White paper
Efficiency
The Crossover Displacement technique is highly effective but does increase the total power
dissipated in the output stage. The dissipation in the Source transistor is increased by the
displacement current flowing through it, while in the sink transistor it is unchanged. There is also
the additional dissipation in the Displacer itself, which is mounted on the same heat sink as the
main output devices.
Table 1 shows the calculated efficiency for the various classes/types of amplifier. The calculations
were not the usual simple theoretical ones that ignore voltage drops in emitter resistors –
transistor saturation voltages and so on – but a lengthy series of SPICE simulations of complete
output stage circuits. The effects of transistor non-linearity and so on are taken into account. The
results are therefore as real as extensive calculations can make them.
For comparison, the ‘classical’ calculations for Class B give a full power efficiency of 78%, but more
detailed simulations show that it is only 73% when typical losses are included. The output stages
were simulated using +/-50V rails, giving a maximum power of about 135W into 8 Ohms. XD
displacement currents were set to give a transition from Class A to B at 5W. All emitter resistors
were 0.1 Ohm.
Full output
Half power
1/10 power
Class B
74%
54%
23%
Class XD push-pull
66%
46%
14%
Class XD constant
57%
39%
11%
Class A
43%
23%
4%
Table 1. Efficiency of amplifier types
Here we have demonstrated that there is some penalty in efficiency when crossover displacement
is used but when compared to Class A a notable improvement is evident.
