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Design How-To

Distortion in power amplifiers, Part
I: the sources of distortion

Douglas
Self

1/2/2008 07:00 PM EST
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For many years I felt that the output
stages of power amplifiers presented very great possibilities for
creative design, and I actually got round to exploring some of them.
The hybrid bipolar-FET output stage, also collected in this volume,
was one of them. One of the first difficulties I met was the problem
of determining how much of the total distortion was due to the
output stage, and how much was being produced in the small-signal
sections. This latter contribution turned out to be larger and more
important than I expected.

Very little reliable information appeared to exist on amplifier
distortion, and I found myself embarked on a major effort to track
down all the sources of distortion in the typical solid-state power
amplifier. The initial investigations were not very illuminating,
until I realised that changing a component value in the typical
power amplifier circuit very often alters two or more distortion
mechanisms simultaneously, making the results hard to interpret.

I was sitting in my armchair late one Spring night when the full
force of this struck home, and thereafter I devised ways to simulate
or measure the distortion mechanisms in isolation. This approach is
well-illustrated in Parts 2 and 3 of the series, dealing with the
input pair and the Voltage-Amplifier Stage, respectively.

Things then began to fall into place, and one day I put together
all the various minor and, apparently insignificant, improvements I
had made to the utterly conventional amplifier circuit I had started
with. The distortion was exhilaratingly low, stability was good, and
I soon felt that I could write a pretty comprehensive guide to
distortion and power amplifier design. Electronics World were good
enough to give me all the space I asked for, and I believe I
succeeded; here, in eight chapters, is the result.

Introduction
It seems surprising that in a
world which can build the Space Shuttle and detect the echoes of the
birth of the universe, we still have to tolerate distortion in power
amplifiers. Leafing through recent reviews and specifications shows
claims for full-power total harmonic distortion ranging more than
three orders of magnitude between individual designs, a wider range
than any other parameter.

Admittedly the higher end of this range is represented by
subjectivist equipment that displays dire linearity, presumably with
the intention of implying that other nameless audio properties have
been given priority over the mundane business of getting the signal
from input to output without bending it.

Given the juggernaut rate of progress in most branches of
electronics this seems to me anomalous, and especially notable in
view of the many advanced analogue techniques used in op-amp design;
after all power amps are only op-amps with boots on. One conclusion
seems inescapable: a lot of power amplifiers generate much more
distortion than they need to.

This series attempts to show exactly why amplifiers distort, and
how to stop them doing it, culminating in a practical design for an
ultra-linear amplifier. It should perhaps be said at the outset that
none of this depends on excessively high levels of negative
feedback. Many of the techniques described here are also entirely
applicable to discrete op-amps, headphone drivers, and similar
circuit blocks. Since we are almost in the twenty-first century I
have ignored valve amplifiers.

Since mis-statements and confusions are endemic to audio, I have
based these articles almost entirely on my own experimental work
backed up with spice circuit simulation; much of the material
relates specifically to bipolar transistor output stages, though a
good deal is also relevant to mosfet amplifiers. Some of the
statements made may seem controversial, but I believe they are all
correct. If you think not, please tell me, but only if you have some
real evidence to offer.

The fundamental reason why amplifier distortion persists is, of
course, because it is a difficult technical problem to solve. A
Science proverbially becomes an Art when there are more than seven
variables, and since it will emerge that there are seven major
distortion mechanisms to the average amplifier, we would seem to be
nicely balanced on the boundary of the two cultures. Given so many
significant sources of unwanted harmonics, overlaid and sometimes
partially cancelling, sorting them out is a nontrivial task.

Make your amplifier as linear as possible before applying
NFB has long been a cliche, (one that conveniently ignores the
difficulty of running a high gain amp without any feedback) but
virtually no dependable advice on how to perform this desirable
linearisation has been published. The two factors are the basic
linearity of the forward path, and the amount of negative feedback
applied to further straighten it out.

The latter cannot be increased beyond certain limits or
high-frequency stability is put in peril, whereas there seems no
reason why open-loop linearity could not, in principle, be improved
without limit, leading us to the Holy Grail of the distortionless
amplifier. This series therefore takes as its prime aim the
understanding and improvement of open-loop linearity. As it proceeds
we will accrete circuit blocks to culminate in two practical
amplifier designs that exploit the techniques presented here.

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re: Distortion in power amplifiers, Part I: the
sources of distortion
HSB
4/22/2008 1:40:51 PM

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Awesome
presentation , Briefly covered the topics in detail.The whole
article can be carried as amplifier hand book. I think, this could
be the best artice available on net.

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re: Distortion in power amplifiers, Part I: the
sources of distortion
atomic
tommy 123   1/4/2008 6:29:49 AM

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good
stuff!learned a lot!

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