A Schmitt trigger is a comparator circuit that incorporates positive feedback. The following schematic shows a typical diagram of Schmitt Trigger Circuit.
This Schmitt trigger circuit is "emitter coupled” and provides a simple comparator action. When the input is higher than a certain chosen threshold, the output is high; when the input is below another (lower) chosen threshold, the output is low; when the input is between the two, the output retains its value. The 2N3069 JFET places very little loading on the measured input. The 2N3565 bipolar is a high hFE transistor so the circuit has fast transition action and a distinct hysteresis loop.