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AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2


Free Running (Astable) Multivibrators


Multivibrators are the most commonly used square wave generators and they belong to a family of oscillators called relaxation oscillators. A multivibrator has two different electrical states, usually high voltage output and low voltage output, and there are three main types of such two-state circuits:

a. Astable multivibrator. This switches continuously between the two states at a constant repetition rate without external triggering. It is therefore a 'free-running' oscillator and it produces continuous square waves at its output.

b. Monostable multivibrator. This circuit has only one stable state. When it is triggered by an external signal it passes into the other state, remains there for a given time and then returns, of its own accord, to the first (stable) state.

It will remain in the stable state until another triggering input pulse is received. This circuit is also referred to as a "one-shot multivibrator" because it goes through a full cycle in response to a single triggering pulse. A more common name however, and the one we shall use in these notes, is "flip-flop".

c. Bistable multivibrator. In this circuit both states are stable, the circuit remaining in either of them indefinitely. It passes from one state to the other only when suitably triggered, and to go through a full cycle two triggering pulses are needed. This circuit is also known as a bistable trigger circuit, a toggle or an Eccles-Jordan circuit.

Note that only the astable multivibrator is free-running. The others must be triggered.


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