324 Chapter 10
voice-coil resistance with heat dissipation, but introduces others, such as control of
the cone resonance. Current amplifi ers therefore appear to be only of use with active
crossovers and velocity feedback from the cone.^18
It is relatively simple to design an amplifi er with any desired output impedance (even a
negative one) and so any compromise between voltage and current drive is attainable.
The snag is that loudspeakers are universally designed to be driven by voltage sources,
and higher amplifi er impedances demand tailoring to specifi c speaker types.^19
10.4.15 The Blomley Principle
The goal of preventing output transistors from turning off completely was introduced by
Peter Blomley in 1971^20 ; here the positive/negative splitting is done by circuitry ahead
of the output stage, which can then be designed so that a minimum idling current can
be separately set up in each output device. However, to the best of my knowledge this
approach has not yet achieved commercial exploitation.
10.4.16 Geometric Mean Class-AB
The classical explanations of Class-B operation assume that there is a fairly sharp transfer
of control of the output voltage between the two output devices, stemming from an
equally abrupt switch in conduction from one to the other. In practical audio amplifi er
stages this is indeed the case, but it is not an inescapable result of the basic principle.
Figure 10.5 shows a conventional output stage, with emitter resistors Re1, Re2 included
to increase quiescent-current stability and allow current sensing for overload protection;
to a large extent, these emitter resistances make classical Class-B what it is.
However, if the emitter resistors are omitted and the stage biased with two matched diode
junctions, then the diode and transistor junctions form atranslinear loop^21 around which
the junction voltages sum to zero. This links the two output transistor currents Ip , In in the
relationshipIn * Ip constant, which in op-amp practice is known as geometric-mean
Class-AB operation. This gives smoother changes in device current at the crossover point,
but this does not necessarily mean lower THD. Such techniques are not very practical for
discrete power amplifi ers; fi rst, in the absence of the very tight thermal coupling between
the four junctions that exists in an IC, the quiescent-current stability will be atrocious,
with thermal runaway and spontaneous combustion a near certainty. Second, the output
device bulk emitter resistance will probably give enough voltage drop to turn the other