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Home>
Audio-design
Design Center > How
To Article

Distortion in power amplifiers, Part VIII: Class A amplifiers

Douglas Self -February 20, 2008

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[Part
1 offers an overview and introduction to the sources of distortion in audio
power amplifiers. Part
2 focuses on distortion in the audio amplifier input stage. Part
3 examines distortion mechanisms in the voltage amplifier stage (VAS). Part 4 focuses on
distortion in the audio power amplifier output stage. Part
5 continues the discussion of distortion in the power amplifier output
stages. Part 6
looks at some of the remaining distortion mechanisms in audio power amplifiers.
Part 7 examines
frequency compensation in audio power amplifiers, with some examples from actual
designs.]

There are two salient facts about Class A amplifiers: they are inefficient;
they give the best possible distortion performance. The quiescent dissipation of
the classic Class A amplifier is equal to twice the maximum output power, making
massive output power impractical.
But the nature of our hearing means that the power of an amplifier must be
considerably increased to sound significantly louder.

It is well known that power in watts must be quadrupled to double sound
pressure level (SPL), but this is not the same as doubling subjective loudness;
this is measured in Sones rather than dB above threshold, and some researchers
have reported that doubling subjective loudness requires a 10 dB rather than 6
dB rise in SPL, implying that amplifier power must be increased tenfold, rather
than merely quadrupled.1 This may help to put worries about amplifier
size into perspective ...

There is an attractive simplicity about class A. Most of the distortion
mechanisms studied so far stem from class B, and we can thankfully forget
crossover and switchoff phenomena (Distortions 3b, 3c), non-linear VAS loading
(Distortion 4), injection of supply-rail signals (Distortion 5), induction from
supply currents (Distortion 6), and erroneous feedback connections (Distortion
7). Beta-mismatch in the output devices can also be ignored.

The art of compromise
The only real disadvantage of class
A is inefficiency, so inevitably efforts have been made to compromise between A
and B. As compromises go, traditional class AB is not a happy one because, when
the AB region is entered, the step change in gain generates significantly
greater high order distortion than that from optimally biased class B. However,
a well-designed AB amplifier will give pure class A performance below the AB
threshold, something a class B amp cannot do.

Another compromise is the so-called non-switching amplifier, with its output
devices clamped to pass a minimum current. However, it is not immediately
obvious that a sudden halt in current-change as opposed to complete turn-off
makes for a better crossover region. Those residual oscillograms that have been
published seem to show that some kind of discontinuity still exists at
crossover.2

A potential problem is the presence of maximum ripple on the supply rails at
zero signal output; the PSRR must be taken seriously if good noise and ripple
figures are to be obtained. This problem can be simply solved by the measures
proposed for class B designs.

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Next: The
art of compromise (cont.)

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