latest Pulse Generator using Op-Amp How to make a Monostable Multi Vibrator using 741 IC ?

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 v_c at v_d during positive excursion.

Under steady-state condition, this circuit will remain in its stable state with the output V_OUT = + V_OUT or + V_z and the capacitor C is clamped at the voltage V_D (on-voltage of diode V_D = 0.7 V). The voltage V_D must be less than β V_OUT for v_in <>OUT – V_D to the non-inverting (+) input terminal.

When a trigger pulse with amplitude greater than β V_OUT – V_D is applied, v_in goes positive causing a transition in the state of the circuit to -V_out. The capacitor C now charges exponentially with a time constant τ = R_fC toward — V_OUT (diode D_l being reverse-biased). When capacitor voltage v_c becomes more negative than – β V_OUT, v_in becomes negative and, therefore, output swings back to + V_OUT (steady- state output). The capacitor now charges towards + V_OUT till v_c attain V_D and capacitor C becomes clamped at V_D. 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. T_P « T. For reliable operation the circuit should not be triggered again before T.

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

v_c = – V_OUT + (V_OUT + V_D)e^-t/τ

At instant t = T, v_c = – β V_OUT

So - β V_OUT =- V_{OUT +} (V_{OUT +} v_D) _e-T/τ or

T = R_fC log_e (1 + V_D/V_OUT)/ 1- β

Usually V_D <<>OUT and if R2 = R3 so that if β = R3/(R2+R3) = ½ then,

T = R_fC log_e 2 = 0.693 R_f C

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