COOKBOOK — PART 7
Ray Marston describes a variety of practical transistor
audio power amplifier and 'accessory' circuits in this
month's penultimate installment of an eight-part series.
by Ray Marston
One of the most popular applications of transistors
is in audio power amplifiers. This month we
describe the operating principles of various circuits of this type and present a selection of practical
audio power amplifier circuit designs. The installment
concludes by presenting a practical 'scratch and rumble'
filter circuit, which can be used to eliminate these
unwanted sounds when playing old-fashioned
records/discs through any type of audio power amplifier
POWER AMPLIFIER BASICS
A transistor power amplifier's job is that of converting
a medium-level medium-impedance AC input signal into
a high-level low-impedance state suitable for driving a low-impedance external load. This action
can be achieved by operating the tran-sistor(s) in either of two basic modes,
known as 'class-A' or 'class-B.'
Figure 1(a) shows a basic class-A
Figure 2. General-purpose high-gain
low-power audio amplifier.
NUTS & VOLTS
Figure 3. Basic circuit (a) and transfer
characteristics (b) of a class-B amplifier.
audio amplifier circuit; Q1 is a common-emitter amplifier
with a loudspeaker collector load, and is so biased that its
collector current has a quiescent value halfway between
the desired maximum and minimum swings of output
current, as shown in Figure 1(b), so that maximal low-distortion output signal swings can be obtained. The circuit
consumes a high quiescent current, and is relatively inefficient; 'efficiency' is the ratio of AC power feeding into the
load, compared with the DC power consumed by the circuit, and at maximum output power is typically about
40%, falling to 4% at one tenth of maximum output, etc.
Figure 2 shows an example of a low-power (up to a
few dozen milliwatts) high-gain general-purpose class-A
amplifier that draws a quiescent current of about 20 mA
and is suitable for driving a medium impedance (greater
than 65Ω) loudspeaker or headset. Q1 and Q2 are wired
as direct-coupled common-emitter amplifiers, and give an overall
voltage gain of about 80 dB.
Q1's base bias is derived (via R2)
from Q2's emitter, which is
decoupled via C3 and thus 'fol-lows' the mean collector voltage
of Q1. The bias is thus stabilized
by DC negative feedback. Input
pot RV1 acts as the circuit's volume control.
A basic class-B amplifier
consists of a pair of transistors,
driven in anti-phase but driving a
load, as shown in
Figure 3(a). In
design, Q1 and
Q2 are wired in
the common-emitter mode and
drive the loudspeaker via
push-pull transformer T2, and the
drive is obtained
Figure 1. Basic circuit (a) and transfer characteristics
(b) of a class-A amplifier.