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Homework Help: Kirchoff's current law for an opamp

  1. Mar 7, 2010 #1
    1. The problem statement, all variables and given/known data

    http://img3.imageshack.us/img3/3939/opamp.jpg [Broken]

    Draw an imaginary boundary around the op-amp, and apply KCL at the five wires that intersect the boundary

    2. Relevant equations

    3. The attempt at a solution

    I am having quite a problem understanding the question above... which boundary should I draw around? Could someone point this out?
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Mar 7, 2010 #2
    You draw a closed line around the op-amp that contains only the opamp and a part of the 5 wires that connect to it. According to KCL the sum of all the currents going into the opamp
    must be 0.

    This is an extremely unhelpful exercise, if the goal is understanding op-amps.
  4. Mar 7, 2010 #3
    so is it:
    i2 + i3 + i7 -i4 - i6 = 0

    is this true?

    based on the number that is there?
  5. Mar 7, 2010 #4
    Yes. Of course, with an ideal op-amp I2 and i3 will be 0.
  6. Mar 7, 2010 #5
    Now what does it mean when the question asks:

    Discuss the significance of the relative current magnitudes??

    what kind of answer should I provide?
  7. Mar 7, 2010 #6


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    Homework Helper

    I2 will be very small, and I3 will be very small .
    which of the other currents would YOU like to be small?
    Tell where you think most of the current comes from, and where most of it goes to .
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