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Bistable Multivibrator

The bistable multivibrator has two absolutely stable states. It will remain in whichever state it happens to be until a trigger pulse causes it to switch to the other state. For instance, suppose at any particular instant, transistor Q1 is conducting and transistor Q2 is at cut-off. If left to itself, the bistable multivibrator will stay in this position for ever. However, if an external pulse is applied to the circuit in such a way that Q1 is cut-off and Q2 is turned on, the circuit will stay in the new position. Another trigger pulse is then required to switch the circuit back to its original state.

In other words a multivibrator which has both the state stable is called a bistable multivibrator. It is also called flip-flop, trigger circuit or binary. The output pulse is obtained when, and why a driving (triggering) pulse is applied to the input. A full cycle of output is produced for every two triggering pulses of correct polarity and amplitude.

Figure (a) shows the circuit of a bistable multivibrator using two NPN transistors. Here the output of a transistor Q2 ​is coupled put of a transistor Q1 through a resistor R2. Similarly, the output of a transistor Q1 is coupled to the base of transistor Q2 through a resistor R1. The capacitors C2 and C1 are known as speed up capacitors. Their function is to increase the speed of the circuit in making abrupt transition from one stable state to another stable state. The base resistors (R3 and R4) of both the transistors are connected to a common source (-VBB). The output of a bistable multivibrator is available at the collector terminal of the both the transistor Q1 and Q2. However, the two outputs are the complements of each other.

Bistable Multivibrator

Let us suppose, if Q1 is conducting, then the fact that point A  is at nearly ON makes the base of Q2 negative (by the potential divider R2 - R4) and holds Q2 off.

Similarly with Q​2​ OFF, the potential divider from VCC to -VBB (RL2​, R​1​, R3) is designed to keep base of Q1 at about 0.7V ensuring that Q1 conducts. It is seen that Q1 holds Q2 OFF and Q​2​ hold Q1 ON.

Suppose, now a positive pulse is applied momentarily to R. It will cause Q2 to conduct. As collector of Q​2​ falls to zero, it cuts Q1 OFF and consequently, the BMV switches over to its other state.

Similarly, a positive trigger pulse applied to S will switch the BMV back to its original state.

Uses:

  1. In timing circuits as frequency divider
  2. In counting circuits
  3. In computer memory circuits

Bistable Multivibrator Triggering

To change the stable state of the binary it is necessary to apply an appropriate pulse in the circuit, which will try to bring both the transistors to active region and the resulting regenerative feedback will result on the change of state.

Triggering may be of two following types:

  1. Asymmetrical triggering 
  2. Symmetrical triggering

(I) Asymmetrical triggering

In asymmetrical triggering, there are two trigger inputs for the transistors Q1 and Q2. Each trigger input is derived from a separate triggering source. To induce transition among the stable states, let us say that initially the trigger is applied to the bistable. For the next transition, now the identical trigger must appear at the transistor Q2. Thus it can be said that the asymmetrical triggering the trigger pulses derived from two separate source and connected to the two transistors Q1 and Q2 individually, sequentially change the state of the bistable.

Figure (b) shows the circuit diagram of an asymmetrically triggered bistable multivibrator.

Asymmetrical triggered bistable multivibrator Circuit Diagram

Initially Q1 is OFF and transistor Q2 is ON. The first pulse derived from the trigger source A, applied to the terminal turn it OFF by bringing it from saturation region to active transistor Q1 is ON and transistor Q2 is OFF. Any further pulse next time then the trigger pulse is applied at the terminal B, the change of stable state will result with transistor Q​2​ On and transistor Q1 OFF.

Asymmetrical triggering finds its application in the generation of a gate waveform, the duration of which is controlled by any two independent events occurring at different time instants. Thus measurement of time interval is facilitated.

(II) Symmetrical Triggering

There are various symmetrical triggering methods called symmetrical collector triggering, symmetrical base triggering and symmetrical hybrid triggering. Here we would liked to explain only symmetrical base triggering (positive pulse) only as given under symmetrical Base Triggering.

Figure (c) shows the circuit diagram of a binary with symmetrical base triggering applying a positive trigger pulses.

Symmetrical base triggering employing positive triggering pulses

Diodes D1 and D2 are steering diodes. Here the positive pulses, try to turn ON and OFF transistor. Thus when transistor Q1 is OFF and transistor Q2 is ON, the respective base voltages and V​B1N, OFF and V​B2N, ON. It will be seen that V​B1N, OFF > V​B1N, ON. Thus diode D​2​ is more reverse-biased compared to diode D1.

When the positive differentiated pulse of amplitude greater than (V​B1N, OFF + Vɣ) appears, the diode D​1​ gets forward biased, and transistor Q1 enters the active region and with subsequent regenerative feedback Q1 gets ON, and transistor Q2 becomes OFF. On the arrival of the next trigger pulse now the diode D2 will be forward biased and ultimately with regenerative feedback it will be in the ON state.