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Opamp problem

  1. Mar 28, 2016 #1
    1. The problem statement, all variables and given/known data

    For part ii) ,
    V+ is 69mV, but how to determine V-? We don't know the resistance of X, so we can't find the p.d. across X, so how to find V-?
    2. Relevant equations
    Vout =A0(V+-V-)
  2. jcsd
  3. Mar 28, 2016 #2
    Isn't the gain of this amp Vout/VR?
  4. Mar 28, 2016 #3
    But Vout and VR are given.
  5. Mar 31, 2016 #4
    I'm sorry, but can you please explain to me why is V-=0 in your eqn for the gain in post #2 , thanks
    Last edited by a moderator: Mar 31, 2016
  6. Mar 31, 2016 #5
    There is no zero in my equation in post #2
    Last edited by a moderator: Mar 31, 2016
  7. Mar 31, 2016 #6
    Then, how did you know that the gain is Vout/VR? Why didn't you take account of V-?
    Last edited by a moderator: Mar 31, 2016
  8. Mar 31, 2016 #7
    That is the general definition of gain.
  9. Mar 31, 2016 #8
    Vout=Ao(V+-V-), am I right?
    V-= ?
  10. Mar 31, 2016 #9
    Why do you want V- ?
  11. Mar 31, 2016 #10
    So that I can use the formula, Vout=A0(V+-V-) ,and get A0
  12. Mar 31, 2016 #11
    The question was what is the gain of the circuit not the op-amp. that's why I commented on the simplicity of the question.
  13. Mar 31, 2016 #12


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    So you are interested in Ao ???
    Up to now I was of the opinion you were looking for the gain of the whole amplifier stage?
  14. Mar 31, 2016 #13


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    Staff: Mentor

    [ Mentor note: this thread has been tidied up ]
  15. Mar 31, 2016 #14
    Ohhh.. I get it now
    Sorry I thought they were asking for A0
    Thanks so much
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