The class-AB amplifier

Chapter The class-AB amplifier

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

  • voltage) curve. This is shown in Fig. 24-5 for an N-channel device. For P-channel, re-verse the polarity signs.440 Amplifiers24-5Various classes ofamplifier operationfor an N-channelJFET.It is important with class-A amplifiers that the input signal not be too strong. Oth-erwise, during part of the cycle, the base or gate voltage will be driven outside of thestraight-line part of the curve. When this occurs, the output waveshape will not be afaithful reproduction of the input waveshape; the amplifier will be nonlinear. This willcause distortion of the signal. In an audio amplifier, the output might sound “raspy” or“scratchy.” In a radio-frequency amplifier, the output signal will contain a large amountof energy at harmonic frequencies, The problem of harmonics, however, can be dealtwith by means of resonant circuits in the output. These circuits attenuate harmonic en-ergy, and allow amplifiers to be biased near, at, or even past cutoff or pinchoff.The class-AB amplifierWhen a class-A amplifier is working properly, it has low distortion. But class-A opera-tion is inefficient. (Amplifier efficiency will be discussed later in this chapter.) This ismainly because the bipolar transistor or FET draws a large current, whether there is asignal input or not. Even with zero signal, the device is working hard.For weak-signal work, efficiency is not very important; it’s gain and sensitivity thatmatter. In power amplifiers, efficiency is a significant consideration, and gain and sensi-tivity are not so important. Any power not used toward generating a strong output sig-nal will end up as heat in the bipolar transistor or FET. If an amplifier is designed toproduce high power output, inefficiency translates to a lot of heat.