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Distortion Correction in Audio Power Amplifiers
An audio power amplifier design technique is presented that has the property of minimising the non-linear distortion that is generated in Class A and Class AB output stages. A modified feedback technique has been identified that is particularly suited to the design of near unity gain stages. The technique can linearise the transfer characteristic and minimise the output resistance of the output stage. Consequently it is possible to design a power amplifier that uses fairly modest overall negative feedback yet attains minimal crossover distortion together with an adequate damping factor. The paper presents initially a generalised feedforward/feedback structure from which a system model is derived that can compensate for both non-linear voltage and non-linear current transfer characteristics. From this theoretical model, several circuit examples are presented which illustrate that only circuits of modest complexity are needed to implement the distortion correction technique. The paper concludes by describing a design philosophy for an audio power amplifier which is appropriate for both bipolar and FET devices, whereby only modest overall negative feedback is necessary.
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