The monostable multivibrator is also called as the one-shot multivibrator. The circuit produces a single pulse of specified duration in response to each external trigger signal. For such a circuit, only one stable state exists. When an external trigger is applied, the output changes its state. The new state is called as a quasi-stable state. The circuit remains in this state for a fixed interval of time. After some time it returns back to its original stable state. In fact, an internal trigger signal is generated which drives the circuit back to its original stable state. Usually, the charging and discharging of a capacitor provide this internal trigger signal.
- The diode connected across the capacitor is called clamping diode. It clamps the capacitor voltage to when the output is at .
- A negative triggering pulse is applied to the Non-inverting terminal of Op-amp through RC differentiator circuit and diode .
Operation of the Circuit
(i). To understand the operation of the circuit, let us assume that the output is at i.e. in its stable state.
(ii). The diode (Connected across Capacitor) conducts and the voltage across the capacitor gets clamped to Volts.
(iii). The voltage at the non-inverting terminal is controlled by voltage divider circuit of and
Voltage at non-inverting terminal
(iv). If , a negative trigger of amplitude is applied to the non-inverting terminal, so that the effective voltage at this terminal is less than then the output of the Op-amp changes its state from to .
(v). The diode is now reverse biased and the capacitor starts charging exponentially to through resistance .
(vi). The voltage at the non-inverting terminal is now . When the capacitor voltage becomes just slightly more negative than , the output of the Op-amp changes its state back to
(vii). The capacitor now starts charging towards through until reaches as capacitor gets clamped to the voltage.
Expression for pulse width
Initial voltage ( at t=0 ) across Capacitor =
Here Role of supply Voltage will play output voltage
Voltage across Capacitor at time is given by the eq
For Monostable operation, the trigger pulse width should be much less than .
The diode is not essential but it is used to avoid malfunctioning if any positive noise spikes are present in triggering line.
It can be seen from the waveform that the voltage does not reach its quiescent value until time . Hence it is necessary that a recovery time () be allowed to elapse before the next triggering signal is applied.