Audio Engineering

(Barry) #1

308 Chapter 9


layouts is that shown in Figure 9.24(a) , in which the transistor, Q 1 , is fed with a fi xed
base voltage—in this case derived from a zener or avalanche diode, although any suitable
voltage source will serve—and the current through Q 1 is constrained by the value chosen
forR 1 in that if it grows too large, the base-emitter voltage of Q 1 will diminish and Q 1
output current will fall. Designers seeking economy of components will frequently
operate several current source transistors and their associated emitter resistors (as Q 1 / R 1 )
from the same reference voltage source.


In the somewhat preferable layout shown in Figure 9.24(b) , a second transistor, Q 2 , is
used to monitor the voltage developed across R 1 due to the current through Q 1 ; when this
exceeds the base emitter turn-on potential (about 0.6 V), Q 2 will conduct and will steal
the base current toQ 1 provided from Vref through R 2. In the very simple layout shown in
Figure 9.24(c) , advantage is taken of the fact that the forward potential of a P-N junction
diode, for any given junction temperature, will depend on the current fl ow through it. This
means that if the base-emitter area and doping characteristics ofQ 1 are the same as those
for the P-N junction inD 1 (which would, obviously, be easy to arrange in the manufacture
of ICs), then the current ( iout ) through Q 1 will be caused to mimic that fl owing through R 1 ,


Figure 9.24 : Constant current sources.

(a)

R 1

ZD1

R 2

0V

iout Vref

Q 1

(b)

R 1

R 2

0V

iout Vref

Q 2

Q 1

(c)

R 1

D 1

0V

iout iref

Q 1
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