[sdiy] VCR - Voltage Controlled Resistor

[Scott Gravenhorst]

"Roy J. Tellason" <rtellason at blazenet.net> wrote:
>On Sunday 18 July 2004 03:15 am, harrybissell wrote:
>> Hello Scott (et al)
>>
>> You are now getting there. The FET distortion is why they are seldom used. 
>> If you add the linearizing resistors, the impedance drops a lot.
>>
>> Usually the more successful FET designs use them in shunt mode... with a
>> fixed series component and a linearized FET to ground. Most FET phasers do
>> this. Guitar waves have very low amplitude after a huge peak... the
>> distortion will be very short and no one will be the wiser. VCO waves are
>> constant amplitude.
>
>I saw an article in EW not all that long ago that talked about distortion when 
>trying to use FETs,  but they weren't clear about why,  this thread has been 
>somewhat more illuminating in that respect.
>
>What about the idea of using one (or a "transmission gate" in effect,  whether 
>it's parts of a 4007, 4016, or 4066) whic gets modulated at some fairly high 
>frequency and duty-cycle controlled?  I've had that thought for a while now 
>and haven't yet gotten around to testing it out.

Yeah, that's a chopper method.  I'm not thrilled with that idea.  I'd rather use
an OTA and make it another WASP clone.  I'm still ruminating on this, FETs are
too distortion prone when used for VCR with large signals.  The idea is to use
the distortion inherent of the MOSFET inverter used as a linear amplifier.  I
wanted the VCR to be distortion free.

>> 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 !
>
>Got a URL handy?

He didn't say, but I think this is what he meant.

http://www5b.biglobe.ne.jp/~houshu/synth/PhaseFet0205.GIF



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