Bistable Multivibrator

 

Construction of Bistable Multivibrator

Two similar transistors Q1 and Q2 with load resistors RL1 and RL2 are connected in feedback to one another. The base resistors R3 and R4 are joined to a common source –VBB. The feedback resistors R1 and R2 are shunted by capacitors C1 and C2 known as Commutating Capacitors. The transistor Q1 is given a trigger input at the base through the capacitor C3 and the transistor Q2 is given a trigger input at its base through the capacitor C4.

The capacitors C1 and C2 are also known as Speed-up Capacitors, as they reduce the transition time, which means the time taken for the transfer of conduction from one transistor to the other.

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

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 QW and Q2 are shown in the following figures.