722 Chapter 20
audio input signal and the feedback signal, which in the
language of operational amplifiers means it is the error
signal. For proper operation, the peak-to-peak value of
the error signal must always be less than the
peak-to-peak value of the triangular waveform at the
other comparator input. This combination of the oscil-
lator, shaper, and comparator forms a pulse-width
modulator. Although the output of the comparator
swings alternately positive and negative, the time spent
in these excursions is in general different. The output
pulse width of the modulator is proportional to the error
signal. This results from the fact that the peak-to-peak
value of the error signal is less than the peak-to-peak
value of the triangular waveform and that the polarity
from the comparator reverses depending on whether the
instantaneous value of the error signal is greater or less
than the instantaneous value of the triangular waveform.
In the variable duty cycle, positive and negative pulses
from the modulator toggle the active output devices
depicted as switches either full-on or full-off. Thus,
constant positive or negative voltage pulses are applied
to the low-pass filter and load for variable intervals of
time. The low-pass filter passes the time average value
of these pulses in the audio band, producing a voltage
across the load proportional to the instantaneous value
of the original incoming audio signal. The high effi-
ciency stems from the fact that there is no power dissi-
pated in an output device when it is nonconducting and
very little power dissipated when it is conducting in
saturation, or fully on, as even though the current
through the device is large, the voltage drop across the
device is very small. Unfortunately, the output devices
are highly specialized in that they must exhibit very fast
switching times with the absence of charge storage
effects. This alone pretty well rules out bipolar power
transistors capable of handling the large currents and
sustaining the high voltages involved. Vertically struc-
tured MOSFETs are usually employed as the switching
elements in this basic simple design. Two other draw-
backs to this simple class D structure are the possibility
of radio-frequency generation and the difficulty of opti-
mizing the low-pass filter for use with more than one
value of load resistance. Loudspeakers are hardly
constant impedance devices, much less constant resis-
tance devices. The most recent designs in switching
amplifiers have addressed these problems.
20.3.3 High-Power Analog Amplifiers
Crown International, principally through the work of
Gerald Stanley, is responsible for a continuing series of
technical innovations in high-power analog amplifiers
involving class AB + B that was first introduced by
Crown. Though not changing the basic efficiency of this
configuration, the innovations have led to high-powered
designs up to several thousands of watts in individual
units. The innovations involve output stage topology,
amplifier cooling, and power transistor safe operating
area assessment as well as control. At the heart of these
innovations is an output stage topology that is a full
bridge configuration with one output terminal always at
ground potential. A simplified view of this configura-
tion is given in Fig. 20-28.In Fig. 20-28 the transistors at points 1, 2, 3, and 4
represent composites of several NPN and PNP bipolar
power transistors constituting AB + B arms of the
bridge. The NPN transistors at 1 are the positive voltage
output stage while the PNP transistors at 4 are the
complementary negative output voltage stage. The NPN
transistors at 2 are the positive bridge balance output
stage and the PNP transistors at 3 are the complemen-
tary negative bridge balance output stage. When a posi-
tive output is required, the transistors at 1 conduct
connecting the left end of the load to the positive
terminal of the supply and the transistors at 3 conduct
connecting the negative terminal of the supply to
ground. When a negative output is required, the transis-
tors at 2 conduct connecting the positive terminal of the
supply to ground and the transistors at 4 conduct
connecting the left end of the load to the negative
terminal of the supply. The control, bias, and driving
circuitry must ensure that at quiescence there is no
voltage drop across the load and, when delivering a
signal, that the voltage division is correct across the
diagonally opposite pairs of transistors that are driven
toward non conduction. This arrangement offers two
very distinct advantages as compared with a conven-
tional complementary symmetry output stage: it
requires only a single power supply voltage VS to
produce a peak-to-peak voltage swing across the load
equal to 2VS and it simultaneously halves the sustaining
voltage requirements of the output devices.Figure 20-28. Crown-grounded full bridge topology.Load23Power
supply
Vs+
14