Path: ...!weretis.net!feeder9.news.weretis.net!news.quux.org!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail
From: john larkin <JL@gct.com>
Newsgroups: sci.electronics.design
Subject: Re: Continuously variable gain amplifier for a low distortion 1kHz Wein bridge sine wave generator.
Date: Fri, 24 Jan 2025 09:08:54 -0800
Organization: A noiseless patient Spider
Lines: 49
Message-ID: <rvh7pjlln0d5pb7plqo6vrd7vgqgv4s3rc@4ax.com>
References: <vmtd1n$1kej0$1@dont-email.me> <aq37pjtha0ombm2k2v5r0m038moo1ku94g@4ax.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Injection-Date: Fri, 24 Jan 2025 18:08:57 +0100 (CET)
Injection-Info: dont-email.me; posting-host="0c555ef57502832090bea85a0ee5fb0f";
	logging-data="2454853"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX18+l2Wkt+//R1XnytC2VZmP"
User-Agent: ForteAgent/8.00.32.1272
Cancel-Lock: sha1:6hAzr6jIG9xpY8yeuCJQ9QSxev0=
Bytes: 3127

On Fri, 24 Jan 2025 08:06:20 -0500, legg <legg@nospam.magma.ca> wrote:

>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.
>>
>>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?
>
>Fiddled with RC4200 at one time. Not sure where you could 
>buy them nowadays. Digikey hands you off to Rochester, which 
>seems to handle 'off-market' or old-stock type sources.
>
>RL

A mosfet run ohmic, with millivolts of swing, should be very linear.
Two fets antiparallel would be better. 

The trick for low distortion is to give the gain control element very
little influence.

A dual-integrator oscillator can add a free -12 dB/octave harmonic
attenuation.

The opamps should be the distortion limit, and I suspect that Spice
models that badly.