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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: An LTSpice test bed for the gain control FET in "low distortion"
 oscillators
Date: Sun, 22 Dec 2024 17:18:02 +1100
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On 22/12/2024 1:39 am, Bill Sloman wrote:
> The Jim Williams low distortion Wein bridge depends on a FET for 
> amplitude control, but the FET isn't the perFectly linear resistor we'd 
> like it to be.
> 
> And minimising the rectifier ripple in the gain control voltage applied 
> to the gate of the FET is tricky.The test bed example below is set to 
> apply 10uV of 2kHz ripple, which is less than any of my circuits can 
> manage - 2mV is closer to what the circuit I've posted could manage.
> 
> I've put together a little LTSpice test bed to try to get some idea of 
> how non-linear the FET channel is.
>
> It looks as if there about 70uV of higher harmonic content in the 10.4mV 
> peak to peak voltage across the FET for the 1V peak to peak 1kHz test 
> signal.

Tinkering got that down a bit to about 18uV with 3.5mV of 2kHz ripple - 
that seems to be the sweet spot where the 2kHz and 3kHz harmonics are 
both about 60dB below the 1kHz fundamental - moving away from that 
pushes one up and the other down.

What's going on in the FET is still a mystery to me. I'd have to dig 
into the FET Spice model to get any idea. The semi-conductor physics 
should be well understood - I've got a textbook on my bookshelf that 
claims to spell it out - but translating that into a hand-on feel for 
what's actually going on isn't something I've ever needed to do.

-- 
Bill Sloman, Sydney