2-21 AMPLIFIER CLASSES OF OPERATION In the previous discussions, we assumed that for every portion of the input signal there was an output from the amplifier. This is not always the case with amplifiers. It may be desirable to have the transistor conducting for only a portion of the input signal. The portion of the input for which there is an output determines the class of operation of the amplifier. There are four classes of amplifier operations. They are class A, class AB, class B, and class C. Class A Amplifier OperationClass A amplifiers are biased so that variations in input signal polarities occur within the limitsof CUTOFF and SATURATION. In a PNP transistor, for example, if the base becomes positive with respect to the emitter, holes will be repelled at the PN junction and no current can flow in the collector circuit. This condition is known as cutoff. Saturationoccurs when the base becomes so negative with respect to the emitter that changes in the signal are not reflected in collector-current flow. Biasing an amplifier in this manner places the dc operating point between cutoff and saturation and allows collector current to flow during the complete cycle (360 degrees) of the input signal, thus providing an output which is a replica of the input. Figure 2-12 is an example of a class A amplifier. Although the output from this amplifier is 180 degrees out of phase with the input, the output current still flows for the complete duration of the input. The class A operated amplifier is used as an audio- and radio-frequency amplifier in radio, radar, and sound systems, just to mention a few examples. For a comparison of output signals for the different amplifier classes of operation, refer to figure 2-15 during the following discussion. Figure 2-15.—A comparison of output signals for the different amplifier classes of operation.
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