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24-5 Various classes of amplifier operation for an N-channel JFET.
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The class-B amplifier 441 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 device into the nonlinear part of the operating curve. A small collector or drain current will flow when there is no input, but it will be less than the no-signal current that flows in a class-A amplifier. This is called class-AB operation. With class-AB operation, the input signal might or might not cause the device to go into cutoff or pinchoff for a small part of the cycle. Whether or not this happens depends on the actual bias point, and also on the strength of the input signal. You can visualize this by imagining the dynamic operating point oscillating back and forth along the 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 of the signal cycle, the amplifier is working in class-AB1. If the device goes into cutoff pinchoff 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 waveshape. But if the wave is modulated, such as in a voice radio transmitter, the waveform of the modulations will come out undistorted. Thus class-AB operation is useful in radio-frequency power amplifiers.
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The class-B amplifier
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When a bipolar transistor is biased exactly at cutoff, or an FET at pinchoff, under zero-input-signal conditions, an amplifier is working in class B. These operating points are 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 signal going into it. (Class-A and class-AB amplifiers draw current even when the input is zero.) When there is an input signal, current flows in the device during exactly half of the cycle. The output waveshape is greatly different from the input waveshape in a class-B amplifier; in fact, it is half-wave rectified. Sometimes two bipolar transistors or FETs are used in a class-B circuit, one for the positive half of the cycle and the other for the negative half. In this way, distortion is eliminated. This is called a class-B push-pull amplifier. A class-B push-pull circuit using two NPN bipolar transistors is illustrated in Fig. 24-6. This configuration is popular for audio-frequency power amplification. It combines the efficiency of class B with the low distortion of class A. Its main disadvantage is that it needs two center-tapped transformers, one at the input and the other at the output. This translates into two things that engineers don t like: bulk and high cost. Nonetheless, the advantages often outweigh these problems. The class-B scheme lends itself well to radio-frequency power amplification. Although the output waveshape is distorted, resulting in harmonic energy, this problem can be overcome by a resonant LC circuit in the output. If the signal is modulated, the modulation waveform will not be distorted. You ll sometimes hear of class-AB or class-B linear amplifiers, especially in ham radio. The term linear refers to the fact that the modulation waveform is not distorted by 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.
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24-6 A class-B push-pull amplifier.
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Class-AB2 and class-B amplifiers take some power from the input signal source. Engineers say that such amplifiers require a certain amount of drive or driving power to function. Class-A and class-AB1 amplifiers theoretically need no driving power, although there must be an input voltage.
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