Bipolar digital ICs

Chapter Bipolar digital ICs

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • The analog multiplexerThe analog multiplexer IC allows several different signals to be combined in a singlechannel via time-division multiplexing, in a manner similar to that used with pulse mod-ulation. (This was discussed in chapter 27.) An analog multiplexer can also be used inreverse; then it works as a demultiplexer. Thus, you’ll sometimes hear engineers talkabout multiplexer/demultiplexer ICs.The comparatorLike an op amp, a comparator IC has two inputs. The comparator does just what itsname implies: it compares the voltages at the two inputs (called A and B). If the inputat A is significantly greater than the input at B, the output will be about + 5 V. This islogic 1, or high. If the input at A is not greater than the input at B, the output voltagewill be about + 2 V. This is designated as logic 0, or low.Voltage comparators are available for a variety of applications. Some can switch be-tween low and high states at a rapid rate of speed; others are slower. Some have low in-put impedance, and others have high impedance. Some are intended for audio orlow-frequency use; others are fabricated for video or high-frequency applications.Voltage comparators are used to actuate, or trigger, other devices such as relaysand electronic switching circuits.Bipolar digital ICsDigital ICs consist of gates that perform logical operations at high speeds. There areseveral different technologies, each with its own unique characteristics. Digital-logictechnology might use bipolar transistors or metal-oxide-semiconductor (MOS) devices.TTLIn transistor-transistor logic (TTL), arrays of bipolar transistors, some with multipleemitters, operate on dc pulses. This technology has several variants, some of which dateback to around 1970. The hallmark of TTL is immunity to noise pulses. A simple TTLgate is illustrated in Fig. 28-6. The transistors are either completely cut off, or else com-pletely saturated. This is the reason why TTL is not very much affected by externalnoise “distractions.”ECLAnother bipolar-transistor logic form is known as emitter-coupled logic (ECL). In ECL,the transistors are not operated at saturation, as they are with TTL. This increases thespeed of operation of ECL compared with TTL. But noise pulses have a greater effect inECL, because unsaturated transistors amplify as well as switch signals. The schematicof Fig. 28-7 shows a simple ECL gate.MOS digital ICsSeveral variants of MOS technology have been developed for use in digital devices. Theyall offer superior miniaturization and reduced power requirements as compared withbipolar digital ICs.MOS digital ICs527