Monostable Multivibrator

Monostable multivibrator is an regenerative assembly which has one stable and one quasi-stable state. In other words, this is relaxation oscillator which when is activated (triggered) it generates one pulse on its output, and it stays blocked until the next trigger pulse is applied on its trigger input. The monostable multivibrator circuit is shown on Picture 1.

In the stable state the transistor Q2 is conducting (in saturation), thus the transistor Q1 is blocked, since the voltage divider R2/R6 provides the base voltage of Q1 to be on lower potential than its emitter. The emitter voltage is determined with the emitter current of the conducting transistor Q2. The assembly can be lead to unstable (quasi-stable) state or state of generating one pulse on its output Vo with applying of positive triggering pulse via the capacitor C2. Since in the stable state, the capacitor C1 is charged approximately to the value of Vcc - Ve, where Ve is the emitter voltage, with activating of the transistor Q1, the negative voltage of C1 is blocking the transistor Q2. The collector voltage of the Q2 increases, thus supporting the conduction of the Q1 in the transitional process until it gets into saturation and without further presence of the triggering pulse. The duration of the quasi-stable state is determined with the moment when the conditions for conducting of Q2 are again created by discharging of the capacitor C1 via the R3 and the conducting Q1.

So, it can be concluded that the successively triggering of the assembly, while it's already in the quasi-stable state, will take no effect. The duration of the quasi-stable mode can be approximately determined by the time constant defined with R3C1, multiplied by some real factor.

Picture 1: Monostable Multivibrator

Time-domain analysis

The results of the transient analysis for this circuit in time domain is shown on Picture 2. On same picture are plotted both wave forms of the output Vo (green line plot) and the triggering voltage VT (blue line plot). As trigger voltage signal, here we use 3 rectangular pulses with amplitude of 5 V and time duration of about 50 ms, where the pause time between pulses or the duration of no trigger signal is about 150 ms. When the trigger signal is on high level (5 V), the output of the circuit is on its high level of about 11 V, while the output low level, during the period when trigger signal is equal to zero, is about 150 mV.

Picture 2: Transient analysis - output voltage Vo and trigger voltage VT wave forms (time-domain)