• Continuously variable gain amplifier for a low distortion 1kHz Wein bri

    From Bill Sloman@21:1/5 to All on Thu Jan 23 23:34:54 2025
    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

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  • From piglet@21:1/5 to Bill Sloman on Thu Jan 23 23:11:35 2025
    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

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  • From legg@21:1/5 to All on Fri Jan 24 08:06:20 2025
    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

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  • From john larkin@21:1/5 to legg on Fri Jan 24 09:08:54 2025
    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.

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  • From Bill Sloman@21:1/5 to john larkin on Thu Jan 30 01:07:06 2025
    On 25/01/2025 4:08 am, john larkin wrote:
    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.

    Why do you think that?

    Two fets antiparallel would be better.

    But you have to provide two separate bias voltages, and since both FETs
    are doing the same job there no obvious way of working out the two
    separate bias voltages. Edward Rawle was posting examples a few months
    ago, but he didn't have any kind of scheme for geting two different bias voltages out of a single output amplitude.

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

    You can buy 0.1% resistors easily enough, but getting better than 1%
    tolerance capacitors involves buying a lot of them and sorting them into well-matched pairs. Trimming potentiometers can take you a long way, but
    it is strictly a small volume production solution, and it isn't cheap.

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

    So post a simulation.

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

    Different Spice op amp models model it more or less precisely. Some of
    them are computationally expensive, and the manufacturers of the op amps
    (who supply most of the models) don't have a lot of motivation to make
    the models all that accurate. Jim Thompson used to bitch that while he
    could produce pretty accurate behavioural models of op amps, hardly
    anybody wanted to pay him to do it for them.

    Your willingness to suspect that all Spice models are inaccurate comes
    from your wish to be seen as some kind of expert without having to
    bother to put in the work it takes to look even moderately expert.

    Looking expert here mainly depends on not carrying on about stuff you
    don't know much about.

    --
    Bill Sloman, Sydney

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