Distorsion Correction

PAPERS

Distortion Correction Circuits for Audio Amplifiers*
M. J. HAWKSFORD

University of Essex, Department of Electrical Engineering Science, Colchester, C04 3SQ, United Kingdom

'

Circuit topologies are introduced which should prove of use to the circuit designer of analog audio amplifiers. The objective is to produce circuits of' modest complexity that overcome the nonlinearitiesinherent in single-transistor and long-tail pair circuits. This allows amplifiers with excellent linearity to be designed without resorting to overall negative feedback with high loop gains. To aid comparison of circuit nonlinear behavior, a parameter called the incrementaldistortion factor (IDF) is introduced and discussed.

0 INTRODUCTION Most modern transistor amplifiers use either a singletransistor or a pair of transistors in the input circuitry. It is argued that if this stage is cascaded with adequate gain, then by the expedience of overall negative feedback, the input devices will operate within the limits for small-signal operation and thus yield good overall linearity. Often a consequence of this design philosophy is poor dynamic performance of the input circuitry, where modestinput overload can result in gross distortion. There are simple circuit modifications that can be introduced: an increase in device operating current, though possibly at the expense of the noise factor; the introduction of local negative feedback (emitter degeneration) which reduces stage gain but enhances linearity and overload performance, again at the expense of the noise factor. Theaim ofthispaper is to introduce circuit topologies that enhance the nonlinear performance of amplifier gain cells without recourse to high overall negative feedback. It is considered by this author that the combination of high loop gain together with its inevitable dynamicperformance (dominant pole)when compounded with nonlinear elements can result in poor transient distortion characteristics, especiallywhen complex signals are being processed. Since the signals being amplified are rendered more complex dueto these nonlinearities falling within a dynamic negative feedback loop, then intermodulation products result which are effectively time smeared. In the limit this must determine the ultimate resolution of an amplifier, which is its ability to transfer fine signal detail in the presence ofcomplex signals, The only rational methodology to minimize these * Manuscript received 1981 January 22.
J. Audio Eng. Soc., Vol. 29, No. 7/8, 1981 July/August

attributes

of nonlinear

distortion

is to use gain cells that

are inherently linear over a wide range of their transfer characteristics and are essentially nondynamic with predictable gain characteristics. Such gain cells can thenbe used with amplifiers with overall negative feedback without detriment to the intermodulation performance. However, the use of linear circuitry may well render the need for high negative feedback unnecessary. Thispaper investigates and catalogsexamples gain of cells that generally exhibit good linearity and dynamic range. The circuits should prove of use to designers of both discrete andintegrated circuitry, although some design examples which are particularly relevant to integrated-circuit fabrication are included. In order to facilitate the comparison of various circuit topologies, a parameter called incremental distortion factor (IDF) is introduced. The IDF is related to the change in slope of the transfer characteristic with the input signal and is useful for quantifyingnonlinearity under large-signal conditions.

I PRINCIPLES Three methods

OF DISTORTION

CORRECTION

are identified in this section to en-

hance the linearity of gain cells that may already use either local or overall negative feedback within an amplifier structure. (See [1-5] for background.) Nonlinear Stages in Cascade

1.1 Complementary

Ifa stage has a predictable nonlinearity, then by...