Chapter Amplifiers

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

  • 24CHAPTERAmplifiersIN THE PRECEDING TWO CHAPTERS, YOU SAW SCHEMATIC DIAGRAMS WITH BIPOLARand field-effect transistors. The main intent was to acquaint you with biasing schemes.Some of the diagrams were of basic amplifier circuits. This chapter examines amplifiersmore closely, but the subject is vast. For a thorough treatment, you should consult abook devoted to amplifiers and amplification.The decibelThe extent to which a circuit amplifies is called the amplification factor. This can begiven as a simple number, such as 100, meaning that the output signal is 100 times asstrong as the input. More often, amplification factor is specified in units called decibels,abbreviated dB.It’s important to keep in mind what is being amplified: current, voltage, or power. Cir-cuits are designed to amplify one of these aspects of a signal, but not necessarily the oth-ers. In a given circuit, the amplification factor is not the same for all three parameters.Perception is logarithmicYou don’t perceive loudness directly. Instead, you sense it in a nonlinear way. Physicistsand engineers have devised the decibel system, in which amplitude changes are ex-pressed according to the logarithm of the actual value (Fig. 24-1), to define relativesignal strength.Gain is assigned positive decibel values; loss is assigned negative values. Therefore,if signal A is at 6 dB relative to signal B, then A is stronger than B; if signal A is at 14 dB relative to B, then A is weaker than B.An amplitude change of plus or minus 1 dB is about equal to the smallest change alistener can detect if the change is expected. If the change is not expected, then thesmallest difference a listener can notice is about plus or minus 3 dB.433Copyright © 2002, 1997, 1993 by The McGraw-Hill Companies. Click here for terms of use.