- EFFECT OF FEEDBACK ON DISTORTION HARMONICS. Updated Nov-2008.
It is well known that a square-law amplifier adds only second harmonics with no feedback, but many high order harmonics with feedback applied, but this is almost entirely misleading, as explained here.
- INPUT STAGE DISTORTION.
Seven of the most common one and two transistor input stages are compared by calculating their levels of intermodulation distortion under identical stage output current levels.
- SLEW RATE AND T.I.D. (Updated June 2005)
The maximum slew rate requirement and the design techniques for low t.i.d. are investigated. Now includes measurement results for maximum CD slew rate.
- MEASURING DISTORTION.
To test our low distortion designs an alternative to buying expensive distortion measuring instruments is to use the bridge-nulling method in which the input and output are directly compared.
- PHASE INTERMODULATION.
Explanation, reduction and measurement of phase intermodulation distortion.
- COMMON-MODE DISTORTION.
Common-mode distortion is examined, a test method described, and the performance of typical input stage transistors tested and compared.
- MEASURING CAPACITOR DISTORTION. (Updated 18.Aug.08)
Capacitors are known to add a little low order harmonic distortion, but does transient testing reveal something a lot worse than this?
- ERROR FEEDBACK AND NESTED LOOPS.
An examination of how effective these methods can be compared to overall negative feedback.
Symmetry can be good or bad, but just aiming for maximum circuit symmetry may add unnecessary complexity with no real benefit.
- FEEDBACK WORKS PART 1.
The idea that high levels of overall negative feedback should be avoided is repeated often, but both theory and measurement lead to a different conclusion.
- FEEDBACK WORKS PART 2.
Feedback is sometimes avoided to prevent the addition of high order harmonics, but this is based on analysis with a single frequency. An analysis based on music signals suggests that a high feedback approach may be more effective.
- FEEDBACK WORKS PART 3.
The effects of phase shift and time delay on distortion reduction are investigated.
- OVERLOAD AND SHORT-CIRCUIT PROTECTION.
A small change to the usual load-line protection circuit may reduce distortion, but is this sort of protection really necessary?
- LOCAL FEEDBACK.
Does local feedback avoid all the problems of overall feedback? Here the addition of high order harmonics is investigated.
- SQUARE WAVE TESTING.
The use of square waves to test for stability can reveal effects which have no relevance to stability, while missing real stability problems.
- The PHYSICS section includes articles about transmission lines and conduction in metals, which may be of interest to some audio enthusiasts.
For further coverage of audio power amplifier design there are a number of website sources. Two I suggest trying are:
Elliot Sound Products. (Rod Elliot).
and The Self Site. (Douglas Self).
Older design notes and design ideas, Oct 2004.
An assortment of links to other sites.