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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Continuously variable gain amplifier for a low distortion 1kHz Wein
 bridge sine wave generator.
Date: Thu, 23 Jan 2025 23:34:54 +1100
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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?

-- 
Bill Sloman, Sydney