Handbook for Sound Engineers

Amplifier Design 721

stages in classes A, AB, or AB + B have poor output
power efficiencies and/or appreciable internal power
dissipation at quiescence. Class A dissipates its rated
power internally at quiescence and only approaches a
power efficiency of 50% when delivering its full output
power. A pure class B stage would have zero internal
quiescent power dissipation and a power efficiency
approaching 78.5% only at full output while being
plagued with unacceptable crossover distortion. Class
AB solves the crossover distortion problem while intro-
ducing some internal quiescent power dissipation along
with a smaller output power efficiency than pure class
B. Class AB + B retains the low quiescent power dissi-
pation of class AB and approaches the power efficiency
of pure class B when operated at full output. Such
amplifiers most often employ conventional power
supplies consisting of a power transformer whose
primary is energized from the ac mains and whose
secondary is applied to a full wave bridge rectifier
connected to a capacitor input filter. This arrangement
can also yield bipolar dc supplies when the secondary of
the transformer is center tapped and two capacitor filters
are employed. Large values of capacitance must be
employed, as the fundamental ripple frequency is
120 Hz. Such supplies inherently suffer from poor
voltage regulation and demand excessive root mean
square (rms) current draw from the ac mains as their
power factors fall in the range of about 0.6 to 0.7. These
power amplifiers most often employ complementary
symmetry bipolar junction transistors and ordinarily
have power limitations that are dictated by the voltage
breakdown properties of the active devices when
conventionally employed. Some clever schemes for
surmounting this limitation will be discussed subse-
quently. The linear power amplifier designs discussed
above might well be referred to as being analog high-
power amplifiers.
The paradigm shift necessary to achieve even higher
output power and performance has affected the design
philosophy of both the amplifier power supply as well
as that of the power amplifier itself. Rather than
employing continuous or analog techniques, the really
high-power units now employ switching techniques in
both the power supply and in the amplifier circuitry.
The concept of having a pair of output devices, one
positive and one negative, each alternately being either
full on or full off, is not new. Audio power amplifiers
employing solid-state active elements operated under
what is termed class D have been around since about

1970. The less than spectacular performance of the
      early efforts was not a result of the failure in operating
      principle but rather the result of the shortcomings of the

available active elements involved. These shortcomings

have been diminished in modern power MOSFETs and

IGBTs to the point that switching amplifiers are not

only viable but also desirable in high-power applica-

tions. Additionally, switching topologies exceeding the

properties of the classic class D have also been evolved.

These will be discussed in a subsequent section.

Class D switching amplifiers have the desirable

property that the output efficiency can approach 100%

independent of the operating power level. This results

because the active output devices ideally are either fully

conducting or fully not conducting—i.e., they act as

switches. This is best explained by reference to Fig.

20-26, which is a functional diagram of a class D ampli-

fier, and Fig. 20-27, which displays pulse width modu-

lation waveforms.

In Fig. 20-26, the continuously running oscillator

operates at a fixed amplitude and with a fixed frequency

usually between 200 kHz and 500 kHz. The shaper

converts this signal into a triangular waveform at the

same fundamental frequency and supplies the resulting

waveform as one input to the comparator. The other

input to the comparator is the summing node of the

Figure 20-26. Functional diagram of a class D amplifier.

Figure 20-27. Pulse width modulation waveforms.

Oscillator Shaper

Comparator Toggle LP filter

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