Digital Electronics: Principles, Devices and Applications

Flip-Flops and Related Devices 361+VCCR c1–VRR C2CR1C1VoQ1Q2R2Figure 10.4Monostable multivibrator.in saturation. However, it conducts some current. The Q 1 collector voltage falls by I c1 R c1 and the Q 2base voltage falls by the same amount, as the voltage across a capacitor (C in this case) cannot changeinstantaneously. To sum up, the moment we applied the trigger, Q 2 went to cut-off and Q 1 startedconducting. But now there is a path for capacitor C to charge from V CC through R and the conductingtransistor. The polarity of voltage across C is such that the Q 2 base potential rises. The moment the Q 2base voltage exceeds the cut-in voltage, it turns Q 2 ON, which, owing to coupling through R 1 , turnsQ 1 OFF. And we are back to the original state, the stable state. Whenever we trigger the circuit intothe other state, it does not stay there permanently and returns back after a time period that dependsupon R and C. The greater the time constant RC, the longer is the time for which it stays in the otherstate, called the quasi-stable state.10.1.3.1 Retriggerable Monostable MultivibratorIn a conventional monostable multivibrator, once the output is triggered to the quasi-stable state byapplying a suitable trigger pulse, the circuit does not respond to subsequent trigger pulses as long asthe output is in quasi-stable state. After the output returns to its original state, it is ready to respondto the next trigger pulse. There is another class of monostable multivibrators, called retriggerablemonostable multivibrators. These respond to trigger pulses even when the output is in the quasi-stablestate. In this class of monostable multivibrators, if n trigger pulses with a time period of T t areapplied to the circuit, the output pulse width, that is, the time period of the quasi-stable state, equals(n − 1)T t + T, where T is the output pulse width for the single trigger pulse and T t