Monostable Multivibrator

The  monostable  multivibrator (called  a ONE-SHOT MULTIVIBRATOR) is a square- or rectangular-wave generator with only one stable condition. With no input signal one amplifier conducts and other is in cutoff. The monostable multivibrator is usually used for pulse stretching. It is used in the computer logic systems  and communication navigation equipment.  The operation of monostable multivibrator is simple relatively. The output changes from one voltage level to different voltage level. The output remains at this voltage level for a certain period of time. Then the circuit reverts automatically to its original condition and remains that way until another the trigger pulse is applied to input. 

Figure 3-10.-Monostable multivibrator block diagram.

The digram for the monostable multivibrator is shown in the figure 3-11. Like astable multivibrator, 1 transistor conducts and the other cuts off when the circuit is provided current.

Figure 3-11.-Monostable multivibrator schematic.

Recall that when astable multivibrator was energized 1st, it was not possible to predict which transistor would initially go to cutoff due to circuit symmetry. The one-shot circuit is not symmetrical like astable multivibrator. Positive voltage V_BB can be applied through R₅ to base of Q₁. This positive voltage makes Q₁ to cut off. Transistor Q₂ saturates because of the negative V voltage applied from CC to its base through R₂. The circuit is illustrated in its stable state.

Figure 3-12.-Monostable multivibrator (stable state).

Let's take detailed look at the circuit conditions in this stable state (figure 3-12). As stated above, Q₁ is cut off, so no current flows through R₁, and collector of Q₁  V is at CC. Q₂ is saturated and has no voltage drop across it practically, so its collector is essentially at 0 volts. R₅ and R₃ form a voltage divider from VBB to ground potential at the collector of Q₂. The tie point between these 2 resistors will be positive. Therefore, the base of Q₁ is held positive, ensuring that Q₁ remains cutoff. Q₂ will remain saturated as the base of Q₂ is slightly negative as a result of the  V_cc voltage drop across R₂. If collector of Q₁ is near CC and base of Q₂ is near ground, C₁ should be charged to nearly V_CC volts with polarity shown. Now that all components and voltages have been described for the stable state, let us see how the circuit operates (see figure 3-13).

Figure 3-13.-Monostable multivibrator (triggered).

The one-shot multivibrator has now been turned on by applying a pulse at the input. It will turn itself off after some time. To see how it does this, look at the figure 3-13 again. Q₁ is held in saturation by negative voltage applied through R₃ to its base, so the circuit can't be turned off here. Notice that base of Q₂ is connected to C₁. The positive charge on C₁ keeps Q₂ cutoff. Remember that a positive voltage change was coupled from the collector of Q₁ when it began conducting to the base of Q₂, placing Q₂ in cutoff. 

Figure 3-14.-Waveforms of a monostable multivibrator (triggered).

The part of the operation not described till now is the short C₁ charge time which occurs right after Q₁ and Q₂ return to their stable states. This is the time required for C₁ to gain electrons on its left side. This charge time can be determined by the R₁C₁ time constant.

Another version of monostable multivibrator is shown in figure 3-15. See (A) is the circuit and see (B) shows related waveforms. In the stable condition (T0), Q₁ is cut off and Q₂ is conducting. The input trigger (positive pulse at T₁) is applied to the collector of Q₁ and coupled by C₁ to the base of Q₂ making Q₂ to be cut off.  V The collector voltage of Q₂ then goes CC. The negative voltage at the collector of Q₂ forward biases Q₁ through R₄. With the forward bias, Q₁ conducts, and the collector voltage of Q₁ goes to nearly 0 volts. C₁ discharges now and makes Q₂ cut off. Q₂ remains cut off until C₁ discharges sufficiently to allow Q₂ to conduct again (T₂). When Q₂ conducts again, the collector voltage of it goes toward 0 volts and Q₁ is cut off. The circuit returns to the quiescent state of it and has completed a cycle. The circuit remains in the stable state till the next trigger arrives (T₃).

Figure 3-15A.-Monostable miltivibrator and waveshapes. Schematic.

3-14 Figure 3-15B.-Monostable miltivibrator and waveshapes. Waveshapes

Note that R₃ is variable to permit adjustment of the gate width. Increasing R₃ increases the time of discharge for C₁ which increases cutoff time for Q₂. Increasing the value of R₃ broadens the gate.  To decrease the width of gate, reduce the  value  of  R₃.  Figure  3-16  shows  relationships in between the output signal and the trigger. View (A) of the figure shows the input trigger; views (B) and (C) show the different gate widths made available by R₃. Although the durations of gates are different, the duration of complete cycle remains the same as the pulse repetition time of the triggers. See (D) of the figure illustrates that the trailing edge of the positive alternation is variable in nature.

Figure 3-16.-Monostable multivibrator waveforms with a variable gate.

The reason monostable multivibrator is also known as one-shot multivibrator can easily be seen. For every trigger pulse applied to multivibrator, a complete cycle, or a positive and negative  alternation  of  output,  is  completed.  

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