggers a reset circuit which resets the integrator to its starting point.
A more sophisticated type of comparator circuit is called a "window comparator"; it produces a high gate output if an input voltage is within a certain range. Window comparators as stand-alone circuits are available from several modular synthesizer manufacturers. A quantizer is basically a chain of window comparator circuits. An unusual application is the LM3914 bar graph driver integrated circuit, which determines how many LED segments should be lit on a bar graph display based on an input voltage, such that the bar graph visually indicates the voltage. The LM3914 can in fact be used, and has been used, to construct quantizer circuits.