[sdiy] Expo converter tempco placement

[René Schmitz]

Hi Justin,

Am 10.07.2014 11:35, schrieb Justin Owen:
> From: René Schmitz [uzs159 at uni-bonn.de]
>
> Hi René,
>
>> I'd like too add, that if you start to design your scaling summer with a
> 100k input resistance, then using a 1k in the feedback doesn't leave you
> with enough drive for the ~18mV per octave required at the base of the
> expo pair.
>
> (At the risk of embarrassing myself...) Can I ask how (or why...) this '~18mV per octave' is a standard that's quoted so often?

As others have said: The 18mV comes from log(2)*kT/q, i.e. the base 
voltage difference for doubling the current. k is the Boltzman constant, 
and q is the electron charge. T is the absolute temperature.

> Wouldn't the current output requirement potentially vary depending on the VCO circuit that the current source was driving?

Yes it does. But we have a reference current in the circuit to which we 
apply a multiplicative factor. And this factor in turn depends 
exponentially to the voltage. You can choose the ref current according 
to the requirements of your CCO core.

Here's more reading: http://www.schmitzbits.de/expo_tutorial/index.html

Cheers,
  René


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