Square Wave Generator using Op-Amp

jojo July 9, 2012 13 Comments

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

op amp astable multi vibrator

op amp astable multi vibrator

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 through feedback resistor R,. The “heart” of the oscillator is an inverting op-amp comparator

The compa­rator 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 an­other 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 – β 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  β 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 β 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  makes  a  transition  from -Vz1 to + Vz 2. when vc equals -βVz 1

Let    Vz1 = Vz 2

The time period, T, of the output square wave is determined using the charging and discharging phenomena of the capacitor C. The voltage across the capacitor, vc when it is charging from – B Vz to + Vz is given by

Vc = [1-(1+β)]e-T/2τ

Where τ = RfC

The waveforms of the capacitor voltage vc and output voltage vout (or vz) are shown in figure.

When t = t/2

Vc = +β Vz or + β Vout

Therefore β Vz = Vz [1-(1+β)e-T/2τ]

Or e-T/2τ = 1- β/1+ β

Or T = 2τ loge 1+β/1- β = 2Rf C loge [1+ (2R3/R2)]

Square Wave Generator Waveforms using 741IC

The frequency, f = 1/T , of the square-wave is independent of output voltage Vout. This circuit is also known as free-running or astable multivibrator because it has two quasi-stable states. The output remains in one state for time T1 and then makes an abrupt transition to the second state and re­mains in that state for time T2. The cycle repeats itself after time T = (T1 + T2) where T is the time period of the square-wave.

The op-amp square-wave generator is useful in the frequency range of about 10 Hz -10 kHz. At higher frequencies, the op-amp’s slew rate limits the slope of the output square wave. The symmetry of the output waveform depends on the matching of two Zener diodes Z1 and Z2. The unsymmetrical square-wave (T1 not equal to t2) can be had by using different constants for charging the capacitor C to +Vout and -Vout

  • karl knudson
    September 13, 2014

    im needing a square wave generator that will produce a square wave of 1,604,000hz, im on a tight budget ,can you assist?

  • Nirmalaa.B
    March 10, 2014

    Hi its very useful for me but i need more information than this…

  • Indraja
    February 16, 2014

    how to design the same for duty cycle of 25%..?

  • January 3, 2013

    what are the values of capacitors and rasistances.

    July 4, 2012

    Old Chips like the 741 never die they only get better!!! Nice Design,,,MR.OHM likes it!!!

  • November 28, 2011

    What is the possible solution to generate a squarewave of frequency 1Hz with output oscillating between +3v and -3v with refrence to ground (i need it to feed a 200mA moter)

  • AMAR
    October 1, 2011

    Pls Send me The Components list for this ckt in detail.

  • seetharaman
    September 12, 2010

    Advantage very simple with very few component count.
    disadvantage limitted to audio frequency range only, you have to use additional buffer stage to avoid loading of the oscillator.

  • kiki
    September 12, 2010


  • Ashwini
    September 10, 2010

    plz send me advantages.disadvantages,application

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