Multivibrators are switching circuits that employ positive feedback by cross-coupling the output of one stage to the input of the other stage, such that if one device is ON, the other is OFF. The interchange of states is possible either by the use of external pulses or by internal capacitive coupling. These circuits are used either to generate waveforms of a desired nature or to store binary information. Multivibrators are of three types—astable, monostable and bistable. An astable multivibrator is basically a square-wave generator. A monostable multivibrator generates a gated output (pulse) of a desired duration. A bistable multivibrator stores binary bits. In this chapter, we focus on astable multivibrators.
A transistor Q1 or Q2 is said to be in the stable state, if it is either ON or OFF permanently. If the state of the device, say Q1, changes from ON to OFF, and automatically returns to the ON state after a time duration, the device is said to be in the quasi-stable state for this specified time interval. The devices in this multivibrator will not remain in any one state (ON or OFF) forever. The change of state in the device occurs automatically after a finite time interval, depending on the circuit components employed. Hence, this circuit has two quasi-stable states.
In an astable multivibrator, if Q1 is ON, then Q2 is OFF; and they will remain in this state for a limited time duration, after which Q1 automatically switches into the OFF state and Q2 into the ON state and so on. The output of the circuit is a square wave with two time periods, T1 and T2. If T1 = T2 = T/2 the circuit is a symmetric astable multivibrator. If T1 = T2, it is called an un-symmetric astable multivibrator. The main application of an astable multivibrator is as a clock in digital circuits.
The astable multivibrator is essentially a square-wave generator (oscillator). We consider two circuits: the collector-coupled astable multivibrator and the emitter-coupled astable multivibrator in the following sections.