358
Digital Electronics
+V
V =V
o2
c2
Q 2
I c2
C
C
R 1
I c1
R c
V =V
o1
c1
–V
R 2
R 2
Q 1
R c
R 1
Figure 10.1 Bistable multivibrator.
induced by applying an appropriate trigger pulse. As we will see in the subsequent pages, the operation
of a bistable multivibrator is identical to that of a flip-flop. Figure 10.1 shows the basic bistable
multivibrator circuit. This is the fixed-bias type of bistable multivibrator. Other configurations are the
self-bias type and the emitter-coupled type. However, the operational principle of all types is the same.
The multivibrator circuit of Fig. 10.1 functions as follows.
In the circuit arrangement of Fig. 10.1 it can be proved that both transistors Q 1 and Q 2 cannot be
simultaneously ON or OFF. If Q 1 is ON, the regenerative feedback ensures that Q 2 is OFF, and when
Q 1 is OFF, the feedback drives transistor Q 2 to the ON state. In order to vindicate this statement, let us
assume that both Q 1 and Q 2 are conducting simultaneously. Owing to slight circuit imbalance, which
is always there, the collector current in one transistor will always be greater than that in the other. Let
us assume that I c2 > I c1 Lesser I c1 means a higher V c1 Since V c1 is coupled to the Q 2 base, a rise in
V c1 leads to an increase in the Q 2 base voltage. Increase in the Q 2 base voltage results in an increase
in I c2 and an associated reduction in V c2 Reduction in V c2 leads to a reduction in Q 1 base voltage and
an associated fall in I c1 , with the result thatV c1 increases further. Thus, a slight circuit imbalance has
initiated a regenerative action that culminates in transistor Q 1 going to cut-off and transistor Q 2 getting
driven to saturation. To sum up, whenever there is a tendency of one of the transistors to conduct more
than the other, it will end up with that transistor going to saturation and driving the other transistor
to cut-off. Now, if we take the output from the Q 1 collector, it will be LOW (= V CE1 sat.) if Q 1 was
initially in saturation. If we apply a negative-going trigger to the Q 1 base to cause a decrease in its
collector current, a regenerative action would set in that would drive Q 2 to saturation and Q 1 to cut-off.
As a result, the output goes to a HIGH (= +V CC state. The output will stay HIGH until we apply
another appropriate trigger to initiate a transition. Thus, both of the output states, when the output is
LOW and also when the output is HIGH, are stable and undergo a change only when a transition is
induced by means of an appropriate trigger pulse. That is why it is called a bistable multivibrator.
10.1.2 Schmitt Trigger
A Schmitt trigger circuit is a slight variation of the bistable multivibrator circuit of Fig. 10.1. Figure
10.2 shows the basic Schmitt trigger circuit. If we compare the bistable multivibrator circuit of Fig. 10.1
Digital Electronics
+V
V =V
o2
c2
Q 2
I c2
C
C
R 1
I c1
R c
V =V
o1
c1
–V
R 2
R 2
Q 1
R c
R 1
Figure 10.1 Bistable multivibrator.
induced by applying an appropriate trigger pulse. As we will see in the subsequent pages, the operation
of a bistable multivibrator is identical to that of a flip-flop. Figure 10.1 shows the basic bistable
multivibrator circuit. This is the fixed-bias type of bistable multivibrator. Other configurations are the
self-bias type and the emitter-coupled type. However, the operational principle of all types is the same.
The multivibrator circuit of Fig. 10.1 functions as follows.
In the circuit arrangement of Fig. 10.1 it can be proved that both transistors Q 1 and Q 2 cannot be
simultaneously ON or OFF. If Q 1 is ON, the regenerative feedback ensures that Q 2 is OFF, and when
Q 1 is OFF, the feedback drives transistor Q 2 to the ON state. In order to vindicate this statement, let us
assume that both Q 1 and Q 2 are conducting simultaneously. Owing to slight circuit imbalance, which
is always there, the collector current in one transistor will always be greater than that in the other. Let
us assume that I c2 > I c1 Lesser I c1 means a higher V c1 Since V c1 is coupled to the Q 2 base, a rise in
V c1 leads to an increase in the Q 2 base voltage. Increase in the Q 2 base voltage results in an increase
in I c2 and an associated reduction in V c2 Reduction in V c2 leads to a reduction in Q 1 base voltage and
an associated fall in I c1 , with the result thatV c1 increases further. Thus, a slight circuit imbalance has
initiated a regenerative action that culminates in transistor Q 1 going to cut-off and transistor Q 2 getting
driven to saturation. To sum up, whenever there is a tendency of one of the transistors to conduct more
than the other, it will end up with that transistor going to saturation and driving the other transistor
to cut-off. Now, if we take the output from the Q 1 collector, it will be LOW (= V CE1 sat.) if Q 1 was
initially in saturation. If we apply a negative-going trigger to the Q 1 base to cause a decrease in its
collector current, a regenerative action would set in that would drive Q 2 to saturation and Q 1 to cut-off.
As a result, the output goes to a HIGH (= +V CC state. The output will stay HIGH until we apply
another appropriate trigger to initiate a transition. Thus, both of the output states, when the output is
LOW and also when the output is HIGH, are stable and undergo a change only when a transition is
induced by means of an appropriate trigger pulse. That is why it is called a bistable multivibrator.
10.1.2 Schmitt Trigger
A Schmitt trigger circuit is a slight variation of the bistable multivibrator circuit of Fig. 10.1. Figure
10.2 shows the basic Schmitt trigger circuit. If we compare the bistable multivibrator circuit of Fig. 10.1