The class-B amplifier

Chapter The class-B amplifier

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

  • When a bipolar transistor is biased close to cutoff under no-signal conditions (Fig.24-4), or when an FET is near pinchoff (Fig. 24-5), the input signal will drive the deviceinto the nonlinear part of the operating curve. A small collector or drain current willflow when there is no input, but it will be less than the no-signal current that flows in aclass-A amplifier. This is called class-AB operation.With class-AB operation, the input signal might or might not cause the device to gointo cutoff or pinchoff for a small part of the cycle. Whether or not this happens de-pends on the actual bias point, and also on the strength of the input signal. You can vi-sualize this by imagining the dynamic operating point oscillating back and forth alongthe curve, in either direction from the static (no-signal) operating point.If the bipolar transistor or FET is never driven into cutoff/pinchoff during any part ofthe signal cycle, the amplifier is working in class-AB1. If the device goes into cutoff pin-choff for any part of the cycle (up to almost half), the amplifier is working in class-AB2.In a class-AB amplifier, the output waveshape is not identical with the input wave-shape. But if the wave is modulated, such as in a voice radio transmitter, the waveformof the modulations will come out undistorted. Thus class-AB operation is useful in ra-dio-frequency power amplifiers.The class-B amplifierWhen a bipolar transistor is biased exactly at cutoff, or an FET at pinchoff, underzero-input-signal conditions, an amplifier is working in class B. These operating pointsare labeled on the curves in Figs. 24-4 and 24-5.In class-B operation, there is no collector or drain current when there is no signal.This saves energy, because the circuit is not eating up any power unless there is a sig-nal going into it. (Class-A and class-AB amplifiers draw current even when the input iszero.) When there is an input signal, current flows in the device during exactly half ofthe cycle. The output waveshape is greatly different from the input waveshape in aclass-B amplifier; in fact, it is half-wave rectified.Sometimes two bipolar transistors or FETs are used in a class-B circuit, one for thepositive half of the cycle and the other for the negative half. In this way, distortion iseliminated. This is called a class-B push-pull amplifier. A class-B push-pull circuit us-ing two NPN bipolar transistors is illustrated in Fig. 24-6. This configuration is popularfor audio-frequency power amplification. It combines the efficiency of class B with thelow distortion of class A. Its main disadvantage is that it needs two center-tappedtransformers, one at the input and the other at the output. This translates into twothings that engineers don’t like: bulk and high cost. Nonetheless, the advantages oftenoutweigh these problems.The class-B scheme lends itself well to radio-frequency power amplification. Al-though the output waveshape is distorted, resulting in harmonic energy, this problemcan be overcome by a resonant LC circuit in the output. If the signal is modulated, themodulation waveform will not be distorted.You’ll sometimes hear of class-AB or class-B “linear amplifiers,” especially in hamradio. The term “linear” refers to the fact that the modulation waveform is not distortedby the amplifier. The carrier wave is, as you’ve seen, affected in a nonlinear fashion,because the amplifiers are not biased in the straight-line part of the operating curve.The class-B amplifier441