Posted by Tittu Thomas , 7/28/2012
ASTABLE MULTIVIBRATOR USING OP AMP
Astable multivibrator is capable of producing square wave for a given frequency, Amplitude and Duty cycle.
The output of OP AMP is forced to swing between positive saturation, +Vsat and negative saturation, -Vsat repeatedly.
Which will results in Square wave output. This circuit is also called as Free Running Oscillator. Because, it doesn't requires any trigger pulse to get active.
The output of an operational amplifier will be in positive saturation Vsat, if the differential input voltage is negative and vice versa.
The differential voltage, Vd=Vc-βVsat.
Where β is the feedback factor and βVsat is the potential at non-inverting terminal of the op amp.
Consider the instant at which Vc=+Vsat. Now the capacitor charges exponentially towards +Vsat through R. Automatically, Vd increases and crosses zero. This happens when Vc=+βVsat. At the moment Vd becomes positive due to further charging of the capacitor. Output changes to -Vsat. Now capacitor starts discharging to zero and recharges towards -Vsat. Now Vd decreases and crosses zero. This happens when Vc=-βVsat. At the moment Vd becames negative again, output changes to +Vsat. This completes one cycle of output.
Astable multivibrator is useful for generating audio frequency signals.
+Vcc and -Vee are Not shown in circuit diagram.
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