Editing Multivibrator (section)

=== Monostable using op-amp ===
[[File:Monostable.jpg|thumb|monostable multivibrator using op-amp]]
The circuit is useful for generating single output pulse of adjustable time duration in response to a triggering signal. The width of the output pulse
depends only on external components connected to the op-amp. A diode D1 clamps the capacitor voltage to 0.7 V when the output is at +Vsat. Let us assume that in the stable
state the output Vo = +Vsat. The diode D1 clamps the capacitor to 0.7 V. The voltage at the non-inverting terminal through the potential divider will be + βVsat. Now a negative trigger of magnitude V1 is applied to the non-inverting terminal so that the effective signal at this terminal is less than 0.7 V. Then the output voltage switches from +Vsat to -Vsat. The diode will now get reverse biased and the capacitor starts charging exponentially to -Vsat through R. The voltage at the non-inverting terminal through the potential divider will be - βVsat. After some time the capacitor charges to a voltage more than - βVsat. The voltage on the non-inverting input is now greater than on the inverting input and the output of the op-amp switches again to +Vsat. The capacitor discharges through resistor R and charges again to 0.7 V.

The pulse width T of a monostable multivibrator is calculated as follows:
The general solution for a  low pass RC circuit is
:<math>V_o = V_f +(V_i - V_f)e^{-t/RC}</math>
where <math>V_f = -V_\text{sat}</math> and <math>V_i = V_d</math>, the diode forward voltage.  Therefore,
:<math>V_c = -V_\text{sat} +(V_d + V_\text{sat})e^{-t/RC}</math>
at <math>t = T</math>,
:<math>V_c = -\beta V_\text{sat}</math>
:<math>-\beta V_\text{sat} = -V_\text{sat} +(V_d + V_\text{sat})e^{-T/RC}</math>
after simplification,
:<math>T = RC\ln\left({1+V_d/V_\text{sat} \over 1 - \beta}\right)</math>
where <math>\beta = {R2 \over R1+R2}</math>
           
If <math>V_\text{sat} >> V_d</math> and <math>R1 = R2</math> so that <math>\beta=0.5</math>, then
<math>T = 0.69RC</math>