Op amp summing non inverting

Discussion in 'Homework Help' started by activee, Jun 11, 2014.

  1. activee

    Thread Starter Member

    Jan 16, 2014
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    Hey I don't understand this summing non inverting OP amp. (I understand the inverting one).
    The final equation is Ve = V1 + v2 + v3 . I don't know how to get that answer.

    Thanks in advance.
     
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
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  3. activee

    Thread Starter Member

    Jan 16, 2014
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    thank you but I don't think I understand still!

    V+ = V1 +V2 +V3 is that correct ?
     
    Last edited: Jun 12, 2014
  4. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Have you try to write nodal equation for Vp node?
     
  5. crutschow

    Expert

    Mar 14, 2008
    12,995
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    No. The plus input is not a summing junction so it has a very high impedance, thus the input voltage is the parallel value of the three voltages through the three resistors. You need to solve that with some type of nodal equation such as Kirchoff's.
     
  6. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    When I had to find the voltage value at the non inverting input I did it as explained in the attached document.

    Please do not complain about the quality of the images. (I am sure that somebody will, anyway).

    I do not have time to redraw them properly. The .pdf was assembled directly from a former .htm file.
     
  7. Fibonacci

    New Member

    May 23, 2014
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    You must follow the reference that makes shteii01; in this case you have three input resistors. Using the superposition theorem, a voltage divider and paralell resistors we find three contributions to v+ equal to Vi/3, i=1,2,3. Therefore, the voltage at noinverting terminal v+ is (V1+V2+V3)/3, and the output voltage is equal to (1+R/R1)(V1+V2+V3)/3.
     
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