Audio Amplifi er Performance 321In principle, movements of the collector voltage on the inner device collectors should not
signifi cantly affect the output voltage, but in practice, Class-G is often considered to have
poorer linearity than Class-B because of glitching due to charge storage in commutation
diodesD 3 , D 4. However, if glitches occur they do so at moderate power, well displaced
from the crossover region, and so appear relatively infrequently with real signals.
An obvious extension of the Class-G principle is to increase the number of supply
voltages. Typically the limit is three. Power dissipation is further reduced and effi ciency
increased as the average voltage from which the output current is drawn is kept closer to
the minimum. The inner devices operate in Class-B/AB as before, and the middle devices
are in Class-C. The outer devices are also in Class-C, but conduct for even less of the time.
To the best of my knowledge, three-level Class-G amplifi ers have only been made in
shunt mode, as described later, probably because in series mode the cumulative voltage
drops become too great and compromise the effi ciency gains. The extra complexity is
signifi cant, as there are now six supply rails and at least six power devices, all of which
must carry the full output current. It seems most unlikely that this further reduction in
power consumption could ever be worthwhile for domestic hi-fi.
A closely related type of amplifi er is Class-G shunt.^10 Figure 10.4 shows the principle;
at low outputs, only Q 3 , Q 4 conduct, delivering power from the low-voltage rails.
Above a threshold set by Vbias3 andVbias4,D 1 or D 2 conduct and Q 6 , Q 8 turn on, drawing
current from the high-voltage rails, with D 3 , 4 protecting Q 3 , 4 against reverse bias. The
conduction periods of theQ 6 , Q 8 Class-C devices are variable, but inherently less than
50%. Normally the low-voltage section runs in Class-B to minimize dissipation. Such
shunt Class-G arrangements are often called “ commutating amplifi ers. ”
Some of the more powerful Class-G shunt PA amplifi ers have three sets of supply rails
to further reduce the average voltage drop between rail and output. This is very useful in
large PA amplifi ers.
10.4.9 Class-H
Class-H is once more basically Class-B, but with a method of dynamically boosting the
single supply rail (as opposed to switching to another one) in order to increase effi ciency.^12
The usual mechanism is a form of bootstrapping. Class-H is used occasionally to describe
Class-G as described earlier; this sort of confusion we can do without.