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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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There is a kind of canonical
sequence of efficiency improvement in class A amplifiers. The simplest kind is
single-ended and resistively loaded, as at Figure 1(a). When it sinks output
current, there is an inevitable voltage drop across the emitter resistance,
limiting the negative output capability, and resulting in an efficiency of 12.5%
(erroneously quoted in at least one textbook as 25%, apparently on the grounds
that power not dissipated in silicon doesn't count). This would be of purely
theoretical interest - and not much of that - except that a single ended design
has recently appeared. This reportedly produces a 10 W output for a dissipation
of 120 W, with output swing predictably curtailed in one direction.3

A better method - constant current class A - is shown in Figure 1(b). The
current sunk by the lower constant current source is no longer related to the
voltage across it, and so the output voltage can approach the negative rail with
a practicable quiescent current. (Hereafter shortened to 'Iq')
Maximum efficiency is doubled to 25% at maximum output; for an example with 20 W
output (and a big fan) see Ref. 4. Some versions (Krell)
make the current source value switchable, controlling it with a kind of noise
gate.

Push-pull operation once more doubles full-power efficiency, producing a more
practical 50%; most commercial class A amplifiers have been of this type. Both
output halves now swing from zero to twice the Iq, and least
voltage corresponds with maximum current, reducing dissipation. There is also
the intriguing prospect of cancelling the even-order harmonics generated by the
output devices.

There are several ways to induce push-pull action. Figures 1(c), (d) show the
lower constant current source replaced by a voltage controlled current source.
This can be driven directly by the amplifier forward path, as in Figure
1(c),5 or by a current control negative feedback loop, as at Figure
1(d).6 The first of these methods has the drawback that the stage
generates gain, phase splitter Tr1 doubling as the VAS; hence
there is no circuit node that can be treated as the input to a unity gain output
stage, making the circuit hard to analyse, as VAS distortion cannot be separated

Figure 1: The major class A configurations. 1(c), 1(d) and
1(e) are push-pull variants, 1(e) being simply a class B stage with higher
Vbias.

from output stage non-linearity. There is also no guarantee that upper and
lower output devices will be driven appropriately for class A if the effective
quiescent varies by more than 40% over the cycle.5

The second push-pull method in 1d is more dependable, and I can vouch that it
works well. The disadvantage with the simple form shown is that a regulated
supply is required to prevent rail ripple from disrupting the current loop
control. Designs of this type have a limited current control range. In Figure
1(d), Tr3 cannot be turned on further once the upper device is
fully off " so the voltage controlled current source will not be able to respond
to an unforeseen increase in the output loading. If this happens there is no way
of resorting to class AB to keep the show going and the amplifier will show some
form of asymmetrical hard clipping.

The best push-pull stage seems to be that in Figure 1(e), which probably
looks rather familiar. Like all the conventional class B stages examined in Chapter 19, this one
will operate effectively in push-pull class A if the bias voltage is
sufficiently increased; the increase over class B is typically 700 mV, dependant
on the value of the emitter resistors. For an example of high biased class B see
Ref. 7. This topology has the great advantage that, when confronted with an
unexpectedly low load impedance, it will operate in class AB. The distortion
performance will be inferior not only to class A but also to optimally biased
class B, once above the AB transition level, but can still be made very low by
proper design.

Although the push-pull concept has a maximum efficiency of 50%, this is only
true at maximum sinewave output. Due to the high peak/average ratio of music,
the true average efficiency probably does not exceed 10%, even at maximum volume
before obvious clipping.

Other possibilities are signal controlled variation of the class A amplifier
rail voltages, either by a separate class B amplifier, or a modulated switch
mode supply. Both approaches are capable of high power output, but involve
extensive extra circuitry, and present some daunting design problems.

A class B amplifier has a limited voltage output capability, but can be
flexible about load impedances, as more current will be simply turned on when
required. However, class A has also a current limitation, after which it enters
class AB, and so loses its raison d'etre. The choice of quiescent value
has a major effect on thermal design and parts cost so a clear idea of load
impedance is important. The calculations to determine the required
Iq are straightforward, though lengthy if supply ripple,
Vce(sat), and Re losses, etc., are all
considered, so I just give the results here. An unregulated supply with 10,000
µF reservoirs is assumed.

A 20W/8Ω amplifier will require rails of approx ±24 V and a quiescent of
1.15A. If this is extended to give roughly the same voltage swing into 4 Ω, then
the output power becomes 37 W, and to deliver this in class A the quiescent must
increase to 2.16A, almost doubling dissipation. If however full voltage swing
into 6 Ω will do, (which it will for many reputable speakers) then the quiescent
only needs to increase to 1.5A; from here on I assume a quiescent of 1.6A to
give a margin of safety.

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