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Op-amp basic circuits questions

Discussion in 'Electronic Basics' started by qwerty, Aug 25, 2006.

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  1. qwerty

    qwerty Guest

    I've been reading about some basic op-amp circuits. The book has the
    circuit and then it gives the analysis. The analysis doesn't make
    much sense to me. For example, it says that when the op-amp is
    working in it's linear area (sorry if the translation from the book's
    lanugage is sloppy) the two inputs have the same voltage.

    But how do we know that the op-amp is working in its linear area? I'm
    trying the circuits on SPICE and it seems that if I use a circuit
    with feedback (http://img58.imageshack.us/img58/4422/opampgm9.png)
    the op-amp will work in its linear area, but if I remove the feedback
    then it won't. Why?

    Should I worry too much about those details or should I just learn
    the circuits and move on?
     
  2. Yes. This is another way to say that an ideal opamp has a very high
    differential gain.
    Its output is not saturated against either of the supply rails, but is
    producing a live, controlled (by the feedback network) output signal.
    If you remove the feedback, it is almost impossible to keep the input
    signal small enough and well enough matched to keep the two inputs
    almost exactly (but not quite) exactly the same. This is what
    negative feedback does. If the two inputs are not almost exactly the
    same, the output voltage changes in the direction that makes the two
    input voltages more alike. When they are almost perfectly matched,
    there is only enough difference between them (when multiplied by the
    high differential gain of the amplifier) to produce the output voltage
    that got them to that state of match.
     
  3. Tim Auton

    Tim Auton Guest

    The bit about the inputs being at the same potential is not a property
    of op-amps. It's a property of op-amp circuits using negative feedback
    (pretty much all of the ones which work well). Without feedback there is
    no way for the op-amp to alter the potentials on its inputs. With
    feedback the output is driven one way or the other (we design the
    circuit so that it's the right way) till there is zero voltage
    differential at the inputs.


    Tim
     
  4. Alan B

    Alan B Guest

    John summed it up quite well. The large open loop gain of the device makes
    even the tiniest difference in potential at the input appear as an enormous
    swing at the output. That's why op amp design (as differentiated from
    comparators) uses degenerative feedback - the gain is thus made
    controllable, and easily determined through mathematical calculation.
    That depends entirely on how well you wish to understand the fundamentals
    of the craft.
     
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