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Wien-bridge oscillator

  1. Jan 6, 2012 #1
    Today I came across this design(as I am studying for my exams :P)

    And looking through my good Malvino, I found it. And I my mind was simply blown out by the concept of this oscillator. (If I got it right)

    http://pokit.org/get/957089cb8862c381d597a745b02c2763.jpg [Broken]

    Malvino went here and there, but in a nutshell is this what is happening:

    We get some thermal noise from that R1. But that part of the op-amp, which has negative feedback is not so much effective like the positive feedback part.

    Thermal noise covers a lot of frequencies(over 1 THz), with that positive feedback resonant circuit, we are picking which frequencies we want.

    As time goes by, more and more voltage we get across the output. Then negative feedback path kicks in and doesn't let it go to infinity, but "drowns" it back to unity.

    For oscillations to continue, 2*R1 must be greater than R2. Then the op-amp feeds itself with that oscillation.

    Now correct me if I am wrong, but did we get a fairly good oscillator(used almost in every car, I read that, I don't know for what) just from the starting thermal noise?

    If it is, I find that most amazing thing I learned about op-amps...
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Jan 6, 2012 #2

    jim hardy

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    Ahhh, Bassalisk...

    i see that you, like me, are enchanted by the behavior of feedback loops.

    the Wein Bridge oscillator is a beautiful real-world application of math.
    It is how the Hewlett-Packard company got started, two guys in a garage shortly after WW2 decided to build a better audio test oscillator...
    note the importance of ratio R2::R1
    to keep output stable an incandescent lamp is often used for one of them. its resistance changes with output voltage, holding output voltage constant.

    it is a cousin to Sallen-Key active filter

    i suggest google on both terms there's plenty of good articles.

    you might enjoy National's appnote introduction to filters....

    if you get a chance take a course in analog computing.

    i have been way too wordy on this board, am embarassed to see my name so much.
    makes me feel like a blabbermouth.
    since nobody else replied to this thread, i wanted to encourage you.
    will pipe down now.

    have fun!

    over and out,
    old jim
    Last edited: Jan 6, 2012
  4. Jan 6, 2012 #3

    Don't be afraid of being blabberyy or whatever. True knowledge is really appreciated from the people like you, Studiot and many others on this forum.

    Why would somebody, give up there free time to tell you things that really don't exist in the books? Book is a one thing, but experience that one has is unique from one individual to another.

    I must ephisize, that my current knowledge of circuits, physics and subjects alike would be nowhere near, without this forum.

    Thank you for your recommendations, I will check them out. Feedback is really something that sends me to thinking corner, very often.
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