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 b = [(R2/R1+R2)]. The positive feedback comparator circuit increases the gain of the op-amp, which helps to switch very fast between the two states of a multivibrator. The positive feedback also provides  hysteresis to the circuit. A capacitor 'C' is connected to the inverting terminal of the Op-Amp with its other end grounded. The capacitor along with the resistor 'R' acts as a RC integrator. The vol;tage across the capacitor is given by Vc.

The below figure demonstrates the working of an Astable Multivibrator using IC 741 Operational amplifier, consisting of a comparator circuit having a positive feedback with a feedback factor.

Now, the op-amp has a voltage of 'Vc' at the inverting terminal and [R2/(R1+R2)]Vo at the non-inverting terminal, giving rise to a voltage equal to +Vsat or -Vsat at the output.

Working
Assume that the output voltage Vo = +Vsat. Then at the non-inverting terminal of the op-amp we will get a voltage of bVo.

ie, Voltage at non-inverting termnal = bVsat = [R2/(R1+R2)]Vsat

On the other hand at the inverting terminal, the capacitor tries to charge to +Vsat, which is available to the capaciror through 'R'. but on the way to +Vsat, when the capacitor reaches at a potential equal to the potential at the non-inverting terminal of the op-amp.

ie, when Vc = bVsat.

Since the op-amp behaves as a comparator circuit, the output changes from +Vsat to -Vsat, because the inverting potential is greater than non-inverting potential.


Now, the output is -Vsat, the feedback voltage reverses the polarity to -bVsat, which is available at the non-inverting terminal of the op-amp. The capacitor holds a voltage of +bVsat initially at the begining of the second stage, since the output voltage Vo = -Vsat which can flow to the capacitor. Due to this potential the capacitor tries to discharge to a maximum of -Vsat, but on the way to -Vsat, when the capacitor reaches a potential equal to the potential of the non-inverting terminal of the op-amp.

ie, when Vc = -bVsat.

The comparator circuit changes the output voltage from -Vsat to +Vsat. Which will repeat in cycles.

 
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