[sdiy] Linear Current Source Math Question

[René Schmitz]

Hi Scott and all,

> I've been reading in chapter 4 of "The Art of Electronics"
> about linear current sources that use a bipolar transistor
> (b-e junction) in the feedback loop of an op-amp.  It states
> that the emitter current is (Vcc-Vin)/R and that Iout is the
> same as Ie.  Is it really that simple that one needs only
> calculate the current in the feedback loop and that current
> will be the output current (Ic)?

It is based on the assumption that base current can be
neglected. I.e. Ic = Ie. That is why you see jFETs or
Darlington BJTs used in that kind of circuit.


Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159