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Schmitt trigger

Schmitt trigger is a comparator with positive feedback. An inverting Schmitt trigger based on op amp is shown on Fig. 1.

An inverting Schmitt trigger based on op amp
Fig. 1. An inverting Schmitt trigger

In a Schmitt trigger circuit, a portion of the op amp output is fed to the non-inverting input and this sets the switching thresholds of the circuit (Fig. 2).

Fig. 2.

A Schmitt trigger can be built using transistors. The Schmitt trigger is an amplifying device with positive feedback, which is achieved by the common resistance in the emitter circuit (Fig. 3). The circuit has two stable states, and the transfer function has hysteresis (Fig. 4).

Fig. 3. Transistor-based Schmitt trigger