[sdiy] Expo converter tempco placement

[Neil Johnson]

Hi,

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

No - you're driving an expo converter, where the output current
doubles in response to a linear step on the input IRRESPECTIVE of what
that current is (within operational limits).  And all silicon BJTs
operate with the same equation thanks to Ebers-Moll:

Ie = Is (exp(Vbe/Vt)-1)

where Vt = kT/q

When you expand it all out, you get down to the "~18mV/octave".

Neil
--
http://www.njohnson.co.uk