Monostable multivibrator

Posted on Feb 6, 2014

In a monostable multivibrator, one of the state is absolutely permanent i. e, stable and the other one is temporary i. e, quasi-stable. When an external trigger pulse is applied to the mono-stable at appropriate point, the mono-stable changes it state from stabe state to quasi-stable state. It stays in the quasi-stable state for a predetermined length of certain interval remains there

Monostable multivibrator
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until another pulse is applied. Thus a mono stable multivibrator can not generate square wave of its own like an astable multivibrator. Only external pulse will cause if to generate the square wave. In other words, a multi vibrator in which one transistor is always conducting (i. e. in the ON state) and the other is non conducting (i. e. in the OFF state) is called mono stable multivibrator. It is also called a single shot or single swing or a one shot multi vibrator. Other names are delay multi-vibrator and univibrator. Figure shows the circuit of a monostable multivibrator using NPN transistor. It consists of two similar transistor Q1 and Q2 with equal collector loads i. e. RL1 = RL2the values of -VBBand R3 are such as to reverse bias Q1 and keep it at cut off. The collector supply Vcc and R2 forward bias Q2 and keep it at saturation. A trigger pulse is given through C2 to obtain the square wave. Let us suppose that in the absense of a trigger pulse and with S closed, initially the circuit is in its stable state i. e. Q1 is OFF (at cut-off) and Q2 is ON (at saturation). If positive trigger pulse is off sufficient amplitude, it will override the reverse bias of the E/B junction of Q1 and give it a forward bias, Hence Q1 will start conducting. As Q1 conducts, its collector voltage falls due to voltage drop across RL1. It means that potential of pointA falls (negative going signal). This negative going voltage is fed to...

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