25.1: Theory Overview

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The maximum output signal, or compliance, of a class A amplifier is determined by its AC load line. The maximum peak level is determined by the smaller of \(V_{CEQ}\) and \(I_{CQ} \cdot r_{Load}\). If either of these levels is hit, the output signal will begin to clip causing greatly increased distortion. Knowing this voltage and the load resistance, the maximum load power may be determined. Dividing this power by the total supplied DC power will yield the efficiency. The maximum theoretical efficiency of an RC coupled class A amplifier is 25% although real-world circuits may be far less. In fact, the power dissipation of the transistor itself \((P_{DQ})\) may be greater than the maximum load power, clearly not a desirable condition. Note that the total supplied power is the product of the total supplied voltage and the average total current. In a class A amplifier that is not clipping, the average supplied current is equal to the quiescent DC current. In the case of a dual supply emitter biased circuit, this is simply the collector current and can be measured with a DC ammeter.