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Design How-To

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

How an
amplifier (really) works

Douglas
Self

1/2/2008 07:00 PM EST
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comments

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How an amplifier (really) works

Figure 1 shows the usual right trusty and well-beloved power
amp circuit drawn as standard is possible. Much has been written
about this configuration, though its subtlety and quiet
effectiveness are usually overlooked, and the explanation below
therefore touches on several aspects that seem to be almost unknown.
It has the merit of being docile enough to be made into a workable
amplifier by someone who has only the sketchiest of notions as to
how it works.

The input differential pair implements one of the few forms of
distortion cancellation that can be relied upon to keep working in
all weathers. This is because the transconductance of the input pair
is determined by the physics of transistor action rather than
matching of variable parameters such as beta; the logarithmic
relation between Ic and Vbe is proverbially accurate over some eight
or nine decades of current variation.

Figure 1(a) Conventional class-B power amp
circuit. The apparent simplicity of circuitry conceals a series of
sophisticated operating mechanisms. The lower drawing (Figure
1(b)) shows an adaptation of the output stage for small signal
modelling.

The voltage signal at the voltage amplifier stage (hereafter VAS)
transistor base is typically a couple of millivolts, looking rather
like a distorted triangle wave. Fortunately the voltage here is of
little more than academic interest, as the circuit topology
essentially consists of a transconductance amp (voltage-difference
input to current output) driving into a transresistance
(current-to-voltage converter) stage. In the first case the
exponential Vbe/Ic law is
straightened out by the differential-pair action, and in the second
the global (overall) feedback factor at LF is sufficient to
linearise the VAS, while at HF shunt negative feedback (hereafter
NFB) through Cdom conveniently takes over
VAS-linearisation while the overall feedback factor is falling.

The behaviour of Miller dominant-pole compensation in this stage
is exceedingly elegant, and not at all just a case of finding the
most vulnerable transistor and slugging it. As frequency rises and
Cdom begins to take effect, negative feedback is
no longer applied globally around the whole amplifier, which would
include the higher poles, but instead is seamlessly transferred to a
purely local role in linearising the VAS. Since this stage
effectively contains a single gain transistor, any amount of NFB can
be applied to it without stability problems.

The amplifier operates in two regions; the LF, where open-loop
gain is substantially constant, and HF, above the dominant-pole
breakpoint, where the gain is decreasing steadily at 6 dB/octave.
Assuming the output stage is unity-gain, three simple relationships
define the gain in these two regions:

LF gain = gm x beta x Rc

At least one of the factors that set this (beta) is not
well-controlled and so the LF gain of the amplifier is to a certain
extent a matter of potluck; fortunately this doesn't matter as long
as it is high enough to give a suitable level of NFB to eliminate LF
distortion. The use of the word 'eliminate' is deliberate, as will
be seen later. Usually the LF gain, or HF local feedback-factor, is
made high by increasing the effective value of the VAS collector
impedance Rc, either by the use of a current
source collector-load, or by some form of bootstrapping.

The other important relations are:

HF gain = gm / (ω x
Cdom)

Dominant pole freq P1 = 1 / (ω x
Cdom x β x Rc)

where ω = 2πf.

In the HF region, things are distinctly more difficult as regards
distortion, for while the VAS is locally linearised, the global
feedback-factor available to linearise the input and output stages
is falling steadily at 6 dB/octave. For the time being we will
assume that it is possible to define an HF gain (say N dB at 20 kHz)
which will assure stability with practical loads and component
variations. Note that the HF gain, and therefore both HF distortion
and stability margin, are set by the simple combination of the input
stage transconductance and one capacitor, and most components have
no effect on it at all.

It is often said that the use of a high VAS collector impedance
provides a current drive to the output devices, often with the
implication that this somehow allows the stage to skip quickly and
lightly over the dreaded crossover region. This is a misconception "
the collector impedance falls to a few kΩ at HF, due to increasing
local feedback through Cdom. In any case it is
very doubtful if true current drive would be a good thing since
calculation shows that a low-impedance voltage drive minimises
distortion due to beta-unmatched output halves,1 and it
certainly eliminates distortion mechanism four described later.

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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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