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From: piglet <erichpwagner@hotmail.com>
Newsgroups: sci.electronics.design
Subject: Re: Continuously variable gain amplifier for a low
 distortion 1kHz Wein bridge sine wave generator.
Date: Thu, 23 Jan 2025 23:11:35 -0000 (UTC)
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Bill Sloman <bill.sloman@ieee.org> wrote:
> We've been messing about using a FET as a variable resistor to try to 
> control the amplitude of a 1kHz Wein bridge sine wave oscillator for 
> months now.
> 
> It works, but it does introduce some harmonic content into the sine wave.
> 
> A good four quadrant analog multiplier can do a better job, but the 
> AD734 isn't cheap. An asymmetric current mirror can do the job more 
> cheaply but with even more components, and seems to introduce even more 
> distortion - not all that much, but enough so that it isn't a good choice.
> 
> All we need is a controllable gain element that can adjust the gain 
> around the Wein bridge to sustain oscillation at a constant amplitude 
> despite component value drift with time and temperature.
> 
> Linear Technology and Burr-Brown both used to sell amplifiers where you 
> could vary the gain continuously with a control voltage - I used both 
> together in one project - the expensive Burr-Brown part managed the 
> signal gain part, and the cheaper and slower Linear Technology part 
> managed the DC offset feedback path.
> 
> The AD8330/1/2/6 parts all seem to do much the same job, as does the 
> AD603. None of them are cheap, and the are all a lot faster than the job 
> requires. Anybody know of anything more suitable?
> 

Back in mid October when I was half following you and Edward I did post two
non fet alternatives. A diode bridge variolosser and minimal current
steering ltp (kinda minimal multiplier). With care and ingenuity maybe
those can be coerced into lower distortion?

-- 
piglet