Multivibrators
A multivibrator is an electronic circuit used to implement a variety of simple two-state systems such as oscillators, timers and flip-flops. The most common form is the astable or oscillating type, which generates a square wave the high level of harmonics in its output is what gives the multivibrator its common name.
Multivibrators are classified into three types. They are:
(i) Astable
(ii) Monostable
(iii) Bistable
Astable Multivibrators:
Astable, in which the circuit is not stable in either state - it continuously oscillates from one state to the other.The astable has two unstable states, being unable to reset in a fixed state.
Monostable Multivibrators:
The monostable has only one permanent stable state and all the monostable multivibrators are timed devices. A monostable multivibrator has only one stable output state. That is, their unstable output state will hold only for a certain minimum amount of time before returning to its stable state. Monostable, in which one of the states is stable, but the other is not - the circuit will flip into the unstable state for a determined period, but will eventually return to the stable state. This circuit is also known as a one shot.
Bistable Multivibrators:
The bistable has two stable states. At switch on, one transistor is ON and the other is OFF. An external pulse makes the circuit change state, with the ON transistor now OFF, and the OFF transistor now ON. The circuit can be flipped from one state to the other by an external event or trigger. Such a circuit is important as the fundamental building block of a register or memory device. This circuit is also known as a flip-flop.