9.4 SCHMITT TRIGGERS

An emitter-coupled bistable multivibrator is also called a Schmitt trigger, named after the designer of the vacuum tube version. In addition to being used as a bistable multivibrator, it has some more important applications.

In the Schmitt trigger circuit shown in Fig. 9.12(a), it is seen that the output of the first transistor is connected to the input of the second transistor through a potential divider network comprising R1 and R2. This is simply an attenuator circuit. Normally R1 and R2 are reasonably large resistors so as to avoid loading the collector of Q1. Further, the emitter resistance RE stabilizes the currents and voltages. Note that the second collector and the first emitter are not involved in the regenerative ...

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