Bistable Multivibrator: Diagram, Working, Operation, Applications

By Neeraj Dubey|Updated : September 21st, 2022

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.

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What is Bistable Multivibrator?

A bistable multivibrator operates in two states and comprises two cross-coupled transistors, one cut off and the other in saturation. This means that the bistable circuit can remain in either stable state indefinitely.

Bistable Multivibrator Diagram

bistable description

To change the bistable from one state to the other, the bistable circuit requires a suitable trigger pulse, and two such pulses are required for a full cycle. It is more commonly known as a "flip-flop" due to its actual operation, in which it flips into a logic state, remains there, then changes back to its original state.

Triggering in the Circuit of Bistable Multivibrator

The kind of triggering can be either asymmetric or symmetric.

  • Asymmetric triggering involves obtaining the trigger for each transistor independently and from separate sources.
  • In the case of symmetric triggering, the trigger for each transistor is obtained from similar sources and is preferred to be linearly dependent on each other.

Design and Working of Bistable Multivibrator

A Bistable circuit runs in two different modes. They require two input signals to switch between them. Additionally, since bistable circuits need user input to change states, they are not self-triggering.

Circuit Diagram of a Bistable Multivibrator

Two NPN transistors, Q1 and Q2, are connected to two load resistors, RL1 and RL2, on the collectors of a bistable multivibrator circuit.

bistable circuit diagram

Using R1, the output terminal of the first transistor is connected to the input terminal of the second transistor. In contrast, R2 connects the second transistor's output to the input of the first transistor. The resistances R1 and R2 are connected in parallel with two capacitors, also known as commutating capacitors, to improve the switching capability of the circuitry.

Bistable Multivibrator PDF

The circuit can also be called a flip-flop or latch. Flip-flops and latches are basic components of any electronic system that stores information as they have two stable states. But there exist a few differences between flip-flop and latch, which helps in deciding the usage of one among the two.

  • As a result of circuit imbalances, one transistor, say Q1, gets switched ON, and the transistor Q2 gets switched OFF. This is a stable state of the Bistable Multivibrator.
  • By applying a negative pulse at the base of transistor Q1, the collector voltage increases, forward biasing transistor Q1. The collector current of transistor Q2, applied at the base of transistor Q1, reverses transistor Q1, thereby making transistor Q1 OFF and transistor Q2 ON. This can be seen as another stable state of the Multivibrator.
  • Changing this stable state again is possible by applying a negative trigger pulse at transistor Q2 or a positive one at transistor Q1.

Operation in a Bistable Multivibrator

In rationality, a mismatch in Q1 and Q2 would make the transistors different and draw different currents. Because of this, one of the transistors would come in saturation first, say Q1.

  • Due to this, the potential at the collector terminal of Q1 will decrease. This Q1 collector is coupled with the base of Q2 through R1, so decreasing VC1 would drive Q2 into cut-off.
  • We will have low VC1 and high VC2.
  • This would be the first stage of a bistable multivibrator where one transistor Q1 is in saturation while Qis in the cut-off.
  • This state will continue until an external trigger is given at points 1 or. For example, giving a positive trigger at 2 or a negative one at 1 would force Q2 towards decreasing voltage VC. Since C2 is coupled with B1, it will drive Q2 into cut-off, and Q2 will be in saturation.
  • We will have high VC1 and low VC2.
  • This marks the second stage of the bistable Multivibrator.
  • To change it all, we must apply a negative trigger at point 2 or a positive one at point 1.

bistable operation

We can take the output from any of the two collector terminals. The only difference is the response we would get will be the inverse of the other collector terminal. We are getting a response with two stable states for two triggers.

Waveform of the Bistable MUltivibrator

The first end of the rectangle waveform depends on the first input signal and varies according to it, and the second relies on the second input signal. Finally, the resultant waveform is drawn in the given figure.

bistable multivibrator waveform

The output waveforms obtained at the terminal O1 and O2 are complementary to each other, always.

Applications of the Bistable Multivibrator

Bistable Multivibrators are widely used in the circuits of latches and counters. It is also used in frequency divider circuits and storage devices.

  • It is used in different storage devices and for counting binary numbers.
  • Used for frequency division in different circuits.
  • It is used for the production of different clock pulses.
  • It is used in different circuits as a toggle switch.
  • It is used for different relay controllers.

Merits and De-merits of a Bistable Multivibrator

A bistable multivibrator can store previous output until no input trigger is provided. Then, however, a triggering pulse is required every time to transition from one stable state to another.

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FAQs on Bistable Multivibrator

  • In a bistable multivibrator, the output voltage oscillates between two states (high and low) when an electronic pulse is an input. Unlike astable multivibrators, this switching behavior does not cause a free-running oscillation.

  • Because Bistable multivibrators are stable in two states, a sequential-type circuit uses them as a latch or flip-flop.

  • The bistable multivibrator, or flip-flop, is widely used in digital communications and computers to reverse the power supply or periodically change the power supply between two circuits.

  • However, this multivibrator has two stable states. They can be changed by providing a negative trigger at the base of Q1 or a positive trigger at the base of Q2.

  • Whenever a stable state changes, a triggering pulse must be used, which can be expensive compared to stables and monostable.

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