[sdiy] Temperature Compensated VCO attempt - help?

[Scott Bernardi]

Ah, I must correct myself. It is the CA3280 which has the Id current
doubled. However, Serge also went into analyzing the LM13600, which he said
showed more distortion as you approached the maximum input signal levels
than the CA3280 counterpart, especially for the single ended input as shown
in the National datasheet (and which my circuit used).
Magnus, this also falls into your discussion about common mode errors with
the National scheme. Jim Patchell's configuration with the two LM13600's
doesn't have this problem.

ASSI wrote:

> On Thursday 30 January 2003 14:12, Scott Bernardi wrote:
> [...]
> > > > http://home.attbi.com/~sbernardi/elec/og2/tempcomp_vco_theory.htm
>
> You mention EN#113 and two extra current mirrors not in the data sheet
> for the LM13700. I've just drawn the CA3080, CA3280 and LM13700
> schematics with just the current mirrors, differential pairs and
> linearizing transistors (tied as diodes) for reference and I'd like to
> include the extra mirrors. I'd think that there should be a npn->pnp
> sink to source configuration (slightly simpler than the one in the
> CA3280), but maybe EN#113 had more specifics?
>
> A note on bandgaps: the PTAT characteristic is a (pretty good) linear
> approximation. Hence the compensation is perfect for only one or two
> temperatures after trimming, depending on where you put the trim
> point(s). You can of course try to remove the curvature by including
> the quadratic term or piecewise linear approximation.
>
> Achim.
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