# How to make an Astable or Free running Multi vibrator using 741 Op-Amp

Posted on Feb 7, 2014

The non-sinusoidal waveform generators are also called relaxation oscillators. The op-amp relaxation oscillator shown in figure is a square wave generator. In general, square waves are relatively easy to produce. Like the UJT relaxation oscillator, the circuit`s frequency of oscillation is dependent on the charge and discharge of a capacitor C thr

ough feedback resistor R, . The heart  of the oscillator is an inverting op-amp comparator The compa ­rator uses positive feedback that increases the gain of the amplifier. In a comparator circuit this offer two advantages. First, the high gain causes the op-amp`s output to switch very quickly from one state to an ­other and vice-versa. Second, the use of positive feedback gives the circuit hysteresis. In the op-amp square-wave generator circuit given in figure, the output voltage vout is shunted to ground by two Zener diodes Z1 and Z2 connected back-to-back and is limited to either VZ 2 or VZ 1. A fraction of the output is fedback to the non-inverting (+) input terminal. Combination of IL and C acting as a low-pass R-C circuit is used to integrate the output voltage vout and the capacitor voltage vc is applied to the inverting input terminal in place of external signal. The differential input voltage is given as vin = vc - ² vout When vin is positive, vout = Vz1 and when vin is negative vout = + Vz 2. Consider an instant of time when vin < 0. At this instant vout = + Vz 2, and the voltage at the non-inverting (+) input terminal is ² Vz 2, the capacitor C charges exponentially towards Vz 2, with a time constant Rf C. The output voltage remains constant at Vz 2 until vc equal ² Vz 2. When it happens, comparator output reverses to Vz 1. Now vc changes exponentially towards-Vz1 with the same time constant and again the output...

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