the circuit that saves two components; in this case, the SPKR forms
part of Q1's collector load, and
directly bootstraps R1.
Figure 15. Simple 1 watt amplifier.
The easiest way to build a class-AB audio amplifier is to do so using
one of the many readily-available
audio ICs of this type. In some cases,
however, particularly when making
'one off' projects, it may be cheaper
or more convenient to use a discrete
transistor design, such as one of
those shown in Figures 15 or 16.
Figure 15 shows a simple class-
AB amplifier that can typically drive 1W into a 3Ω speaker. Here, common-emitter amplifier Q1 uses collector load
LS1-R1-D1-RV2, and drives the Q2-Q3 complementary
emitter follower stage. The amplifier's output is fed (via
C2) to the LS1-R1 junction, thus providing a low impedance drive to the loudspeaker and simultaneously bootstrapping the R1 value so that the circuit gives high voltage gain. The output is also fed back to Q1 base via R4,
thus providing base bias via a negative feedback loop. In
use, RV1 should be trimmed to give minimal audible
cross-over distortion consistent with low quiescent current consumption (typically in the range 10 mA to 15
Figure 16 shows a rather more complex audio power
amplifier that can deliver about 10W into an 8Ω load when
powered from a 30 V supply.
This circuit uses high-gain quasi-complementary output stages (Q3 to Q6) and uses an adjustable amplifier
diode (Q1) as an output biasing device. The Q2 common
emitter amplifier stage has its main load resistor (R2) bootstrapped via C2, and is DC biased via R3, which should set
the quiescent output voltage at about half-supply value (if
not, alter the R3 value). The upper frequency response of
the amplifier is restricted via C3, to enhance circuit stability, and C5-R8 are wired as a Zobel network across the
output of the amplifier to further enhance the stability.
In use, the amplifier should be initially set up in the
way already described for the Figure 15
back via R1-R2, thus enabling the
circuit to be used over a wide range
of supply voltages.
The feedback resistors can be
AC-decoupled (as shown) via C2 to
give increased gain and input impedance, at the expense of increased distortion. Q1 can be a Darlington type,
if a very high input impedance is
Figure 18 shows an alternative
configuration of driver stage. This
design uses series DC and AC feedback, and gives greater gain and
input impedance than the basic
Figure 7 circuit, but uses two transistors of opposite polarities.
Finally, to complete this look at
audio power amplifiers, Figure 19 shows a circuit that has
direct-coupled ground-referenced inputs and outputs, and
uses split power supplies. It has a long-tailed pair input
stage, and the input and output both center on zero volts
if R1 and R4 have equal values. The circuit can be used
with a single ended power supply by grounding one supply
line and using AC coupling of the input and output
signals. This basic circuit forms the basis of many IC
Figure 16. 10 watt audio amplifier.
In the basic Figure 7 circuit, the Q1
driver stage uses parallel DC and AC voltage feedback via potential divider network
R2-R3. This circuit is simple and stable, but
suffers from fairly low gain and very low
input resistance, and can be used over
only a very limited range of power supply
voltages. A simple variation of this circuit
is shown in Figure 17. It uses current feed-
Figure 17. Driver stage with
decoupled parallel DC feedback.
Figure 18. Driver stage with
series DC feedback.