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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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The class A output stage
I consider here only the high biased class B topology, because it is probably the most popular approach, effectively solving the problems presented by the others. Figure 2 shows a Spice simulation of the collector currents in the output devices versus output voltage for the emitter follower configuration, and also the sum of these currents. This sum of device currents is, in principle, constant, but need not be so for low THD. The output signal is the difference of device currents and is not inherently related to the sum. However, a large deviation from this constant sum condition means

Figure 2: How output device current varies in push-pull class A. The sum of the currents is near-constant, simplifying biasing.

inefficiency, as the stage is conducting more quiescent than it needs to for some part of the cycle. The constancy of this sum is important because it shows that the voltage measured across Re1 and Re2 together is also effectively constant so long as the amplifier stays in class A. This in turn means that Iq can be simply set with a constant voltage bias generator, in very much the same way as class B.

Figures 3, 4, 5 show Spice gain plots for open loop output stages, with 8 Ω loading and 1.6 A quiescent; the circuitry is exactly as for class B in Part 4. The upper traces show class A gain, and the lower traces gain under optimal class B bias for comparison. Figure 3 shows an emitter follower output, Figure 4(a) simple quasi complementary stage, and Figure 5(a) CFP output.

We would expect class A stages to be more linear than B, and they are. Harmonic and THD figures for the three configurations, at 20 V peak, are shown in Table 1. There is absolutely no gain wobble around OV, and push-pull class A genuinely does cancel even order distortion. Class B only does this in the crossover region, in a partial and unsatisfactory way.

It is immediately clear that the emitter follower has more gain variation, and therefore worse linearity, than the CFP, while the quasi complementary

Figure 3: Gain linearity of the class A emitter-follower output stage. Load is 8 Ω, and quiescent current (Iq) is 1.6A. Upper trace class A, lower trace optimal class B.

circuit shows an interesting mix of the two. The more curved side of the quasi gain plot is on the negative side, where the CFP half of the quasi circuit is passing most of the current. However we know by comparing Figure 3 and Figure 5 that the CFP is the more linear structure. Therefore it appears that the shape of the gain curve is determined by the output half that is turning off, presumably because this shows the biggest gm changes. The CFP structure maintains gm better as current decreases, and so gives a flatter gain curve with less rounding of the extremes.

The gain behaviour of these stages is reflected in their harmonic generation; Table 1 reveals that the two symmetrical topologies give mostly odd order harmonics as expected. The asymmetry of the quasi comp version causes a large increase in even order harmonics, and this is reflected in the higher THD figure. Nonetheless the THD figures are still two to three times lower than for their class B equivalents.

If this factor of improvement seems a poor return for the extra dissipation of class A, this is not so. The crucial point about the distortion from a class A output stage is not just that is low, but that it is low order, and so benefits much more from a typical NFB factor that falls with frequency than does high order crossover distortion.

Figure 4: Gain linearity of the class A quasi-complementary output stage. Conditions as in Figure 3. Upper trace class A, lower class B.

The choice of class A output topology is now simple. For best performance, use the CFP. Apart from greater basic linearity, the effects of output device temperature on Iq are servoed out by local feedback, as in class B. For utmost economy, use the quasi complementary with two NPN devices: these need only a low Vce(max) for a typical class A amp, so here is an opportunity to recoup some of the money spent on heatsinking.

The rules are different from class B; the simple quasi configuration will give first class results with moderate NFB, and adding a Baxandall diode to simulate a complementary emitter follower stage makes little difference to linearity.7

It is sometimes assumed that the different mode of operation of class A makes it inherently short circuit proof. This may be true with some configurations, but the high biased type shown here will continue delivering current until it bursts. Overload protection is no less necessary.

Figure 5: Gain linearity of the class A CFP output stage. Upper trace class A, lower trace class B.

Table 1: THD is calculated from the first nine harmonics, though levels above the fifth are very small

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