White paper
This is the basic Class XD principle, and it’s a very simple one, develop a topology that displaces
the crossover point to one side of zero crossing.
The essence of the Crossover Displacement principle is the injection of an extra current, into the
output point of a conventional Class B amplifier. This is shown in Fig 3.
Fig 3: On the left is a conventional Class B output stage, showing drivers and bias voltage source.
At the right the stage has been modified by adding a displacer system that draws current from the
output and sinks it into the negative rail.
For convenience we have called the current-injection subsystem The Displacer. Similarly the upper
transistor is The Source, while the lower is The Sink. The displacement current does not directly
alter the voltage at the output - the output stage inherently has low output impedance, and this is
further lowered by the use of global negative feedback. What it does do is alter the pattern of
current flowing in the output devices. The displacement current in the version shown here is sunk to
V- from the output, rather than sourced from V+, so the crossover region is displaced downward
rather than being pulled upwards. This is arbitrary as the direction of displacement makes no
difference, either could be used.
