[sdiy] Expo converter tempco placement

[Neil Johnson]

Justin Owen wrote:
 >> Can anyone give any insight into the differences between the
 >> two main locations for tempco placement in expo converter circuits?
 >>
 >> Seems like the schems I have are divided fairly evenly
 >> between those who use a tempco (generally 2K) in the feedback
 >> loop of the CV scaling op amp that feeds into the base of the
 >> expo pair and those that use a tempco (generally 1K) as part
 >> of a voltage divider at the output of the CV scaling op amp.
 >>
 >> Secondly, if you were using a tempco int he feedback loop of
 >> the CV scaling op amp - would there be any problems using
 >> that tempco in parallel or series with another standard R to
 >> achieve a certain R value?
 >>
 >> Thoughts?

I think there are various reasons.  First that comes to mind is that 
early tempco resistors tended to be wire-wound resistors, with a 
correspondingly high inductance.  You don't want an inductor in the 
feedback circuit of an op-amp unless you want to make an oscillator!

Another reason is simply down to engineering.  If you can get 2k tempcos 
easier/faster/cheaper than 1k then, notwithstanding the inductance 
issue, you'd design the circuit around the available components.

Finally, there's the blinkered-vision explanation.  If a designer has 
always thought along one track then they'll adapt all problems to fit 
that one solution...

David G Dixon wrote:
> The answer to your question undoubtedly lies in mathematical modeling.  I
> haven't done it for tempcos as I don't use them -- I use the famous 2164
> expo and tempco scheme in my VCOs (and sometimes in VCFs), and I've modeled
> the crap outta that.

I guess if your model includes the parasitic inductance of wirewound 
tempcos, and/or pricing/delivery/availability of said component, then 
yes modelling might provide one answer.

Neil
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