Op-Amp Question

Discussion in 'Homework Help' started by dli286, Mar 9, 2009.

  1. dli286

    Thread Starter New Member

    Mar 9, 2009
    6
    0
    For the op-amp below, I am solving V_o in as a function of V_i.

    [​IMG]

    Here are the equations (using KCL) I have so far.

    V_i - V+ = I_1 * R_1
    V + = I_1 * R_2
    V_i - V- = I_2 * 10k
    V_o - V- = I * 10k

    I am not sure how to apply KCL at the node between the two 10k resistors. Thanks for your help.
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Assume that the op amp is ideal and thus no current flows in its inputs.

    Thus,

    Current through right 10K:

    Io=[Vo-(V-)]/10K

    Current through left 10K:

    Ii=[(V-)-Vi]/10K

    Then V- equals V+ which equal:

    (V-)=(V+)=Vi*R2/(R1+R2)

    Make the substitutions and hopefully you will find it.
     
  3. dli286

    Thread Starter New Member

    Mar 9, 2009
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    0
    Thanks a lot for your fast reply. Can I let Io=Ii?
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Of course, that is the reason you assume no input current.
     
  5. dli286

    Thread Starter New Member

    Mar 9, 2009
    6
    0
    Thank you very much for your help.
     
  6. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    What is your answer?
     
  7. Ratch

    New Member

    Mar 20, 2007
    1,068
    3
    dli286,

    By now you should have found that Vo/Vi = (R2-R1)/(R1+R2) . If you did not, then you should ask for more help.

    Ratch
     
  8. dli286

    Thread Starter New Member

    Mar 9, 2009
    6
    0
    My answer is V_o = V_i ((2kR_2 - R_1 - R_2) / (R_1 + R_2)), which is the same thing as V_o = V_i (R_2 - R_1) / (R_2 + R_1).
     
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