[sdiy] VCR - Voltage Controlled Resistor

[Scott Gravenhorst]

One comment inline.

HOSHUYAMA <houshu at muj.biglobe.ne.jp> wrote:
>Hello Scott and List,
>
>Maybe I don't follow all the discussion in the weekend.
>But, I think I can contribute something.
>
>Scott Gravenhorst wrote:
>> harrybissell <harrybissell at prodigy.net> wrote:
>>>I suggest you hie hither to Osamu Hoshuyama's site. He has some ideas there on
>>>FET linearization iirc.  One of them is truly a new idea that has not been
>>>commonly used !
>> 
>> I've been there, he has an SVF using 4069UB integrators, but with an M5222 dual
>> expo OTA (unobtainium), essentially as WASP type filter.  He also has a 4069UB
>> integrator SVF using MOSFET OTAs built from more 4069UB (but says they are tricky
>> to make them work).  He doesn't seem to use FET VCRs in any of his filters. 
>> Perhaps I know why now.
>
>Actually, at the beginning of my CMOS VCF project, I tried to
>design some VCF structures using 4049 as shunt VCRs, but
>failed in those days.
>
>The reason I used inverters as OTAs in my CMOS VCF is that I was
>inspired by Pearl phaser PH-03, where an MC14069U is used as VCRs
>based on an OTA technique.  I thought the structure is the only
>way to obtain "smooth" and "wide-range" sweep.
>
>  http://www5b.biglobe.ne.jp/~houshu/synth/VcfMos0206.GIF
>  http://www5b.biglobe.ne.jp/~houshu/synth/CmosOta.GIF
>
>
>I've seen a schematic that uses 4007 as VCRs in the SVF.  It's an
>envelope-controlled filter of Electroharmonics, which may be
>useful, though I don't think it has sufficiently wide sweep-range
>for a synth VCF.
>
>If I have "matched" and "floating" FETs (MOSFETs), I'd try a 4-pole
>VCF based on the LPF structure in the following schematic replacing
>the OTA by a FET, and the opamp by a CMOS inverter.
>
>  http://www5b.biglobe.ne.jp/~houshu/synth/Vcf0210.GIF

I saw this the other day and it's implication went whizzing right over my little
wooden head.

This technique (I assume as depicted in the LPF inset) looks like a H11F1(s) could
be used, or perhaps a 4007UB to make a SVF from 2 of these LPF blocks.  (or am I
being terribly naive?)  I will not claim to understand this structure.  It is not in
Lancaster's Active-Filter Cookbook.  I am not at all sure exactly where the summing
node is!  It's almost is if there are two virtual grounds (which could explain the
low Vds possibility) because of the dual feedback configuration.  My guess is that
the source of the MOSFET would connect to the capacitor/amplifier input node.

Would a SVF made this way have a 12 dB slope?



>In the structure, the signal level across the FET is small
>thanks to negative feedback, which leads to less distortion
>in pass band.  My linearizing technique can also be applied.
>
>  http://www5b.biglobe.ne.jp/~houshu/synth/PhaseFet0205.GIF
>
>Linearizing the relationship between CV and the resistance of
>the FET is possible if master-slave technique is applied.
>
>I'm busy these days.  So, don't expect quick response please.
>Writing English is not easy job for me  ;-)
>
>Sam Hoshuyama
>Saitama, Japan
>
>

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