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Design How-To

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

The
seven distortions

Douglas
Self

1/2/2008 07:00 PM EST
2
comments

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The seven distortions
In the
typical amplifier THD is often thought to be simply due to the
Class-B nature of the output stage, which is linearised less
effectively as the feedback factor falls with increasing frequency.
However the true situation is much more complex as the small-signal
stages can generate significant distortion in their own right in at
least two different ways. This can easily exceed the output stage
distortion at high frequencies. It seems inept to allow this to
occur given the freedom of design possible in the small-signal
section.

Include all the ills that a class-B stage is prone to and then
there are seven major distortion mechanisms.

Distortion in power amplifiers arises from:

1. Non-linearity in the input stage. If this is a
carefully-balanced differential pair then distortion is typically
only measurable at HF, rises at 18 dB/octave, and is almost pure
third harmonic. If the input pair is unbalanced (which from
published circuitry it usually is) then the HF distortion emerges
from the noise floor earlier. As frequency increases, it rises at 12
dB/octave as it is mostly second harmonic.

2. Non-linearity in the voltage amplifier stage
surprisingly does not always figure in the total distortion. If it
does, it remains constant until the dominant-pole frequency P1 is
reached, and then rises at 6 dB/octave. With the configurations
discussed here, it is always second harmonic.

Usually the level is very low due to linearising negative
feedback through the dominant-pole capacitor. Hence if you crank up
the local VAS open-loop gain, for example by cascoding or
putting more current-gain into the local VAS/Cdom
loop, and attend to mechanism four below, you can usually ignore VAS
distortion.

3. Non-linearity in the output stage, which is naturally
the obvious source. This, in a Class-B amplifier, will be a complex
mix of largesignal distortion and crossover effects, the latter
generating a spray of high-order harmonics, and in general rising at
6 dB/octave as the amount of negative feedback decreases.
Large-signal THD worsens with 4 Ω loads and worsens again at 2 Ω.
The picture is complicated by dilatory switch-off in the relatively
slow output devices, ominously signalled by supply current
increasing in the top audio octaves.

4. Loading of the VAS by the non-linear input impedance of the
output stage. When all other distortion sources have been
attended to, this is the limiting distortion factor at LF (say below
2 kHz). It is simply cured by buffering the VAS from the output
stage. Magnitude is essentially constant with frequency, though
overall effect in a complete amplifier becomes less as frequency
rises and feedback through Cdom starts to
linearise the VAS.

5. Non-linearity caused by large rail-decoupling capacitors
feeding the distorted signals on the supply lines into the signal
ground. This seems to be the reason many amplifiers have rising
THD at low frequencies. Examining one commercial amplifier kit, I
found that rerouting the decoupler ground-return reduced THD at 20
Hz by a factor of three.

6. Non-linearity caused by induction of Class-B supply
currents into the output, ground, or negative-feedback lines.
This was highlighted by Cherry3 but seems to remain
largely unknown; it is an insidious distortion that is hard to
remove, though when you know what to look for on the THD residual,
it is fairly easy to identify. I suspect that a large number of
commercial amplifiers suffer from this to some extent.

7. Non-linearity resulting from taking the NFB feed from
slightly the wrong place near where the power-transistor Class-B
currents sum to form the output. This may well be another common
defect.

Having set down what Mao might have called The Seven Great
Distortions " Figure 2 shows the location of these mechanisms
diagrammatically " we may pause to put to flight a few Paper Tigers.
The first is common-mode distortion in the input stage, a spectre
that tends to haunt the correspondence columns.

Since it is fairly easy to make an amplifier with less than <
0.00065%="" thd="" (1="" khz)="" without="" paying="" any=""
special="" attention="" to="" this,="" it="" cannot="" be="" too=""
serious="" a="" problem.="" a="" more="" severe="" test="" is=""
to="" apply="" the="" full="" output="" voltage="" as="" a=""
common-mode="" signal,="" by="" running="" the="" amplifier="" as=""
a="" unity-gain="" voltage-follower.="" if="" this="" is="" done=""
using="" a="" model="" (see="" below="" for="" explanation)=""
small-signal="" version="" of="" figure="" 1,="" with="" suitable=""
attention="" to="" compensation,="" then="" it="" yields="" less=""
than="" 0.001%="" at="" 8="" v="" r.m.s.="" across="" the=""
audio="" band.="" it="" therefore="" appears="" that="" the=""
only="" real="" precaution="" required="" against="" common-mode=""
distortion="" is="" to="" use="" a="" tail="" current-source=""
for="" the="" input="" pair.="">

Figure 2: A topology of distortion: the
location of the seven distortion mechanisms.

The second distortion conspicuous by its absence in the list is
the injection of distorted supply-rail signals directly into the
amplifier circuitry. Although this putative mechanism has received a
lot of attention,4 dealing with Distortion five above by
proper grounding seems to be all that is required...

Once again, if triple-zero THD can be attained using simple
unregulated supplies and without specifically addressing power
supply rejection ratio, (which it reliably can be) then much of the
work done on regulated supplies may be of doubtful utility. However,
PSRR does need some attention if the hum/noise performance is to be
of the first order.

A third mechanism of doubtful validity is thermal distortion,
allegedly induced by parameter changes in semiconductor devices
whose instantaneous power dissipation varies over a cycle. This
would presumably manifest itself as a distortion increase at very
low frequencies, but it simply does not seem to happen. The major
effects would be expected in Class-B output stages where dissipation
can vary wildly over a cycle. However drivers and output devices
have relatively large junctions with high thermal inertia. Low
frequencies are of course also where the NFB factor is at its
maximum.

The advantages of being conventional
The input
pair not only provides the simplest way of making a d.c.
coupled amp with a dependably small output offset voltage, but
can also (given half a chance) completely cancel the
second-harmonic distortion which would be generated by a
single-transistor input stage. One vital condition must be
met; the pair must be accurately balanced by choosing the
associated components so that the two collector currents are
equal. (The 'typical' component values shown in Figure 1 do
not bring about this most desirable state of affairs.)

The input devices work at a constant and near-equal Vce,
giving good thermal balance.

The input pair has virtually no voltage gain so no
low-frequency pole can be generated by Miller effect in the
Tr2 collector-base capacitance. All the voltage
gain is provided by the VAS stage, which makes for easy
compensation. Feedback through Cdom lowers
VAS input and output impedances, minimising the effect of
input-stage and output stage capacitance. This is often known
as pole-splitting;2 the pole of the VAS is moved
downwards in frequency to become the dominant pole, while the
input-stage pole is pushed up in frequency.

The VAS Miller compensation capacitance smoothly transfers
NFB from a global loop which may be unstable, to the VAS local
loop that cannot be. It is quite wrong to state that
all the benefits of feedback are lost as the frequency
increases above the dominant pole, as the VAS is still being
linearised. This position of Cdom also
swamps the rather variable Ccb of the VAS
transistor.

To return to our list
of the unmagnificent seven, note that only Distortion three is
directly due to O/P stage non-linearity, though numbers 4"7
all result from the Class-B nature of the typical output stage.

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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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