[sdiy] Temperature stable lin-exp converter with a CA3086or CA3046

[Neil Johnson]

> All free-electron-like metals have the correct tempco.

Indeed, but don't forget that this also depends on the purity of the
metal, and can vary over a wide range.  For example, standard Pt100 RTDs
are specified (DIN) as having a temp.co of 3850 ppm/'C, but higher purity
Pt (eg. 99.999% pure) can go as high as 3923ppm/'C (from info on the web).

I guess the old Q81s were wire-wound copper because its easier to make
that vapourizing platinum onto a ceramic substrate??  And standard copper
has a temp.co of 3930ppm/'C.

Indeed, in Tim Orr's Transcendent 2000 design he describes making a copper
temp.co from handwound copper around a high-R resistor substrate, then
bulking out the temp.co to the desired value with a low-value metal-film
resistor (from memory something like 820R copper, 180R metal-film).

Certainly for op-amps, inductors in the feedback path is bad news (unless
you specifically design for it) as it can lead to loop instability, such
as overshoot.

I tend to use platinum surface-mount temp.cos, as (a) they're quite cheap
and available, and (b) easier to glue to the side of tranny-pairs.  As
they are not wire-wound I could put them in either of the two places
(feedback or divider) but convenience suggests the divider, directly
across the base-emitter pair on one of the transistors (with maybe a
padding resistor to adjust the temp.coeff).  Even though, as Rene pointed
out, its not *quite* as perfect is being in the feedback path :-)

<flame>
Heck, if you want "perfect", go build a digital oscillator!!
</flame>

Confuscious say: "A bank of analogue oscillators should be like bamboo --
pure, simple, drifting in the breeze"

Neil

--
Neil Johnson :: Computer Laboratory :: University of Cambridge ::
http://www.njohnson.co.uk          http://www.cl.cam.ac.uk/~nej22
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