Drive and overdrive

Chapter Drive and overdrive

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

  • Problem 24-6An FET amplifier is 60 percent efficient. If the power output is 3.5 W, what is the dcdrain power input?“Plug in” values to the formula eff(%) 100 Pout/PD. This gives60100× 3.5/PD60350/PD60/3501/PDPD350/605.8 WEfficiency versus classClass-A amplifiers are the least efficient, in general. The efficiency ranges from 25 to 40percent, depending on the nature of the input signal and the type of bipolar or field-ef-fect transistor used.If the input signal is very weak, such as might be the case in a shortwave radio re-ceiver, the efficiency of a class-A circuit is near zero. But in that application, the circuitis not working as a power amplifier, and efficiency is not of primary importance.Class-AB amplifiers have better efficiency. A good class-AB1 amplifier might be 35to 45 percent efficient; a class-AB2 amplifier will be a little better, approaching 60 per-cent with the best designs.Class-B amplifiers are typically 50 to 60 percent efficient, although some radio fre-quency PA circuits work up to 65 percent or so.Class-C amplifiers are the best of all. This author has seen well-designed class-C cir-cuits that are 75 percent efficient.These are not absolute figures, and you shouldn’t memorize them as such. It’s suf-ficient to know ballpark ranges, and that efficiency improves as the operating pointmoves towards the left on the curves shown in Figs. 24-4 and 24-5.Drive and overdriveClass-A power amplifiers do not, in theory, take any power from the signal source in or-der to produce a significant amount of output power. This is one of the advantages ofclass-A operation. The same is true for class-AB1 amplifiers. It is only necessary that acertain voltage be present at the control electrode (the base, gate, emitter, or source).Class-AB2 amplifiers need some driving power to produce ac power output. Class-Bamplifiers require more drive than class-AB2, and class-C amplifiers need still more drive.Whatever kind of PA is used in a given situation, it is important that the driving signalnot be too strong. If overdrive takes place, there will be distortion in the output signal.An oscilloscope can be used to determine whether or not an amplifier is being over-driven. The scope is connected to the amplifier output terminals, and the waveshape ofthe output signal is examined. The output waveform for a particular class of amplifieralways has a characteristic shape. Overdrive is indicated by a form of distortion knownas flat topping.In Fig. 24-8A, the output signal waveshape for a properly operating class-B ampli-fier is shown. It looks like the output of a half-wave rectifier, because the bipolar tran-sistor or FET is drawing current for exactly half (180 degrees) of the cycle.Drive and overdrive445