Output stages for feedback amplifiers are compared on the basis of the resulting closed-loop small-signal performance and intermodulation distortion. The variables in the discussion are the internal resistance of the output device and its configuration [common emitter (CE) or common collector (CC)]. Constants are the combined gain of all stages except the output stage, and the overall closed-loop gain. It is concluded that there is nothing to choose between CE and CC systems in regard to small-signal performance; output impedance, high frequency response, and stability of the feedback loop with a capacitive load are the same for each. It is also concluded that the effects of nonlinearity in the output device itself are the same in CE and CC systems, although CE systems have an advantage if there is nonlinearity in the other stages. In all cases high-internal-resistance output devices are preferred to low-internal-resistance devices.
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