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Inverting Non Ideal Op Amp

  1. Jul 22, 2015 #1

    GRW

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    [ mod note: thread title corrected ]

    Hi All,

    My first post in the forums, hopefully I'll be able to give out as much help as I receive. I'm currently stuck on an Op Amp gain derivation.

    Attached is the question and diagram, the derivation I am struggling with is part B.

    I understand the gain for this Op Amp were it ideal would be 1+ Rf/R1 but I cannot seem to derive the equation if the input resistance is not ideal.

    Thanks in advance,

    George


    Capture2.PNG Capture.PNG
     
    Last edited by a moderator: Jul 22, 2015
  2. jcsd
  3. Jul 22, 2015 #2

    NascentOxygen

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    Staff: Mentor

    Hi GRW. :welcome:

    You won't be able to derive the equation in (b) until you have completed part (a), drawing the equivalent circuit with ideal components.
     
  4. Jul 22, 2015 #3

    GRW

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    Thanks for your reply Nascent,

    I have already completed part A but I can't see how is it that relevant to part B? I know the equivalent circuit and that Rin is in the Op Amp internally and that A is the Op Amp gain.

    Any further guidance on how the two relate / where to start with part B would be much appreciated.
     
  5. Jul 22, 2015 #4

    NascentOxygen

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    Staff: Mentor

    Can you attach your completed circuit diagram for (a), with RI and RF arranged around it as in Fig Q1.
     
  6. Jul 22, 2015 #5

    GRW

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    Sorry but I don't have any way of uploading my paper working to a computer. It is as in Fig 1, with Rin linked between the + and - terminals of the Op Amp.
     
  7. Jul 22, 2015 #6

    NascentOxygen

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    Staff: Mentor

    That is the circuit you analyze, but this time without a reliance on the dodge of a "virtual earth".
     
  8. Jul 22, 2015 #7

    GRW

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    It's the circuit analysis for the derivation here that I am struggling with. Sorry if my initial explanation was poor.
     
  9. Jul 22, 2015 #8

    NascentOxygen

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    Staff: Mentor

    You draw a large circuit diagram. Mark vin. Label the voltage at the input to the amplifier proper, at the junction of the 3 resistances, I called it va.

    How will you label the output of the amplifier?
     
  10. Jul 22, 2015 #9

    LvW

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    Question to you: The input voltage is connected (via a resistor) to the inverting terminal of the opamp. Wouldn`t you expect a negative gain in this case?
    (Your ideal gain value (1+Rf/R1) is not correct!)
    You have stated: "I understand that the gain ....".
    Did you really "understand"? I don`t hope that you understood a false result! That would be fatal!.
     
    Last edited: Jul 22, 2015
  11. Jul 23, 2015 #10

    Zondrina

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    Homework Helper

    a) This you must draw on your own. If you could somehow take a picture and upload it, we could verify if it is correct. What do you think the ideal values for ##A##, ##R_{in}##, and ##R_{out}## are? Hint: Think about preserving the signal. Do you need a large input impedance or a small one?

    b) For the closed loop gain, what equation is relevant to op-amp operation? Use this equation, and the virtual ground between ##R_1## and ##R_F##.
     
  12. Jul 23, 2015 #11

    GRW

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    Thanks for all your help guys, after a decent sleep last night I woke up this morning and after re reading the suggestions I was able to derive it no trouble. Thanks again and I look forward to being part of the forum!
    GRW
     
  13. Jul 24, 2015 #12

    LvW

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    GRW, to be honest - did you derive the correct result really without any help?
     
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