[sdiy] problem with 13700 flter upper range

[Scott Gravenhorst]

simon.oo.o at xs4all.nl wrote:
>harrybissell at wowway.com schreef:
>> You really want to drive the OTA Gm inputs with a current source rather
>> than voltage. Try looking for the EFM VCF1 that was adapted from the
>> LM13700 data sheet... that would do it.
>
>As the Gm pins of the OTA are connected to current mirrors the voltage on
>them will be practically constant; about 0.7V above the negative rail. The
> control voltage range is *much* larger than the variation of that
>voltage, therfore the Gm current will be proportional to the control
>voltages with little error. I don't see how using a "real" current source
>could improve much. An exponential converter however...

That is what I thought too, however when I first started playing with LM13600, I did
some experiments playing around using a resistor to feed the Iabc pin - it was anything
but linear.  When I switched to a real current source, the device did what it was
supposed to do and quite accurately.


-- ScottG
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-- Scott Gravenhorst
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