Audio Engineering

200 Chapter 7

Fortunately, because the output of the compact disc player is at a high level, typically
2 V rms, and requires no signal or frequency response conditioning prior to use, the gain
control can be sited directly at the input of the preamp. Nevertheless, this still leaves the
possibility that signal peaks may occur during use that are some 56 greater than the
mean program level, with the consequence of the following amplifi er stages being driven
hard into overload.

This has refocused attention on the design of solid-state voltage amplifi er stages having a
high possible output voltage swing and upon power amplifi ers that either have very high
peak output power ratings or more graceful overload responses.

7.13 Voltage Amplifi er Design ......................................................................................

The sources of nonlinearity in bipolar junction transistors have already been referred
to, in respect to the infl uence of collector load impedance, and push–pull symmetry in
reducing harmonic distortion. An additional factor with bipolar junction devices is the
external impedance in the base circuit, as the principal nonlinearity in a bipolar device
is that due to its input voltage/output current characteristics. If the device is driven from
a high impedance source, its linearity will be substantially greater, since it is operating
under conditions of current drive.

This leads to the good relative performance of the simple, two-stage, bipolar transistor
circuit of Figure 7.30 in that the input transistor, Q 1 , is only required to deliver a very
small voltage drive signal to the base of Q 2 so that the signal distortion due to Q 1 will

Ein Eout

0V

V

C 1

C 2

R 1

R (^2) R
3
R 5
R 4
Q 2
Q 1
Figure 7.30 : A two-stage transistor voltage amplifi er.