Types of Multivibrators

The Multivibrator circuits are classified in 3 general categories.

1-       Astable multivibrators.

 Astable multivibrators circuits contain no stable state but still keep changing from one state to the other, and so the named as multivibrator. There useful application is as clocks or oscillators. These are also used as a 555 as an astable multivibrator.

2-        Bistable Multovibrators.

These circuits can be easily induced to move from 1 state to another, and can remain in either state permanently after the input signals have been removed. They remain thus stable in both of their allowed states. The more commonly used term for bi-stable multi-vibrator is flip-flop.

3-        Mono-stable Multi-vibrators.

These circuits exist only in a single stable state. They can only be forced to move out of their stable state by a trigger pulse, but they return to their original state after a very limited period of time. The main use for mono stables is to produce pulses of known duration from triggering pulses of shorter, longer, or of variable duration. Mono-stables are widely applied to generate ‘gate’ signals for counter circuits. They are generally considered to be hybrid analog/digital chips in that the digital output is normally determined by means of the RC time constant of an external analog circuit linked to the chip.

Another term used for mono-stable multi-vibrator is 1-shot, since it is a device that ‘shoots’ only once (that is issues an output pulse) each time it receives an input signal. Among mono-stables, the 555 timer is the top choice for pulse widths ranging from milliseconds to hours, & for applications in which the pulse width have to be stable to better than 0.05%.

In case of digital designs, mono-stables such as the model 74121 are favored for pulse widths ranging from about 40 ns to 10 ms, & will operate up to tens of seconds, but we can not predict their pulse widths as predictable or stable.