Current Source Question

[Scott Gravenhorst]

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?

Thanks as usual...

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