[sdiy] VCR - Voltage Controlled Resistor

[HOSHUYAMA]

Hello Scott,

Scott Gravenhorst wrote:
> One comment inline.
> 
> HOSHUYAMA <houshu at muj.biglobe.ne.jp> wrote:
>>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.

Exactly.

If the M5222 is replaced by a VCR, and R10 and R12 have the same
value "R", the cutoff frequency of the structure, "fc" is

   fc = 1 / { 2 * pi * C4 * ( R + 2*VCR ) }.

Sam Hoshuyama
Saitama, Japan