Automatic Op-amp gain adjust

Discussion in 'The Projects Forum' started by Shagas, Jun 1, 2013.

  1. Shagas

    Thread Starter Active Member

    May 13, 2013
    802
    74
    Hello again

    I'm facing a simple challenge in designing an audio oscillator :

    My wien bridge sine-wave generator has a range of about 30-6300 Hz . I used a diode stabilization method.
    Scroll down to the variable frequency Wien with diode stabilization

    The problem is that at about 30 hz the output is at about 200mV and approaching 6000 hz the output is about 1volt

    I need a constant output amplitude.

    I was thinking of doing a simple RC low pass filter and then feeding it to an op amp to bring up the output to say ...2 volts pk-pk , but the problem is that there is going to be a phase shift in the filter isn't there? It doesnt matter that much but still i'd like to make it without the phase shift .

    Question:


    Is there a way of doing with an op amp that will give me a constant amplitude output regardless of the input amplitude ?? (assuming ofc that VIN < VOUT)
    Some sort of output feedback that keeps the output amplitude constant?

    I was thinking (and in the process of trying out ) something like this :
    stabijfet.png
    A crude schematic , but just to give the idea. Using a JFET which will vary the resistance of RIN and consequently the gain .

    Another method similar to the one above and easier would be to use a JFET optocoupler . I've got a few H11F1 's lying around but they are costly and I need them for other things .

    If anyone has any input on this i'll be glad to hear , thanks in advance !
     
  2. Shagas

    Thread Starter Active Member

    May 13, 2013
    802
    74
    no one? :)
     
  3. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    Jim Williams designed a Wien Bridge amplitude stabilized oscillator - see his app note AN43 about page 30.
     
    Shagas likes this.
  4. Shagas

    Thread Starter Active Member

    May 13, 2013
    802
    74
    Thanks , I'll check it out !

    But i'd also like find a solution using a second stage op amp amplifier (as mentioned above) to stabilize the amplitude for use in future circuits when I come up with the same problem
     
  5. Ramussons

    Active Member

    May 3, 2013
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