Audio Engineering

202 Chapter 7

of the type shown in Figure 7.32. In this, if the two-input transistors are reasonably well
matched in current gain and if the value of R 3 is chosen to give an equal collector current
fl ow through both Q 1 and Q 2 , the DC offset between input and output will be negligible,
which will allow the circuit to be operated between symmetrical (  and  ) supply rails
over a frequency range extending from DC to 250 kHz or more.

Because of the improved rejection of odd harmonic distortion inherent in the input
“ push–pull ” layout, the THD due to this circuit, particularly at less than maximum output
voltage swing, can be extremely low, which probably forms the basis of the bulk of linear
amplifi er designs. However, further technical improvements are possible, which are
discussed next.

7.14 Constant-Current Sources and “Current Mirrors” ................................................

As mentioned earlier, the use of high-value collector load resistors in the interests of high
stage gain and low inherent distortion carries with it the penalty that the performance of
the amplifying device may be impaired by the low collector current levels that result from
this approach.

Advantage can, however, be taken of the very high output impedance of a junction
transistor, which is inherent in the type of collector current/supply voltage characteristics

15 V

15 V

0V 0V

C 1

10 μ

1K

100 K

R 1

100 K

R 3

56 K

R 2

4K7

R 6

1K5

R 4

R 5

Q 1 Q 2

Ein Eout

Figure 7.32 : Improved two-stage feedback amplifi er.