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operational amplifiers

Discussion in 'Electronic Basics' started by Simon Ward, Jul 29, 2003.

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  1. Simon Ward

    Simon Ward Guest

    I have been given the following problem for a college assignment. I
    was hoping that someone with more knowledge than I could help me out;

    Operational amplifier (op-amps), which have negative feedback applied,
    can have their overall gain fixed using a simple resistor network in
    the feedback path. Explain why this is so.

    Cheers
     
  2. Sam Wormley

    Sam Wormley Guest

    What have you done to determine the answer to date?
     
  3. FrediFizzx

    FrediFizzx Guest

    | I have been given the following problem for a college assignment. I
    | was hoping that someone with more knowledge than I could help me out;
    |
    | Operational amplifier (op-amps), which have negative feedback applied,
    | can have their overall gain fixed using a simple resistor network in
    | the feedback path. Explain why this is so.

    Theoretically, an op amp with no feedback would have infinite gain.

    FrediFizzx
     
  4. Take a look at application notes for Op-Amps. Here are a couple links
    to get you started.

    http://merchant.hibbertco.com/servlet/mtrlext.MtrlExtServlet?tp=archsearch
    http://www.national.com/
    http://www.onsemi.com/home
     
  5. Roy McCammon

    Roy McCammon Guest

    have you heard of the parable of
    the ass and the carrot and the stick?


    --
    Achilles: I wish my wish would not be granted.
    < an undescribable event occurs >
    Achilles: What happened? Where's my Genie?
    Tortoise: Our context got restored incorrectly.
    Achilles: What does that cryptic comment mean?
    Tortoise: The system crashed.


    To email me send to :

    rb <my last name> AT ieee DOT org
     
  6. DarkMatter

    DarkMatter Guest


    And no control. And "infinite" has starvation points elsewhere in
    the chip and circuit. It would be a single, always on device. Hell,
    a transistor works in that instance. Feedback loops are critical to
    control circuitry. They don't just guess. They have to be
    responsive, active control elements, and that requires data from what
    they are controlling. We call it feedback. Why does that word scare
    some people? One monitors a point, and utilizes the data to control
    the system that controls the point being monitored.

    Feed forward should even be more scary, then. ;]

    If one lives in the basement of a house or building in a wet region
    of the world, are they considered to be "critically damped"?
     
  7. Jim Backus

    Jim Backus Guest

    What's your college attendance record?

    --
    Jim Backus OS/2 user
    bona fide replies to jimb-thecirclethingy-jita-dp-demon-dp-co-dp-uk
    or remove "NOT" from address
    remove dashes and make the obvious substitutions for valid email
    address
     
  8. Douglas Eagleson





    You are asking the question called why does the op-amp's internal
    circuit implement the defined external ampilfier. And it is
    designed that way using transistors.

    And when the instructor claims infinite causality of gain by the
    added feedback short, it is an incorrect claim in physics. The
    gain is always limited by the transistor's hfe.

    So the defined op-amp is an abstract theory of its specific
    circuit.
     
  9. Simon, you do understand that this group has little use for students using
    them in lieu of doing their "homework"? Having said that, picture an
    amplifier with two inputs. One of these inverts the input signal and the
    other does not. If the output signal, or some part of it, is fed back to
    the inverting input, this is called negative feedback. If all of it is fed
    back, the gain is 1. In the history of electronics, negative feedback looms
    as one of big, big ideas.

    Anyway, if the amplifier has lots of gain and if the entire signal is fed
    back to the inverting input, the result is an amplifier that follows the
    input with no gain. Most times, only a part of the output is fed back to
    the input.

    The classic model used to understand and analyze op-amps is that the
    difference between the two inputs is zero when the amplifier is dealing with
    normal signals. So, if you connect a voltage divider between the output and
    the inverting input, you get an easy to analyze circuit that predicts the
    gain to be equal to ratio of two resistors. If the feedback resistor is
    10,000 ohms and the input resistor is 1,000 ohms the gain is Rf/Ri or 10 in
    this case.

    Hope that helps.
     
  10. CWatters

    CWatters Guest

    The amp on it's own has huge gain so in normal use the two inputs are at
    more or less the same voltage. In the classic circuit the + input is
    connected to 0V so the circuit behaves as if the -ve input was also at 0V.

    Now think about current flows in the circuit....

    The current going through the input resistor (Rin) is Vin/Rin

    The current flowing in the feedback resistor (Rfb) is Vout/Rfb

    Since the input impedance of the opamp is very high these two currents are
    equal (it has nowhere else to go..

    Vin/Rin = Vout/Rfb

    Rearrange this to give

    Vout/Vin = Rfb/Rin

    eg Gain = Rfb/Rin
     
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