Audio Engineering

206 Chapter 7

Because of this, there is a marked preference on the part of IC design engineers for the
use of circuit layouts known as “ current mirrors, ” of which a simple form is shown in
Figure 7.37.

7.14.1 IC Solutions

These are not true constant current sources in that they are only capable of generating an
output current ( Lout ) that is a close equivalent of the input or drive current ( Lin ). However,
the output impedance is very high, and if the drive current is held to a constant value, the
output current will also remain constant.

A frequently found elaboration of this circuit, which offers improvements in respect to
output impedance and the closeness of equivalence of the drive and output currents, is
shown in Figure 7.38. Like the junction FET-based constant current source, these current
mirror devices are available as discrete, plastic-encapsulated, three-lead components,

Iin

Q 1 Q 2

0V

Iout

Figure 7.37 : Simple form of a current mirror.

Iin

V

Q 1

Q 2

Q 3

0V

Iout

Figure 7.38 : Improved form of a current mirror.