Pulse Generator using Op-Amp


Posted on Feb 7, 2014

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


Pulse Generator using Op-Amp
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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 ² VOUT for vin < 0. The circuit can be switched to the other state by applying a negative pulse with amplitude greater than ² VOUT VD to the non-inverting (+) input terminal. When a trigger pulse with amplitude greater than ² 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 = RfC toward ” VOUT (diode Dl being reverse-biased). When capacitor voltage vc becomes more negative than ² 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...




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