A new result in negative‐feedback theory, and its application to audio power amplifiers
Edward M. Cherry · International Journal of Circuit Theory and Applications · 1978
Abstract A nest of differentiating loops is described by means of which a very large amount of negative feedback can be applied to a specified stage in an amplifier (normally the output stage) with progresively less being applied to other stages. The effects of nonlinearity (other than hard limiting) in the specified stage can be virtually eliminated. The structure enables Bode's limit for loop‐gain roll‐off to be exceeded for the specified stage; both the frequency up to which loop gain is maintained constant and the frequency at which loop gain falls through unity are free design variables. As a result, practical difficulties associated with excess phase shift in low‐frequency power transistors are minimized. The structure does not increase the susceptibility to transient overload and the resulting intermodulation. A practical amplifier using output transistors with fT∼2 MHz has an output‐stage loop gain of 25,000 (88 dB) at 20 kHz and 0.002 per cent harmonic distortion.