204 Chapter 7
source, independent of supply lines or reference potentials, and which can be used at
either end of the load chain.
The current output from this type of circuit is controlled by the value chosen for R 1 ,
and this type of constant current source may be constructed using almost any available
junction FET, provided that the voltage drop across the FET drain-gate junction does not
exceed the breakdown voltage of the device. This type of constant current source is also
available as small, plastic-encapsulated, two-lead devices, at a relatively low cost, and
with a range of specifi ed output currents.
All of these constant current circuit layouts share the common small disadvantage that
they will not perform very well at low voltages across the current source element. In the
case of Figures 7.33 and 7.34 , the lowest practicable operating potential will be about 1 V.
The circuit of Figure 7.35 may require, perhaps, 2–3 V, and this factor must be considered
in circuit performance calculations.
The “ boot-strapped ” load resistor arrangement shown in Figure 7.36 , and commonly used
in earlier designs of audio amplifi er to improve the linearity of the last class ‘ A ’ amplifi er
stage ( Q 1 ), effectively multiplies the resistance value of R 2 by the gain which Q 2 would be
deemed to have if operated as a common-emitter amplifi er with a collector load of R 3 in
parallel withR 1.
This arrangement is the best confi guration practicable in terms of available rms output
voltage swing as compared with conventional constant current sources, but has fallen into
R 1Q 2Q 1R 20VV IcoutputFigure 7.34 : Two-transistor constant current source.