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course avoids this small pitfall (because the transistors are always on) but at the expense of a lot
of heat generation. Managing this heat and power dissipation inevitably means that Class A
designs are much more expensive to implement and often of lower power output so as to minimise
the heat as much as possible.
It is indisputable that Class A power amplifiers have the potential to give the best linearity when
well designed, but they are usually impracticable, in reasonably priced equipment at least.
Alternatively, optimal Class B linearity can of course be very good when well designed and is the
preferred method we and most other amplifier designers use in our core products.
We won’t cover these two design methodologies in more detail here as they are generally well
understood and covered in other sources. It is important that it should not be assumed that in
doing this work we intended to replace the Class B or A approaches, both have very real
advantages at different ends of the market and both can be made to sound wonderful when well
implemented. What we were seeking with the development of Class XD was a way of incorporating
a lot of the advantages and sound quality of Class A at a far lower price level than normal and
without the inefficiency inherent in that method.
Well designed Class B amplifiers can in fact achieve extremely low distortion levels of <0.001%-at 1
kHz [Ref 2]. The Class B approach however, does have its ultimate limitations, as we have said
Class B inherently generates crossover distortion, and inconveniently displays this non-linearity at
the zero-crossing, where it is always in evidence no matter how low the signal amplitude. At one
unique value of quiescent current the distortion produced is a minimum, and this is what
characterises optimal Class B; however at no value can it be made to disappear. It is in fact
inherent in the classical Class B operation of a pair of output transistors.
Fig 1 shows a simulation of an output stage that illustrates the heart of the ‘problem’. The diagram
plots the incremental gain of the output stage against output voltage; in other words the gain for a
very small signal. A complementary-feedback pair (CFP) output stage was used. Both 8 and 4 Ohm
