Current Sources and Voltage References

72

Using the BJT as a current source

then to determine the reference current, I

REF

in either NPN or PNP circuit:

Also, because the reference current is composed of both Q

1

’s collector current and

both base currents, then:

Therefore, the output current, I

C2

, is determined by:

The output resistance (r

o

) for this particular configuration is given by the internal col-

lector-to-emitter resistance (r

ce

) of Q

2

, derived from the BJT’s small-signal model:

where r

ce

is determined by:

As a working design example, the previous equations can be used to design two sim-

ple, practical 500-µA current sources in both NPN and PNP, with the following charac-

teristics: