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Astable multivibrator

Astable Multivibrator
Astable multivibrator with transistors

Certain areas in electronics use timing circuits to start, stop, or synchronize various circuits in a system. One method of producing these timing pulses is with multivibrators. A multivibrator is basically a two-stage RC-coupled amplifier with regenerative feedback.

A multivibrator is a form of relaxation oscillator. A relaxation oscillator is one that makes use of the transient represented by the charge and discharge of a capacitance or inductance through a resistance.

Circuit operation
Assume Q1 conducts first. The voltage on the collector becomes negative (about zero). This is felt on the base of Q2 and drives it beyond cutoff. The collector of Q2 becomes positive in regard Vcc and is felt on the base of Q2, which causes Q1 to go to saturation. As Q2's collector becomes positive, C1 charges through the emitter to the base resistance of Q1 and R2.

C2 discharges through R4 and the forward-conducting resistance of Q1, which keeps Q2 cut off. As C2 discharges, the base voltage of Q2 rises toward cutoff. Q2 starts conducting, its collector becomes negative and drives Q1 into cutoff. As Q1 goes into cutoff, its collector becomes positive. The change is coupled to the base of Q2, driving it into maximum conduction. As the collector of Q1 becomes positive, it causes C2 to charge through R1 and the emitter-base resistance of Q2.

C1 discharges through R3 and the conducting resistance of Q2. The pulse width of the output waveform depends upon the RC time in the base circuit and how long each transistor is cut off.

If R1 equals R2, R3 equals R4, C1 equals C2, and Q1, Q2 are matched, then the output waveform will be a symmetrical waveform.