Pulse Generator using Op-Amp

jojo February 25, 2010 No Comments

How to make a Monostable Multi Vibrator using 741 IC ?

op-amp monostable multivibrator

op-amp monostable multivibrator

As already explained , a monostable multivibrator (MMV) has one stable state and one quasi-stable state. The circuit remains in its stable state till an external triggering pulse causes a transition to the quasi-stable state. The circuit comes back to its stable state after a time period T. Thus it generates a single output pulse in response to an input pulse and is referred to as a one-shot or single shot.

Monostable multivibrator circuit illustrated in figure  is obtained by modifying the astable multivibrator circuit  by connecting a diode D1 across capacitor C so as to clamp vc at vd during positive excursion.

Under steady-state condition, this circuit will remain in its stable state with the output VOUT = + VOUT or + Vz and the capacitor C is clamped at the voltage VD (on-voltage of diode VD = 0.7 V). The voltage VD must be less than β VOUT for vin < 0. The circuit can be switched to the other state by applying a negative pulse with amplitude greater than β VOUT – VD to the non-inverting (+) input terminal.

When a trigger pulse with amplitude greater than β VOUT – VD is applied, vin goes positive causing a transition in the state of the circuit to -Vout. The capacitor C now charges exponentially with a time constant τ = RfC toward — VOUT (diode Dl being reverse-biased). When capacitor voltage vc becomes more negative than – β VOUT, vin becomes negative and, therefore, output swings back to + VOUT (steady- state output). The capacitor now charges towards + VOUT till vc attain VD and capacitor C becomes clamped at VD. The trigger pulse, capacitor voltage waveform and output voltage waveform are shown in figures  respectively.

The width of the trigger pulse T must be much smaller than the duration of the output pulse generated i.e. TP « T. For reliable operation the circuit should not be triggered again before T.

During the quasi-stable state, the capacitor voltage is given as

vc = – VOUT + (VOUT + VD)e-t/τ

At instant t = T,                       vc = – β VOUT

So – β VOUT =- VOUT + (VOUT + vD) e-T/τ or

T = RfC loge (1 + VD/VOUT)/ 1- β

Usually VD << VOUT and if R2 = R3 so that if β = R3/(R2+R3) = ½ then,

T = RfC loge 2 = 0.693 Rf C

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