Cambridge Audio Azur 840A White Paper page 12

White paper
attenuated by the very low impedance of the basic power amplifier and its global negative feedback,
so complex control circuitry is not required. The efficiency of this configuration is greater, because
the output current of the displacer does not increase as the output moves more positive. The
voltage across the current source increases and so its dissipation is still increased - but by a
smaller amount. Likewise, the source transistor is passing less current on positive excursions so
its power dissipation is less.
Push-pull displacement
Having moved from a simple resistor displacer idea to a constant-current source implementation,
we then considered whether we could even modulate the current source. Our next step was to
move from a constant current to a voltage-controlled current source (VCIS) whose output is
modulated by the signal to further improve efficiency. The most straightforward way to do this is to
make the displacement current proportional to the output voltage. Thus, if the displacement current
is 1 Amp with the output at quiescent at 0V, it is set to increase to 2 Amps with the output fully
negative, and to reduce to zero with the output fully positive. The displacer current is set by the
equation:
Id = Iq (1 - Vout/Vrail)
where Iq is the quiescent displacement current (i.e. with the output at 0V) and Vrail is the bottom
rail voltage, a constant which must be inserted as a positive number to make the arithmetic work.
Depending on the design of the VCIS, a scaling factor is required to drive it correctly; see Fig 7.
Since an inversion is also necessary to get the correct mode of operation, active controlling circuitry
is necessary.