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[Part 1 offers an overview and introduction to the sources of distortion in power amplifiers. Part 2 focuses on distortion in the audio amplifier input stage. Part 3 examines distortion mechanisms in the voltage amplifier stage (VAS) of audio power amplfiers.]
The almost universal choice in semiconductor power amplifiers is for a unity gain output stage, and specifically a voltage follower. Output stages with gain are not unknown,1 but they are not common. Most designers feel that controlling distortion while handling large currents is hard enough without trying to generate gain at the same time.
The first three parts of this series have dealt with one kind of distortion at a time, due to the monotonic transfer characteristics of small signal stages, which usually, but not invariably, work in class A.2 Economic and thermal realities mean that most output stages are class B, and so we must now consider crossover distortion, which remains the thorniest problem in power amplifier design, and HF switchoff effects.
It is now also necessary to consider what kind of active device is to be used; jfets offer few if any advantages in the small current stages, but power fets are a real possibility, providing that the extra cost brings with it tangible benefit.
The class war
The fundamental factor in determining output stage distortion is the class of operation. Apart from its inherent inefficiency, class A is the ideal operating mode, because there can be no crossover or switchoff distortion.
However, of those designs which have been published or reviewed, it is notable that the large signal distortion produced is still significant. This looks like an opportunity lost, as of the distortion mechanisms discussed in the first part of this series, we now only have to deal with Distortion 1 (input stage), Distortion 2 (VAS), and Distortion 3 (output stage large signal nonlinearity). Distortions 4, 5, 6 and 7, as mentioned earlier, are direct results of class B operation and therefore can be thankfully disregarded in a class A design. However, class B is overwhelmingly of the greater importance, and is therefore dealt with in detail.
Class B is subject to misunderstanding. The statement is often made that a pair of output transistors operated without any bias are working in 'class B', and therefore 'generate severe crossover distortion'. In fact, with no bias each output device is operating for slightly less than half the time, and the question arises as to whether it would not be more accurate to call this class C and reserve class B for that condition of quiescent current which eliminates, or rather minimises, the crossover artifacts.
A further complication exists; it is not generally appreciated that moving into what is usually called class AB, by increasing the quiescent current, does not make things better. In fact, the THD reading will increase as the bias control is advanced, with what is usually known as 'gm doubling' (i.e. a voltage gain increase caused by both devices conducting simultaneously in the centre of the output voltage range) putting edges into the distortion residual that generate high order harmonics in much the same way that underbiasing does. This important fact seems almost unknown, presumably because the gm doubling distortion is at a relatively low level and is completely obscured in most amplifiers by other distortion mechanisms.
The phenomenon is demonstrated in Figures 1(a), (b), (c) which shows spectrum analysis of the distortion residuals for under biasing, optimal, and over biasing of a 150W/8 Ω amplifier at 1 kHz.
Figure 1: Spectrum analysis of class B & AB distortion residual. 1(a) Underbiased class B; 1(b) Optimal class B; 1(c) class AB.
As before, all non-linearities except the unavoidable Distortion 3 (output stage) have been effectively eliminated. The over biased case had its quiescent current increased until the gm doubling edges in the residual had an approximately 50:50 mark/space ratio, and so was in class A about half the time which represents a rather generous amount of quiescent for class AB. Nonetheless, the higher order odd harmonics in Figure 1(c) are at least 10 dB greater in amplitude than those for the optimal class B case, and the third harmonic is actually higher than for the under-biased case as well. However the under biased amplifier, generating the familiar sharp spikes on the residual, has a generally greater level of high-order odd harmonics above the 5th; about 8 dB higher than the AB case.
Bearing in mind that high order odd harmonics are generally considered to be the most unpleasant, there seems to be a clear case for avoiding Class AB altogether, as it will always be less efficient and generate more high order distortion than the equivalent class B circuit, class distinction therefore seems to resolve itself into a binary choice between A or B.
It must be emphasised that these effects can only be seen in an amplifier where the other forms of distortion have been properly minimised. The r.m.s. THD reading for case 1a was 0.00151%, for case 1b 0.00103%, and for case 1c 0.00153%. The tests were repeated at the 40 W power level with very similar results. The spike just below 16 kHz is interference from the test gear VDU.
This may seem complicated enough, but there are other and deeper subtleties in class B.
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