Pulse Generator using Op-Amp

Description: A monostable multivibrator (MMV) features one stable state and one quasi-stable state. The circuit remains in its stable state until an external triggering pulse prompts a transition to the quasi-stable state. After a designated time period T, the circuit returns to its stable state, producing a single output pulse in response to an input pulse, thus referred to as a one-shot or single shot. The monostable multivibrator circuit, as illustrated, is derived from modifying the astable multivibrator circuit by connecting a diode D1 across capacitor C to clamp the voltage across the capacitor (vc) at voltage (vd) during positive excursions. Under steady-state conditions, the circuit maintains its stable state with the output voltage (VOUT) at + VOUT or + Vz, while the capacitor C is clamped at voltage VD (the on-voltage of diode VD = 0.7 V). The voltage VD must be less than ² VOUT when vin < 0. Transitioning the circuit to the quasi-stable state can occur by applying a negative pulse with an amplitude greater than ² VOUT + VD to the non-inverting (+) input terminal. When a trigger pulse exceeding this threshold is applied, vin becomes positive, resulting in a state transition to -VOUT. The capacitor C then charges exponentially with a time constant equal to RfC towards + VOUT (with diode D1 being reverse-biased). Once the voltage across the capacitor vc becomes more negative than ² VOUT, vin turns negative, and the output returns to + VOUT (steady-state output). The capacitor subsequently charges towards + VOUT until vc reaches VD, at which point capacitor C is clamped at VD. The trigger pulse, capacitor voltage waveform, and output voltage waveform are illustrated in the accompanying figures. The width of the trigger pulse T must be significantly smaller than the duration of the output pulse generated, i.e., TP << T. For reliable operation, the circuit should not be retriggered before the time period T elapses.

The monostable multivibrator circuit is a crucial component in various timing applications and pulse generation. It can be implemented using discrete components such as resistors, capacitors, and operational amplifiers, or integrated circuits like the 555 timer in monostable mode. The design relies on the precise timing interval defined by the resistor Rf and capacitor C, which together determine the time period T that the circuit remains in the quasi-stable state.

In practical applications, the MMV can be employed in scenarios such as pulse-width modulation, timer circuits, and signal conditioning. The ability to generate a single output pulse in response to a triggering event makes it suitable for applications where precise timing and control are necessary.

The diode D1 plays a vital role in ensuring the quick return to the stable state by clamping the capacitor voltage, thereby preventing any unintended oscillations or noise from affecting the output. Proper selection of component values is essential to achieve the desired timing characteristics, and the circuit must be designed to accommodate the specific voltage levels and current ratings of the components used.

In summary, the monostable multivibrator is a versatile and reliable circuit that can be tailored to meet various electronic timing and pulse generation needs, ensuring accurate and predictable performance in a wide range of applications.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 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 circuit should not be triggered again before T.