726 Chapter 20
truly represents the expression, “thinking outside of the
box.”
The innovation put forth by the Swedish firm
Lab.gruppen has resulted in the class TD, which is a
tracking class D amplifier. The objective was to produce
a very powerful audio power amplifier in which the
entire signal path is that of an analog-type amplifier.
This is accomplished by feeding the original audio
signal to both an analog amplifier as well as a classic
class D amplifier in parallel. The class D amplifier is
structured in the form of a half-bridge as depicted in Fig.
20-26 except that instead of having a loudspeaker as the
load on this amplifier, it is the output stage of the analog
amplifier that serves as a load of the class D amplifier.
The effect of this arrangement is to produce a class AB
- B amplifier that has continuously varying positive and
negative rail supplies that track the needs of the audio
signal of the moment. The output stage of the analog
amplifier operates with high efficiency as the voltage
across the output devices at any instant exceeds the
amplifier output voltage only by the amount necessary
to insure active operation of the devices. This topology
is illustrated in simplified form in Fig. 20-35. The
overall system is powered by a switch-mode power
supply. This design is said to retain the distortion and
noise characteristics of an analog amplifier while closely
approaching the output efficiency of a class D amplifier.
In Fig. 20-35 T 1 and T 4 represent several parallel N
channel power MOSFETs that represent the switching
elements of a class D amplifier each with an associated
low-pass filter typical of class D operation. Sand-
wiched in between these two parts of the half-bridge are
the output devices of the analog amplifier that constitute
the load for the class D amplifier. T 2 and T 3 represent
several parallel complementary NPN and PNP bipolar
power transistors connected as emitter followers that
directly drive the loudspeaker load.20.3.6 Class D in Full BloomInstead of trying to skirt the problems associated with
class D, Gerald Stanley and his design group at Crown
Audio, Inc. have faced them head-on and over a period
of time have evolved wide-ranging innovative solutions
from the ac power cord to the amplifier output
terminals.
The innovations begin in the switch-mode power
supply that can operate, without any necessity for
internal changes, from line sources ranging from 85 Vac
to 277 Vac 50–60 Hz. Full power is obtained for supply
voltages ranging from 120 Vac to 240 Vac. The bulk dc
supply is a full-wave bridge rectifier and capacitor filter
combination. This is followed by a unique dc-to-dc
converter consisting of two half-bridges that are oper-
ated in a novel way in that the high-frequency switching
signals to the two halves of the converter are phase shift
modulated. This mode of operation impresses across the
primary of the high-frequency step-up transformer an
alternating square wave voltage whose duty cycle can
be varied from zero to 50%. The rail voltages are
derived through full wave rectification and filtering of
the secondary high-frequency voltage from the step-up
transformer. The positive and negative rail voltages can
range from zero to some maximum value as the duty
cycle of the primary square wave ranges from zero to
50%. A control loop constantly compares the rail
voltage and current with reference values and adjusts
the duty cycle in such a fashion that voltage regulation
of the rail supply output is maintained for both changes
in load as well as raw supply and all the while main-
taining an overall power factor close to 0.95. Protection
mechanisms are included to handle internal amplifier or
overload problems.
The output stage is a far cry from the classic class D
design as it employs an innovative topology as well as
an advanced form of pulse width modulation. The
output stage topology is termed BCA, standing for
balanced current amplifier. Fig. 20-36 is a bare bones
Figure 20-35. Class TD or tracking class D amplifier. illustration of the BCA output stage.
Positive supplyT 1T 2T 3T 4Negative supplyAmplifier outBias and drive from
complementary
class AB stage