A monostable multivibrator is one in which there is a stable state and a quasi-stable state. This circuit has two devices Q1 and Q2, one being in the OFF state, say Q1, and the other, Q2, in the ON state—preferably in saturation. These devices remain in this state forever and only on the application of a trigger, the multivibrator goes into the quasi-stable state, i.e., Q1 is ON and Q2 is OFF. After a time interval T, the multivibrator will return to the stable state, i.e., Q1 is OFF and Q2 is ON. Thus, this circuit generates a pulse of duration T. Let us consider two types of monostable multivibrators:
- Collector-coupled monostable multivibrator
- Emitter-coupled monostable multivibrator
The output of this circuit is a pulse of time duration, T called the pulse duration, pulse width or gate width. Some other circuits can also be controlled with the help of this output for a finite time duration. Its main application is as a gate.