Audio Engineering

Audio Amplifi ers 309

which is labelediref. This particular action is called a current mirror, and several further
versions of these are shown in Figure 9.25.

9.11.2 Current Mirror Layouts

Current mirror (CM) layouts allow, for example, the output currents from a long-tailed
pair to be combined, which increases the gain from this circuit. In the version shown in
Figure 9.25(a) , two matched transistors are connected with their bases in parallel so that
the current fl ow through Q 2 will generate a base-emitter voltage drop that will be precisely
that which is needed to causeQ 1 to pass the same current. If any doubt exists about the
similarity of the characteristics of the two transistors, as might reasonably be the case for
randomly chosen devices, the equality of the two currents can be assisted by the inclusion
of equal value emitter resistors ( R 1 , R 2 ) as shown in Figure 9.25(b). For the perfectionist,
an improved three-transistor current mirror layout is shown in Figure 9.25(c). Commonly
used circuit symbols for these devices are shown in Figures 9.25(d) and 9.25(e).

9.12 Circuit Oddments

Several circuit modules have found their way into amplifi er circuit design, and some
of the more common of these are shown in Figure 9.26. Both the DC bootstrap, shown

Figure 9.25 : Current mirror circuits.

(a)

Q 2

0V

iout iin

Q 1

(b)

R 1 R 2

Q 2

0V

iout iref

Q 1

(c)

Q 2

Q 3

0V

iout V iref

Q 1

Symbols

(d)

Constant

current source

(e)

Current

mirror