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Double-pole compensation and the push-pull transimpedance stage in discrete audio frequency power amplifiers

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By Michael Kiwanuka, B.Sc. (Hons) Electronic Engineering

Some time ago, a fascinating debate ensued in respect of improvements that might be made to the venerable double cascaded differential stages (DCDS) gain block recommended by Hitachi for use with their power MOSFETs.

Alas, there was much missing of the point, as the suggested modifications did not accommodate the essential linearity-enhancing techniques detailed by Douglas Self in his seminal works ‘Distortion in Power Amplifiers, 1: The Sources of Distortion’, ‘Distortion in Power Amplifiers, 2: The Input Stage’ and ‘Distortion in Power Amplifiers, 3: The Voltage-amplifier Stage’. It’s demonstrated here that such a topology need not compromise on linearity; however, the increase in complexity and cost may be relatively significant.

The transient and steady-state conduct of an amplifier is inextricably linked to the choice of frequency stabilisation. Prominence is therefore given to a first-order analysis of double-pole compensation as a desirable and cost-effective means of reducing forward-path error. A brief overview of loop transmission and its determination in context is also provided. A first-principals approach is preferred and adhered to whenever appropriate.

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