[sdiy] VCR - Voltage Controlled Resistor

[rizacan]

I am looking for thiese chips in my (sh.ty)third world country (Turkey)
But never  found yet
HOSHUYAMA wrote:

>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
>
>
>
>  
>
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