Bistable multivibrator

 Bistable multivibrator is a type of two-state device similar to a monostable and stable multivibrator. However, the two states are stable, and a trigger is required to change them. In the case of a Bistable multivibrator, the output state is maintained indefinitely until an external trigger is applied to change the state.

In a bistable multivibrator, a driving pulse is applied to the input, and the output pulse is contained when and why the triggering pulses are applied. Each output cycle is produced after two triggering pulses with the correct polarity and amplitude.

The following figure shows the circuit diagram of a self-biased Bistable Multivibrator.

Bistable Multivibrator Diagram

Operation of Bistable Multivibrator

When the circuit is switched ON, due to some circuit imbalances as in Astable, one of the transistors, say Q₁ gets switched ON, while the transistor Q₂ gets switched OFF. This is a stable state of the Bistable Multivibrator.

By applying a negative trigger at the base of transistor Q₁ or by applying a positive trigger pulse at the base of transistor Q₂, this stable state is unaltered. So, let us understand this by considering a negative pulse at the base of transistor Q₁. As a result, the collector voltage increases, which forward biases the transistor Q₂. The collector current of Q₂ as applied at the base of Q₁, reverse biases Q₁ and this cumulative action, makes the transistor Q₁ OFF and transistor Q₂ ON. This is another stable state of the Multivibrator.

Now, if this stable state has to be changed again, then either a negative trigger pulse at transistor Q₂ or a positive trigger pulse at transistor Q₁ is applied

Output Waveforms

The output waveforms at the collectors of Q₁ and Q₂ along with the trigger inputs given at the bases of Q_W and Q₂ are shown in the following figures.