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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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A new class A design

The full circuit diagram shows a 'blameless' 20W/8Ω class A power amplifier.
This is as close as possible in operating parameters to the previous
class B design to aid comparison. In particular the NFB factor remains 30 dB
at 20 kHz. The front end is as for the class B version, which should not be
surprising as it does exactly same job, input Distortion 1 being unaffected by
output topology.

Figure 7: A Blameless 20 W class A power amplifier, using
the novel current-control system.

As before the input pair uses a high tail current, so that
R2,3 can be introduced to linearise the transfer
characteristic and set the transconductance. Distortion 2 (VAS) is dealt with as
before, the beta enhancer Tr12 increasing the local feedback
through Cdom. There is no need to worry about Distortion 4
(non-linear loading by output stage) as the input impedance of a class A output,
while not constant, does not have the sharp variations shown by class B.

The circuit uses a standard quasi output. This may be replaced by a CFP stage
without problems. In both cases the distortion is extremely low but,
gratifyingly, the CFP proves even better than the quasi, confirming the
simulation results for output stages in isolation.

The operation of the current regulator Tr13,15,16 has
already-been described. Using a band gap reference, it holds a 1.6 A
Iq to within ±2mA from a second or two after switch on.
Looking at Table 2, there seems no doubt that the new system is effective.

As before an unregulated power supply with 10,000 µF reservoirs was used, and
despite the higher prevailing ripple, no PSRR difficulties were encountered once
the usual decoupling precautions were taken.

Performance
The closed loop distortion performance (with
conventional compensation) is shown in Figure 8 for the quasi comp output stage,
and in Figure 9 for a CFP output version. The THD residual is pure noise for
almost all of the audio spectrum, and only above 10 kHz do small amounts of
third harmonic appear. The suspected source is the input pair, but this so far
remains unconfirmed.

The distortion generated by the class B and A design examples is summarised
in Table 3, which shows a pleasing reduction as various measures are taken to
deal with it. As a final tweak, two pole compensation was applied to the most
linear (CFP) of the class A versions, reducing distortion to 0.0012% at 20 kHz,
at some cost in slew rate (Figure 10). While this may not be the fabled straight
wire with gain, it must be a near relation ...

Figure 8: Class A amplifier THD performance with quasi-comp
output stage. The steps in the LF portion of the trace are measurement
artifacts.

Figure 9: Class A distortion performance with CFP output
stage.

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