Monostable Multivibrator

Objective
To study and perform the Monostable Multivibrator.

Introduction

A multivibrator is an electronic circuit used to implement a variety of simple two-state systems such as oscillators, timers and flip-flops. It is characterized by two amplifying devices (transistors, electron tubes or other devices) cross-coupled by resistors or capacitors. The name "multivibrator" was initially applied to the free-running oscillator version of the circuit because its output waveform was rich in harmonics. There are three types of multivibrator circuits depending on the circuit operation:


Astable Multivibrator : in which the circuit is not stable in either state, it continually switches from one state to the other. It functions as a relaxation oscillator.

Monostable Multivibrator : in which one of the states is stable, but the other state is unstable (transient). A trigger pulse 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 Multivibrator : in which the circuit is stable in either state. It can be flipped from one state to the other by an external trigger pulse. This circuit is also known as a flip flop. It can be used to store a bit of information.

Monostabel Triggering
To change the monostble multivibrator state from the stable to quasi-state the external trigger pulses are to be applied. In general the negative triggering has grater sensitivity, because here the negative pulse amplitude should be enough, so as to bring the operating point from saturation to active region. Secondly when the base emitter voltage of a junction changes from forward bias to reverse bias, its input impedance is continuously rising, which avoids the loading of the triggering source. It should be further noted that the monostable period is affected by this method.
The positive pulse triggering has sensitivity, because to turn of the transistor from the OFF state, it is necessary to feed the excess stored charge in the base such that the amplitude of triggering pulse is enough and is derived from a low impedance source, which can supply a peak demand current to turn on.