Current Source Question

[>>>marjan<<<]

Hi Scott,


> Given the op-amp transistor type current source (b-e junction
> is part of the negative feedback loop), why would one want
> to use a Darlington over a single transistor?  I've noticed
> that the FatMan VCO uses the Darlington, but I've seen other
> designs that use a single transistor.
> 
> I know that the Darlington configuration causes the current
> gains of the transistors to multiply, giving much higher gain,
> but how is that helpful?  I'll make a naive guess: the expected
> input current can be very small, so the Darlington's additional
> gain puts this current in a more usable range for this VCO
> design?
>

Nope... IIRC Fatman uses PNPs, right? What you have in there is
V/I converter and you want to force current that's proportional
to converter input voltage to the ocs. That current is Ie, but
you can't do that, so you force Ic. And Ic is equal to Ie minus
Ib, so you see that Ib is a error generator here, so what you'll
do: you know that Ib (here you expect slow signals like CV)
is Ic/beta (or more correctly h21E), so you see larger beta is
you get smaller error, and darlington has high beta.


Cheers,


marjan