# Flip-flops and related devies

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10
Flip-Flops and Related Devices

Having discussed combinational logic circuits at length in previous chapters, the focus in the present
chapter and in Chapter 11 will be on sequential logic circuits. While a logic gate is the most basic
building block of combinational logic, its counterpart in sequential logic is theflip-flop. The chapter
begins with a brief introduction to different types of multivibrator, including the bistable multivibrator,
which is the complete technical name for a flip-flop, the monostable multivibrator and the astable
multivibrator. The flip-flop is not only used individually for a variety of applications; it also forms
the basis of many more complex logic functions. Counters andregisters, to be covered in Chapter 11
are typical examples. There is a large variety of flip-flops having varying functional tables, input
clocking requirements and other features. In this chapter, we will discuss all these basic types of
flip-flop in terms of their functional aspects, truth tables, salient features and application aspects. The
text is suitably illustrated with a large numberof solved examples. Application-relevant information,
including a comprehensive index of flip-flops and related devices belonging to different logic families,
is given towards the end of the chapter. Pin connection diagrams and functional tables are given in the
companion website.

10.1 Multivibrator
Multivibrators, like the familiar sinusoidal oscillators, are circuits with regenerativefeedback, with the
difference that they produce pulsed output. There are three basic types of multivibrator, namely the
bistable multivibrator, the monostable multivibrator and the astable multivibrator.

10.1.1 Bistable Multivibrator
A bistable multivibrator circuit is one in which both LOW and HIGH output states are stable.
Irrespective of the logic status of the output, LOW or HIGH, itstays in that state unless a change is

Digital Electronics: Principles, Devices and Applications Anil K. Maini
© 2007 John Wiley & Sons, Ltd. ISBN: 978-0-470-03214-5

Digital Electronics

358

+V
Ic1

C

Ic2

C

Rc

Rc
Vo1=Vc1

Vo2=Vc2
R1

R1

Q1

Q2
R2

R2

–V
Figure 10.1 Bistable multivibrator.

induced by applying an appropriate trigger pulse. As we will seein 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. 10.1functions as follows.
In the circuit arrangement of Fig. 10.1 it can be proved that both transistors Q1 and Q2 cannot be
simultaneously ON or OFF. If Q1 is ON, the regenerative feedback ensures that Q2 is OFF, and when
Q1 is OFF, the feedback drives transistor Q2 to the ON state. In order to vindicate this statement, let us
assume that both Q1 and Q2 are conducting simultaneously. Owing to slightcircuit imbalance, which
is always there, the collector current in one transistor will always be greater than that in the other. Let
us assume that Ic2 > Ic1 Lesser Ic1 means a higher Vc1 Since Vc1 is coupled to the Q2 base, a rise in
Vc1 leads to an increase in the Q2 base voltage. Increase in the Q2 base voltage results in an increase
in Ic2 and an associated reduction in Vc2 Reduction inVc2 leads to a reduction in Q1 base voltage and
an associated fall in Ic1 , with the result thatVc1 increases further. Thus, a slight circuit imbalance has
initiated a regenerative action that culminates in transistor Q1 going to cut-off and transistor Q2 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...