Audio Engineering

Audio Amplifi ers 289

Q 2 will be relatively low because of the high effective source resistance seen by the Q 2
base. A similar low level of distortion is given by the amplifi er layout (bipolar transistor
with constant current load) described earlier because of the very high stage gain of the
amplifying transistor and the consequent utilization of only a very small portion of its
Ic / Vb curve.

9.4 Basic Junction Transistor Circuit Confi gurations .................................................

As in the case of the thermionic valve, there are a number of layouts, in addition to the
simple single transistor amplifi er shown in Figure 9.4 or the two-stage amplifi er of Figure
9.5 , that can be used to provide a voltage gain or to perform an impedance transformation
function. There is, for example, the grounded base layout of Figure 9.6 , which has a very
low input impedance, a high output impedance, and a very good HF response. This circuit
is far from being only of academic interest in the audio fi eld in that it can provide, for
example, a very effective low input impedance amplifi er circuit for a moving coil
pick-up cartridge. I showed a circuit of this type, dating from about 1980, in an earlier
book ( Audio Electronics , Newnes, 1995, p. 133).

R 3 R (^6) C 3
Q 2
Q 1
Output
R 5
R 4
R 2
R 1
R 7
C 2
C 1
0V 0V
Input
Vcc
Figure 9.5 : NPN/PNP feedback pair.