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OP Amp Oscillations/Feedback

Discussion in 'Electronic Basics' started by FS, Sep 13, 2004.

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  1. FS

    FS Guest

    When employing an op amp in a negative feedback configuration, phase
    shifts due to the feedback loop, and the op amp itself can lead to
    oscillation. I am experiencing something like this with the 741. Of
    course I can play some tricks with capacitors, but my first question
    is: Where can I find a simple resource describing the differential
    equation for this scenario? Perhaps I have overlooked one on the web,
    or maybe a good textbook will do (Horowitz and the Jung "Cookbook"
    didnt'help here)

    Thanks for any tips
    Fritz
     
  2. What kind of "feedback" are you using? If it is just resistive it is
    unlikely that the circuit becomes unstable because there is only one pole,
    created by the OPAMP transfer function characteristics (-6dB/oct slope) and
    the resistor in the feedback.
    Gene
     
  3. Ban

    Ban Guest

    Google for "opamp stability criterion" or similar. You can do that by
    analyzing the actual circuit inside the opamp together with stray
    capacitances from the PCB layout, or by the bode-plot resulting from it.
    Whenever the open loop gain is greater than 1 *and* the phase-shift gets
    180°, the amp will oscillate.
    When you use reactive compensation elements, these are not dirty tricks, but
    scientifically chosen components which tailor the transfer function in such
    a way to insure stability.
     
  4. FS

    FS Guest

    Thanks for your response Gene-
    I have the inverter-input connected to ground. The non-inverter is
    connected to a photodiode- a little tricky here-the photo diode
    receives light from a laser on the op-amp output to create the loop
    optically. When I put the scope on either the diode or laser I get
    oscillations-I think around 1 micro-second but I need to check when I
    get back to the lab. I presume that the output, when attempting to
    re-zero the difference on the inputs, overshoots, resulting in
    oscillation. Further, I would expect laser and photo diodes to have
    some inductance and capacitance (they are in close proximity so there
    is no lightspeed lag here).
    I can play with a decade capacitor box but my main interest is
    setting up the diff-E for this loop so I can estimate the required
    compenstation analytically. I thought perhaps this is described in a
    textbook somewhere-I tried to do it with taylor series but didn't come
    out the way I expected, and I am sure there is more to the picture
    that a good text would add.

    Thanks
    Fritz
     
  5. FS

    FS Guest

    Thanks for your reply Ban-
    as I mentioned I am trying to analytically calculate the oscillation
    rather than resort to "tricks" or trial-and-error here. I did not find
    anything on the web which describes negative feedback in terms of a
    second order differential equation however I presume the electronics
    terminlogy here is "transfer function" and maybe I am looking to much
    at the mathematical approach to this kind of problem. I will head out
    to the library and see what I can find on feedback loops.

    Thanks
    Fritz
     
  6. Hi Fritz,

    the discipline that studies this topic is called "Control Theory" and it is
    quite complex. A book I could recommend is "Automatic Control Engineering"
    by Francis Raven. In any case, a loop that is unstable( oscillations) is
    indicative of excessive phase lag and gain above unity. I do not know if you
    have the capability to perform an open-loop Bode-plot, but if you were it
    would tell you the degree of instability of the loop and the way to
    stabilize it. Adding capacitors will not solve the problem. Considering your
    lack of knowledge in this field I recommend the following:
    1. Reduce the gain of the OPAMP until the loop has good stability.
    2. Is the performance accettable? if yes, leave it. If not go to the next.
    3. Is there a tracking error problem? If yes you could try to to put a
    capacitor in SERIES with the feedback resistor that sets the OPAMP gain.
    Start with a big one and step by step decrease its value until the loop
    become unstable. Mark the capacitor value when this happens. Intall a cap
    that has double of the capacitance. Now the loop should be stable, with
    virtually zero tracking error.
    4. If you have dynamic tracking problem, then it is problematic to improve
    the situation without having control theory knowledge.

    Das Glueck ist dem Kuehnen hold!

    Gene
     
  7. I agree 100%. The theory can lead to the "ballpark", but then good lab
    investigation and analysis finalizes the design.
    Gene
     
  8. Ban

    Ban Guest

    Fritz, the EEs do not use differential equations for their transfer
    functions, but the Laplace transforms with the frequency operator S= j
    f/f_nominal. This way the whole thing can be solved with simple algebra and
    can be overlooked easily.
     
  9. john jardine

    john jardine Guest

    I don't know about the rest of the world out there, but before spice
    simulators, I never ever found a circuit that could be nicely stabilised by
    the application of cold theoretical methods.
    Disregarding of course, those trivial textbook examples, or the cop-out of
    slugging the response to that of treacle, or having available a roomfull of
    gain/phase measuring equipment, or having a pocketfull of those nice,
    dominant poles that everyone except me seems to have in their toolkit.
    The impossible to define parasitics seem always a major factor. The
    semiconductor data sheets give too little info. The source and load Z's are
    usually nothing like those estimated.
    That 20nH of unseen, inductive cross-coupling may be impossible to analyse,
    measure, estimate, simulate or (sometimes) even understand but is resulting
    in 200megs oscillation, massive current drain and signal distortions, random
    device failures and exciting artefacts in the preceding electronics.
    Any solution must come from an in-the-flesh-on-the-bench-suck-it-and-see
    approach. With experience it gets easier to home in on the sweet spots but
    it's still ad-hoc and unscientific.

    As the best *fixes* usually seem to involve the odd strategic R or C (or
    god forbid, an occasional L), to me it still most definitely feels as if I'm
    cheating or playing a dirty trick on the circuit, when just by adding that
    critical 2p cap' the response instantly changes from a monstrous nightmare,
    to that of pure sweetness and light.

    Unless others know better :)

    regards
    john
     
  10. FS

    FS Guest

    OK and I thank you all.
    I checked back and I see 50 microsecond Oscillations. Again the
    feedback is optical (e.g. there is no feedback resistor). I found that
    placing a .04 uf capacitor from the output to input removed the
    osciallations and I am now able to modulate the laser as needed.
    I appreciate the book reference from Gene and will get a copy (I
    have D'azzio but I presume it is somewhat dated).

    Viele Dank,
    Fritz
    Woods Hole Oceanographic
     
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