[sdiy] Phasors FET vs OTA?

[harry]

It was Osamu Hoshuyama who posted the new j-fet linearization
scheme.  VERY nice imho...

H^) harry

patchell wrote:

>     I don't think it matters how you use OTA's, voltage controlled resistors or
> not, if they are controlling the signal characteristics, they will introduce
> noise.
>
>     Some things you need to keep in mind about noise.  If you increase the
> number of stages by 4, you only increase the amount of noise by 2, so adding
> stages is not quite the big deal you may think (noise addes by the square root
> sum of the squares, or something like that).  Also, the offset voltage on the
> CA3280 is pretty good, you might be able to get away with quite a few stages
> before you need offset trim.
>
>     Also, on FETs, you can improve the voltage control characteristic of the fet
> by using feed back around the fet.  This also improves the distortion of the FET
> Voltage controlled resistor as well (talking about J-FETs here).  In fact,
> didn't somebody just post a bunch of schematics of experimental JFET VCResistors
> just a little while ago?  I was impressed with some of the novel solutions.
>
> Jay Schwichtenberg wrote:
>
> > Howdy One and All,
> >
> > I've been thinking lately about doing some work with phasors, at least 6
> > stages maybe more. In general I'm more into quiet vs freq tracking for this
> > type of device. After doing some research it seems to me there are about 4
> > different general architectures for phasor circuits. FET resistors, OTA all
> > pass, transistor ladders (Arp Omni) and LDRs. I'll ignore LDRs and
> > transistor ladders simply because of size (I'm into small and SMT) so that
> > leaves FETs and OTAs. In my mind (believe me it's a scary place) there are
> > issues with both. With a number of OTA stages I think there would be issues
> > of noise (OTAs aren't the quietest parts) and offset unless diode biasing
> > (LM13X00 & CA3280) and offset trim was used. Using the diodes wouldn't be a
> > problem but by the time you 6 or more trim pots on a board you've sucked up
> > a lot of space. Using a FET to ground at the op-amp input starts to seem
> > better noise wise. The major problem that I can think of with the FETs is
> > the control/freq range. A lot of FETs have a very small gate voltage for
> > full off/on operation some in the order of 0.1-0.2 volts. You can find other
> > FETs that are more in the volt range which might be better suited for this
> > type of circuit. Some other ideas for discussion might be AC coupling stages
> > and using OTAs as VC resistors like the FET but not directly in the audio
> > path.
> >
> > I know that there are people out there that have some ideas and opinions.
> >
> > Thanks for the discussion everyone.
> > Jay
>
> --
>  -Jim
> ------------------------------------------------
> * Visit:http://www.silcom.com/~patchell/
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> *I'm sure glad Merry Christmas comes just once a year
> * -Yogi Yorgensen
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