Multivibrator

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A multivibrator is an electronic circuit that switches rapidly by means of positive feedback between two or more states. There are three types of multivibrator circuit depending on the circuit operation:

  • astable, in which the circuit is not stable in either state —it continually switches from one state to the other. It does not require an input such as a clock pulse.
  • monostable, in which one of the states is stable, but the other state is unstable (transient). A trigger causes the circuit to enter the unstable state. After entering the unstable state, the circuit will return to the stable state after a set time. Such a circuit is useful for creating a timing period of fixed duration in response to some external event. This circuit is also known as a one shot.
  • bistable, in which the circuit is stable in either state. The circuit can be flipped from one state to the other by an external event or trigger. The bistable multivibrator is simply a latch (flip-flop); it is added to this classification only for completeness.

A multivibrator consists of two main components — two passive networks and a bistable circuit, connected in a common feedback loop. The networks can be both resistive-capacitive (in the case of an astable circuit), a resistive-capacitive and a resistive (monostable), and both resistive (bistable). There are two versions of this basic arrangement (considered below) in the case of astable and monostable multivibrator (bistable multivibrators are implemented only according to the first version).

Multivibrators find applications in a variety of systems where square waves or timed intervals are required. For example, before the advent of low-cost integrated circuits, chains of multivibrators found use as frequency dividers. A free-running multivibrator with a frequency of one-half to one-tenth of the reference frequency would accurately lock to the reference frequency. This technique was used in early electronic organs, to keep notes of different octaves accurately in tune. Other applications included early television systems, where the various line and frame frequencies were kept synchronized by pulses included in the video signal.


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