[sdiy] Temperature stable lin-exp converter with a CA3086 or CA3046

[Ian Fritz]

At 08:47 PM 5/27/2002, patchell wrote:
>     The way I see the issue of where to put the Tempco resistor is 
> basically, six
>of one, half dozen of the other.

Agreed, probably not such a big deal.


>     If you put the tempco in the divider, the compensation is not exact, 
> but, the
>offset voltage of the opamp is a smaller portion of the output signal.
>
>     If you put the tempco in the feedback of the opamp (and since it is 
> difficult
>to get large values because of the fact that the tempcos are usually wire 
>wound
>using copper wire), the offset voltage makes up a large portion of the signal,
>creating potential for more errors...although, this problem can be solved 
>by using
>a premium quality opamp.

Isn't there also an issue with the higher source impedance of the scheme 
with the tempco in the voltage divider? I haven't put in numbers for this, 
but the 2k feedback scheme with about 500R for the output divider will act 
more like a pure voltage source.

BTW, weren't the old Q81's made with a high-resistivity alloy (Nichrome? 
Anybody remember?)

   Ian