Audio Engineering

Preamplifi ers and Input Signals 201

be low. Q 2 , however, which is required to develop a much larger output voltage swing,
with a much greater potential signal nonlinearity, is driven from a relatively high source
impedance, composed of the output impedance ofQ 1 , which is very high indeed, in
parallel with the base-emitter resistor, R 4. R 1 , R 2 , and R 3 / C 2 are employed to stabilize the
DC working conditions of the circuit.

Normally, this circuit is elaborated somewhat to include both DC and AC negative
feedback from the collector ofQ 2 to the emitter of Q 1 , as shown in the practical amplifi er
circuit of Figure 7.31.

This is capable of delivering a 14-V p-p output swing, at a gain of 100, and a bandwidth
of 15 Hz to 250 kHz, at  3-dB points; largely determined by the value of C 2 and the
output capacitances, with a THD fi gure of better that 0.01% at 1 kHz.

The practical drawbacks of this circuit relate to the relatively low value necessary for
R 3 —with the consequent large value necessary for C 2 if a good LF response is desired,
and the DC offset between point ‘ X ’ and the output, due to the base-emitter junction
potential ofQ 1 , and the DC voltage drop along R 5 , which makes this circuit relatively
unsuitable in DC amplifi er applications.

An improved version of this simple two-stage amplifi er circuit is shown in Figure 7.32 , in
which the single input transistor has been replaced by a “ long-tailed pair ” confi guration

Ein Eout

0V

15 V

C 1

C 2

0.47μ

R 2

12 K

R 1

10 K

R 4

6K8

R 3

100 R R 6

1K5

‘X’ R 5

Gain 100

Q 2

Q 1

10 K

Figure 7.31 : A practical two-transistor feedback amplifi er.