August 2005
Intermediate to advanced
608 pages
18h 12m
English
Content preview from Current Sources and Voltage References
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Widlar current sources
Using BJTs to Create Current Sources
79
Therefore, the output current, I
C2
, is determined by:
The output resistance (r
o
) for this three-transistor current source is given by the inter-
nal collector-to-emitter resistance (r
ce
), derived from the BJT’s small-signal model:
where again r
ce
is determined by:
As another working design example, the previous equations can be used to design a
simple, practical NPN 250-µA current sink with the following characteristics: Transis-
tors = MMPQ3904 (2N3904 type), a general-purpose NPN quad amplifier, in a SOIC-
16 surface-mount package. The +VCC equals 9 volts; V
BE
equals 0.65 volt; and h
FE
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