Amplifier Design 723The composite transistors at 1 and 2 are mounted
directly without electrical insulators in order to ensure
good thermal contact to one heat sink that itself is elec-
trically insulated. The composite transistors 3 and 4 are
mounted directly to a second electrically insulated heat
sink. The heat sinks themselves, rather than the usual
heavy metal extrusions, involve many thin metal fins
such as those employed in refrigeration and air condi-
tioning technology or automobile radiators. This greatly
increases the surface area exposed to a forced air stream
for cooling purposes and allows the power devices to
operate with lower junction temperatures than would
otherwise be the case.
The final contributor to high-power operation of this
circuitry involves what amounts to a small, dedicated
analog computer that emulates the operation of the
output stage consistent with the operating conditions
that exist in the amplifier in real time. This allows the
output stage control circuitry to restrict the drive to the
output such that the output devices remain always in
what is the safe operating area of the moment.
Another approach to high-power operation, which
also offers an efficiency advantage over class AB while
basically employing AB circuitry, involves changing the
supply voltage from a nominal value to a higher value
when larger output swings are called for. This can be
accomplished with a fixed output stage configuration
powered by a variable switching power supply, by
switching the supply voltage from a fixed nominal value
and a fixed higher value, or by using two sets of output
devices with one set powered by a nominal voltage
supply and the other set powered by a higher voltage
supply and switching between the sets of output
devices. This last method of operation is termed class G.
QSC Audio was the first manufacturer to introduce such
an amplifier for employment in the sound reinforcement
industry. This design can still fall in the analog category
as the switching between the sets of output devices is
purely accomplished by analog techniques as devised
by Pat Quilter of QSC. Fig. 20-29 is a simplified sche-
matic of the positive half of a complementary-symmetry
class G output stage that illustrates the principal of
operation.
For small-amplitude drive signals, the emitter
follower in the lower part of Fig. 20-29 is powered by
the supply with voltage V and operates as a normal AB
stage consistent with this value of the supply voltage.
The upper transistor is not forward biased and is not
conducting so the supply with 2V plays no role. For
large amplitudes of the drive signal, when the value of
the drive signal approaches V, the lower transistor
approaches saturation and sufficient forward bias is
applied to the upper transistor to bring it into conduc-
tion, thus bringing into play the supply of voltage 2V.
For even greater values of drive voltage, the supply of
voltage V is disconnected by the diode on the right
because this diode is now reverse biased and the lower
transistor is in full saturation. Under this condition, the
upper transistor is operating as an emitter follower with
a supply of voltage 2V. The operation of the negative
half of the output stage is the same except all polarities
are reversed.
A further step in this same direction also employed
by QSC as well as other manufacturers is that of the
class H topology for the power stage. Instead of two sets
of devices permanently connected to two different
voltage supplies on the positive half as well as the
complementary negative half of the output stage, class
H employs a single set of devices on the positive half
and a complementary set on the negative half. The
supply voltages to these devices are switched to
different values according to the requirements of the
audio signal at the moment. Such an arrangement is
illustrated in Fig. 20-30.
In Fig. 20-30 the positive and negative class H
output stages usually consist of paralleled bipolar power
transistors and an associated driver with the negative
half being complementary with the positive half. The
class of operation of each half is basically that of class
B except for the very lowest signal levels. Efficiency is
improved by maintaining as low as possible voltage
drop across the active devices when they are delivering
current to the load. Consider for the moment that the
output signal is swinging positive and its instantaneous
value is approaching the fixed value of +V. Switch S+ is
a voltage comparator–operated switch. This switch is
closed when the output signal exceeds a fixed positive
reference voltage whose value is chosen so that the
stage never goes into saturation. With S+ closed, the
output signal can now increase if required up to slightly
less than the rail limit of +2V whereas the voltage across
the active transistors themselves is always less than +V.Figure 20-29. Simplified positive half of a class G output
stage.++ +Bias and drive
LoadVV V
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