Monostable Multivibrator using Op-amp

Monostable Multivibrator using Op-amp

The monostable multivibrator circuit using op-amp is shown in the Fig. 3.14.1.

The diode D₁ is clamping diode connected across C. The diode clamps the capacitor voltage to 0.7 V when the output is at +V_sat. A narrow negative triggering pulse V_t is applied to the noninverting input terminal, through diode D₂.

Let us see the operation of the circuit.

To understand the operation of the circuit, let us assume that the output Vo is at +V_sat i.e. in its stable state. The diode D₁ conducts and the voltage across the capacitor C i.e. V_C gets clamped to 0.7 V. The voltage at the non-inverting input terminal is controlled by potentiometric divider of R₁R₂ to β V_o i.e. + 3 V_sat in the stable state.

Now if V_t, a negative trigger of amplitude V_t is applied to the non-inverting terminal, so that the effective voltage at this terminal is less than 0.7 V (+ β V_sat + (- V_t)) then the output of the op-amp changes its state from + V_sat to - V_sat.

The diode is now reverse biased and the capacitor starts charging exponentially to - Vsat through the resistance R. The time constant of this charging is τ = R_C.

The voltage at the non-inverting input terminal is now - β V_sat. When the capacitor voltage V_C becomes just slightly more negative than - β V_sat, the output of the op-amp changes its state back to + V_sat.

The capacitor now starts charging towards + V_sat through R until V_C reaches 0.7 V as capacitor gets clamped to the voltage.

The waveforms are shown in the Fig. 3.14.2.

1. Expression for Pulse Width T

For a low pass RC circuit let,

V_i = Initial value of the voltage

V_f = Final value of the voltage

Then the general solution is given by,

V_o = V_f + (V_i - V_f) e^-t/RC  ...(3.14.1)\

Now for the monostable multivibrator discussed above, the values of V_f  and V_i  are,

V_f = - V_sat and V_i = V_D1 (diode forward voltage)

while V_o = Output = Capacitor voltage = V_C

V_C -V_sat + (V_D1-[-V_sat])e-^t/RC   …. (3.14.2)

at  t = T, V_C = -β  V_sat …. (3.14.3)

This is obtained by absorbing negative sign inside the natural logarithm. The potential divider decides the value of 3 given by,

β = R₂ / R₁ + R₂  … (3.14.6)

If V_sat >> V_D1 and R₁ = R₂ so that β = 0.5, then

T = 0.69 RC  …. (3.14.7)

For monostable operation, the trigger pulse width T_p should be much less than T, the pulse width of the monostable multivibrator.

The diode D₂ is not essential but it is used to avoid malfunctioning if any positive noise spikes are present in the triggering line. It can be seen from the waveform that the voltage V_C does not reach its quiescent value V_D1 until time T' > T. Hence it is necessary that a recovery time T' - T be allowed to elapse before the next triggering signal is applied.

Review Question

1. Draw the circuit of monostable multivibrator and obtain expression for pulse width.

Dec.-07, 09, Marks 8