[sdiy] linearize OTA for VCF?

[jh.]

> > If you look at the LM13600/13700 datasheets, the filter designs are
simply
> > CA3080 designs transferred to the different chip. Using the input diodes
> > means dropping the whole circuits input impedance to 10K, which can
affect
> > other parts of the circuit. It's a pain.
>
>
> I agree, those data sheets are pretty lame. You have to learn how to
design
> independently with the diodes biased.
>

I wrote an email to National's Bob Pease recently, asking for the theory
behind the resistor-biased linearisation diodes of the 13700. The data
sheet speaks of current sources (as if it were the ordinary Gilbert method),
but as Jörgen Bergfors found out, replacing some of the resistors with
current sources destroys the 13700's linearity. With RAP's recent
articles on expo stuff, I thought I'd give it a try. Unfortunately, his
reply
was less than helpful. He spoke of an obsolete product, and basically
asked me to analyze the circuit myself. (;->)

JH.