op-amp

Discussion in 'Homework Help' started by uofmx12, Mar 8, 2011.

  1. uofmx12

    Thread Starter Member

    Mar 8, 2011
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    if I have a ideal op-amp of an instrumentation amplifier with 2R1, R2, R3, and R4. with v1=Vcm-Vd/2 and v2=Vcm+Vd/2.

    Finding Ad and Acm:
    Ad=.5[(R4/(R2+R3) ) (R4/R1 + 1) + (R4 / R1) ]
    Acm=(R4/ (R2+R3) ) (R4/R1 + 1) ( R4/R1)

    Would those equations be correct?

    or Acm=0?
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    It would help us better understand your equations if you would post a diagram of your circuit.

    hgmjr
     
  3. Ron H

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    Agreed. I am baffled by posts like these. What are they thinking?
     
  4. uofmx12

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    Mar 8, 2011
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    I figured everyone knew what an ideal op-amp instrumentation amplifier looked like. Let me google it for you:

    http://www.electronics-tutorials.ws/opamp/opamp21.gif

    With R1=2R1.
     
    Last edited: Mar 8, 2011
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    Link does not work. How about a schematic with all those resistors placed and labeled?
     
  6. uofmx12

    Thread Starter Member

    Mar 8, 2011
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    what about now? I used the insert link button.
     
  7. Ron H

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    Before you post, click on the Go Advanced button, post your link, then click on the Preview button and test your link before you click on Submit Reply.
     
  8. Ron H

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    I know what the circuit looks like, but I haven't memorized the resistor reference designators. In fact, I doubt that they are standard.
     
  9. uofmx12

    Thread Starter Member

    Mar 8, 2011
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    my link works when I click on it in my post. So not sure what the problem is.
     
  10. Ron H

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    When I click on your link, I get a blank page with this text in the upper left corner:
     
  11. Ron H

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    OK, I got the schematic.
    In your original post, what does 2R1 mean?
    You can find the gain equation on the Internet.
     
  12. uofmx12

    Thread Starter Member

    Mar 8, 2011
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    Ok, so if 2R1=4k, R2=100k, R3=10k, and R4=10k, Find differential gain and common mode gain, would I use those equations in first post? Assumming V1 and V2 what I put.
     
  13. Ron H

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    I still don't get the 2R1 thing. Why didn't you just say R1=2k?
     
  14. uofmx12

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    Mar 8, 2011
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    that is the case, by my circuit has 2R1 instead of R1. SO R1=2k
     
  15. Ron H

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    Have you searched for the gain equation?
     
  16. uofmx12

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    Mar 8, 2011
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    No, Acutally I believe Ad=R4/R3(1+R3/R2). I have a problem in my book that states.

    For instrumentation amplifier a designer proposes R2=R3=R4=100k, 2R1=10k. For ideal components, what is difference mode gain, common mode gain.

    The solution uses Ad above to get 21V/V and Acm=0. So, not sure what Acm equation is but doesn't look like my equations on original post are correct.
     
  17. Ron H

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    That equation doesn't even contain R1, so it can't be correct.
    The correct equation is all over the internet.
     
  18. uofmx12

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    Mar 8, 2011
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    Ad=R4/R3(1+R2/R1), wrote the equation wrong
     
  19. Ron H

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    Ad=R4/R3(1+2R2/R1)
     
  20. uofmx12

    Thread Starter Member

    Mar 8, 2011
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    how is it 2R2? Confused now, as the solution that was provided for the problem I mentioned in previous post, we used the Ad I posted. With Acm=0, so would Acm=0 here?
     
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