[sdiy] Expo converter ??

[jhaible]

> >What are the advantages/disadvantages  of the NPN/PNP expo converter
> >favored by ARP synths... vs the more well known NPN pair and opamp servo
> >???
> >
> >For what parameters are the NPN / PNP matched ???
>
> Harry --
>
> I've always wondered about that too.  Hope you get some good answers.  :-)
>
> The critical part about matched pairs seems to me to be the cancellation
of
> the exponential prefactors.  I've never understood how that can be made to
> work well for different transistor types.
>
> Hopefully somebody can tell us!


Big advantage: no servo loop required.

Ian, I think it was _you_ who discovered that the "standard" expo converter
with opamp and npn pair (or pnp pair) is prone to having a tiny bit
of HF oscillation that is hard to detect on a scope, but which might
cause a little bit of HF mistracking. I think you suggested to add some
extra compensation to get rid of this problem, but of course this will also
make the expo converter slower for audio rate modulation and such.
(from some old EN article, as I remember it.)

Now with a npn + pnp configuration, you get a fairly constant current
in your "reference" transistor just with a resistor, without an opamp
and servo loop. (As Don pointed out, it's not ideal. But it's close,
because the expo transistor just eats current from the reference transistor
with its base, not with its emitter. And the voltage change across the
resistor is small compared to the DC bias, so it's almost constant current.)

JH.