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Design How-To

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

The
performance

Douglas
Self

1/2/2008 07:00 PM EST
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The performance
The THD curve
for the standard amplifier is shown in Figure 3. As usual the
distortion increases with frequency and, as we shall see later,
would give grounds for suspicion if it did not. The flat part of the
curve below 500 Hz represents non-frequency-sensitive distortion
rather than the noise floor, which for this case is at about the
0.0005% level. Above 500 Hz the distortion rises at an increasing
rate, rather than a constant number of dB/octave, due to the
combination of Distortions 1, 2, 3 and 4. (In this case Distortions
5, 6 and 7 have been carefully eliminated to keep things simple.
This is why the distortion performance looks good already, and the
significance of this should not be overlooked.)

Figure 3: The distortion performance of the
class-B amplifier shown in Figure 1(a).

It is often written that having distortion constant across the
audio band is a good thing. This is a most unhappy conclusion as the
only practical way to achieve this with a Class-B amplifier is to
increase the distortion at LF, for example by allowing the
VAS to distort significantly.

It should now be clear why it can be hard to wring linearity out
of a snake-pit of contending distortions. A circuit-value change is
likely to alter at least two of the distortion mechanisms, and
probably change the open-loop gain as well. In the coming articles I
shall demonstrate how each of these mechanisms can be measured and
manipulated separately.

Determining open-loop linearity
Improving something
demands its measurement, and so it is essential to examine the
open-loop linearity of typical power-amp circuits. This cannot in
general be done directly, so it is necessary to measure the NFB
factor and calculate open-loop distortion from the usual closed-loop
data. It is assumed that the closed-loop gain is fixed by
operational requirements.

Finding the feedback-factor is at first sight difficult, as it
means determining the open-loop gain. The standard methods for
measuring op-amp open-loop gain involve breaking feedback-loops and
manipulating closed-loop gains, procedures that are likely to send
the average power-amplifier into fits.

However, the need to measure this parameter is inescapable, as a
typical circuit modification " e.g. changing the value of
R2 " will change the open-loop gain as well as the
linearity, and to prevent total confusion it is necessary to keep a
very clear idea of whether the observed change is due to an
improvement in open-loop linearity or merely because the open-loop
gain has risen. It is wise to keep a running check on the
feedback-factor as work proceeds, and so the direct method of
open-loop gain measurement shown in Figure 4 was evolved.

Direct open-loop gain measurement
Since the amplifier
shown in Figure 1 is a differential amplifier, its open-loop gain is
simply the output divided by the voltage difference between the
inputs. If the output voltage is kept effectively constant by
providing a swept-frequency constant voltage at the +ve input, then
a plot of open-loop gain versus frequency is obtained by measuring
the error-voltage between the inputs, and referring this to the
output level.

This yields an upside down plot that rises at HF rather than
falling, as the differential amplifier requires more input for the
same output as frequency increases, but the method is so quick and
convenient that this can be lived with. Gain is plotted in dB with
respect to the chosen output level (+16dBu in this case) and the
actual gain at any frequency can be read off simply by dropping the
minus sign. Figure 5 shows the plot for the amplifier in Figure 1.

Figure 4: Test circuit for measuring open-loop
gain directly. The measurement accuracy depends on the test gear
CMRR.

The HF-region gain slope is always 6 dB/octave unless you are
using something special in the way of compensation and, by the
Nyquist rules, must continue at this slope until it intersects the
horizontal line representing the feedback factor provided that the
amplifier is stable. In other words, the slope is not being
accelerated by other poles until the loop gain has fallen to unity,
and this provides a simple way of putting a lower bound on the next
pole P2; the important P2 frequency (which is usually somewhat
mysterious) must be above the intersection frequency if the
amplifier is seen to be stable.

Given test gear with a sufficiently high
common-mode-rejection-ratio balanced input, the method of Figure 4
is simple; just buffer the differential inputs from the cable
capacitance with TL072 buffers, placing negligible loading on
the circuit if normal component values are used. Be particularly
wary of adding stray capacitance to ground to the -ve input, as this
directly imperils amplifier stability by adding an extra feedback
pole. Short wires from power amplifier to buffer IC can usually be
unscreened as they are driven from low impedances.

Figure 5: Open-loop gain versus frequency plot
for Figure 1. Note that the curve rises as gain falls, because the
amplifier error is the actual quantity measured.

The test gear input CMRR defines the maximum open-loop gain
measurable; I used an Audio Precision System-1 without any special
alignment of CMRR. A calibration plot can be produced by feeding the
two buffer inputs from the same signal; this will probably be found
to rise at 6 dB/octave, being set by the inevitable input asymmetry.
This must be low enough for amplifier error signals to be above it
by at least 10 dB for reasonable accuracy. The calibration plot will
flatten out at low frequencies, and may even show an LF rise due to
imbalance of the test gear input-blocking capacitors; this can make
determination of the lowest pole P1 difficult, but this is not
usually a vital parameter in itself.

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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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