[sdiy] Temperature stable lin-exp converter

[Jay Schwichtenberg]

Tim,

I don't think that you can just add the compensation into the expo input.
Looks like you have to multiply, possibly by the inverse of the temp sensor.
If my notes are right:

Io = Iref * (e ^ ((q * Vin) / (k * T)))

So to me it looks like you need to multiply Vin by (k * T) / q  instead of
adding.

What I don't know is (k * T) / q inverted from what you'd get from the sense
transistor?

I have a couple of thoughts on how to do this divider thing. One is to look
into an OTA in the feedback path of the expo summming amp being controlled
by q / (k * T). The other is to use an OTA as the summing amp taking the
compensation and run it into Id (diode bias in LM13X00, CA3280). There has
been talk about using Id to make a divider but it isn't speced well for the
LM13X00 and CA3280.

Jay

> -----Original Message-----
> From: owner-synth-diy at dropmix.xs4all.nl
> [mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of Tim Ressel
> Sent: Thursday, May 30, 2002 12:10 AM
> To: SynthDIYmailinglist
> Subject: [sdiy] Temperature stable lin-exp converter
>
>
> Yo,
>
> A while back someone posted an EDN article showing a
> temperature-compensated expo converter. The design
> used a third matched tranny to generate a reference
> voltage. The circuit as shown wasn't readily adaptable
> to a VCO, but it got me to thinking. If one were to
> take a third transistor and run a constant current
> through it, the Vbe would vary with temperature. You
> could then amplify the Vbe to a useful level and
> polarity, and inject it into the summing node of the
> expo converter input. I think this would give the
> needed temp compensation. What do you think folks?
>
> --TR
>