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

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