Re: Continuously variable gain amplifier for a low distortion 1kHz Wein bridge sine wave generator.

On Thu, 23 Jan 2025 23:34:54 +1100, 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.
>

A discrete 4 quadrant multiplier  can easily be constructed from
discrete components which will be "faultless" when used in this
application (ie. H2, H3 below -155dBc over the audio band).  A 2
quadrant multiplier could of course also be used.  A simpler gain
controlled current mirror could also be employed with no crossover
distortion.

I'll send you examples at the weekend if I remember, and I'm still
standing after the lastest storm visits tomorrow.

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?

There's plenty of good transistor arrays from the likes of THAT,
suitable for tranlinear designs, and listening to the following fellow
is a half hour well spent.

https://www.youtube.com/watch?v=LQNJVtcFrCc&t=2s