9 Digital Circuits
9.1 Transistor Switches
9.2 DTL and TTL Logic Circuits9.3 CMOS and Other Logic Families9.4 Learning Objectives9.5 Practical Application: A Case Study—Cardiac Pacemaker, A Biomedical Engineering
ApplicationProblemsThe use of nonlinear devices (BJT and FET) in constructing linear amplifiers has been studied in
Chapter 8. Although these devices are inherently nonlinear, their operation was confined to the
linear portions of their characteristics in order to produce linear amplification of a signal. When
these devices are operated in the nonlinear regions of their characteristics, the primary use lies in
electronic switches for use in computers and other digital systems. Digital electronic circuits are
becoming of increasing importance. Revolutionary advances in integrated-circuit (IC) technology
have made it possible to place a large number of switches (gates) on a chip of very small size.
Besides the advantage of utilizing less space, another significant advantage of digital systems
over analog systems is their inherent immunity to noise and interference.
Digital building blocks (made up of transistor circuits) and computer systems have been
presented in Chapter 6. Digital circuits are almost always purchased as ready-made IC building
blocks. However, users of digital hardware must decide which technology is best suited for each
case because competing technologies known aslogic familiesare available. Digital circuit design
is of course of great interest to engineers in the IC industry for designing and building digital
blocks. The users of digital technology will certainly benefit from knowing what is inside the
blocks they use.
A fundamental circuit of digital logic is the transistor switch, or inverter, which is considered
in Section 9.1. Basic logic circuits are developed in Section 9.2 using bipolar technology.
Section 9.3 deals with other logic families, particularly the FET-based family known as CMOS
(complementary metal-oxide semiconductor) technology.422