[sdiy] Linear Current Source Math Question

Scott Gravenhorst music.maker at gte.net
Sun Feb 23 17:33:08 CET 2003

Thanks to all who helped me with this.  I think I finally understand how
this cicuit works.  If I may:

Ie is really the sum of Ic and Ib, something not mentioned in A of E,
they probably thought this was obvious.  But because of the transistor's
current gain (beta, I believe), Ib will be a small fraction of Ie and Ic
will be the bulk.  This is what causes the small error in output current
and is why using a darlington further minimizes this error due to the
current gain being much larger, the product of the gains of the two

Further thinking here has me note that this curcuit requires that the
collector be connected to it's load, otherwise, the opamp will try to
push all of the current through the b-e junction of the transistor.  Most
of the times I've seen this circuit, it has no resistor to limit base
current at all.  In the FatMan, there's a 47 ohm in series with the base
and 1K in series with the emitter, but the total being 1047 means a
possible base current of about 11.5 milliamps.  So then I looked at the
data sheet I have for a 2N3906, and I see no maximum base current
parameter listed.  ??  There must be *some* limit, all diodes have a
maximum forward current...

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-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
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