Distortion In Power Amplifiers

The distortion produced by a typical solid-state Class-B power amplifier is shown to consist of eight mechanisms, all of which may coexist and whose distortion products overlap to give a complex result. Methods for isolating each mechanism for study, and minimising its contribution, are given. If the avoidable distortions are designed out, Class-B amplifiers of unusually low distortion (Below 0.0005% at 1kHz, 0.003% at 10 kHz) may be designed as a matter of routine, and without significant extra cost. Such amplifiers define a distortion benchmark, and I have named them "Blameless" amplifiers. CLICK BELOW TO GO DIRECT TO SECTION. CLICK ON FIGURES FOR FULL-SIZE VERSION. C O N T E N T S 1: The Generic Amplifier Configuration. 2: The Eight Distortions. 3: Three Non-existent Distortions. 4: Techniques for Amplifier Investigation. 5: The Distortion Mechanisms. 6: The Concept of the Blameless Amplifier. 7: Conclusions. References. Much more detail on distortion and other matters can be found in the book I finally got round to writing: 0. INTRODUCTION. Considering the economic importance of audio power amplifiers, surprisingly little reliable information has been published on their design. Distortion in particular has been neglected, although it is the most variable feature of amplifier performance. You may have two units placed side, one giving 2% THD and the other 0.0005% at full power, and both claiming to provide the ultimate audio experience. I investigated the origins of distortion in the period 1992-94, and determined that power amplifier distortion, traditionally a difficult and mysterious thing to grapple with, was the amalgamation of eight basic mechanisms, superimposed and sometimes partially cancelling, giving a complex result. I evolved ways of measuring and minimising each distortion mechanism separately, and the result is a design methodology for making Class-B or Class-A amplifiers with distortion performance so good that two or three years ago it would have been regarded as impossible. 0.0008% at 1 kHz and 0.003% at 10 kHz are easily obtained. The methodology gives reliable and repeatable results with moderate amounts of negative feedback; increases in complexity and cost are insignificant. 1. THE GENERIC AMPLIFIER CONFIGURATION. Fig 1a shows the generic Lin power amplifier circuit, with the now universal differential input stage, representing something like 98% of the amplifiers ever built. It is the obvious starting point for amplifier investigation. [1] Fig 3 shows its distortion plot; there are two distortion regimes. Below 1 kHz THD is low at 0.002% but not zero, the noise floor being 0.0006% approx. Above 1 kHz, THD quadruples with each octave and reaches 0.5% before 20 kHz. The basic topology is a transconductance amplifier (voltagedifference input, current output) driving a transimpedance (current-tovoltage converter) Voltage Amplifier Stage, followed by a unity-gain power buffer. The voltage at the VAS transistor base is typically only a couple of millivolts, and is of little interest in itself; it is the current passed from the input stage to the VAS that counts. This topology has many advantages, Distortion In Power Amplifiers file:///C:/www_schematicsforfree_mattsoft_net/web%20site%20waiting/... 1 of 38 9/1/2009 8:04 PM including simple compensation. [2] Top |