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DC Imperfections of a Real Op Amp

  1. Oct 7, 2011 #1
    The problem statement, all variables and given/known data
    A certain op amp has the maximum offset voltage magnitude of Vos = 2 mV, the maximum bias current of Ib = 100 nA and the maximum offset current magnitude of Ios = 10 nA. The op amp is used in the noninverting amplifier shown in the diagram below. Find the worst-case DC output voltage range if vi = 0 and Rbias = 0. What should be the value of Rbias to cancel the effect of bias currents? Repeat part 1 for vi = 0 and Rbias calculated in part 2.

    ReOp_P2_Diagram2.aspx.png

    The attempt at a solution

    I have no idea with the question but I calculated output voltage as 22mV using
    V0 = -(1+R2/R1)*Voff
     
  2. jcsd
  3. Oct 7, 2011 #2

    berkeman

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    That's a good start. But you also need to include the effects of the input offset current.

    Figure out visually how to get the worst case offsets each way (both positive and negative) -- which way should the input offset current point in order to add to a positive input offset voltage? Calculate the maximum output voltage that results. Then which way should theinput offset current point in order to add to a negative input offset voltage? Calculate the maximum output voltage that results. Then subtract the two output voltages to see what the maximum output deviation is due to the finite input offset voltage and input offset current.

    Then the 2nd part of the question is a classic circuit question in opamp circuit design. Think about why the input offset current generates an input offset voltage term...

    EDIT -- BTW, it may help you to look at a typical opamp datasheet, to see how the input offset voltage and current are defined...
     
  4. Oct 7, 2011 #3
    I'm still confused can you guide through what to do as the first step
     
  5. Oct 7, 2011 #4

    berkeman

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    Please link to a typical opamp datasheet, and post your specific questions about what you don't understand about input offset voltages and currents...

    This all works best if you put a lot of effort forth in working on your schoolwork problems...
     
  6. Oct 7, 2011 #5
    That's because I barely understand these problems and how to approach them
     
  7. Oct 8, 2011 #6

    rude man

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    To compute the effects of bias and offset currents, pretend the op amp offset voltage = 0 and compute the output voltage based on elementary Kirchhoff equations.

    Remember that Ios = Ib2 - Ib1.

    The total effect of Ios, Ib and Eos is then the addition of these individual effects,worst-cased.
     
  8. Oct 8, 2011 #7
    Can you specify where is Ib1 and Ib2?
     
  9. Oct 8, 2011 #8

    rude man

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    Sure. Ib1 is the current into (or out of) the - input lead, and Ib2 is the current into (or out of) the + input lead.

    Ideally, Ib1 = Ib2 but because of imperfections, Ib2 - Ib1 = Ios.
     
  10. Oct 8, 2011 #9
    Isn't Ib1 = 0 because resistance is infinity
     
  11. Oct 8, 2011 #10

    rude man

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    No. This is a dc current. The ac impedance may be (essentially) infinite, but there is still a dc current flowing all the time. That's to keep the input transistors alive. This current does not change with either dc or ac input voltage (as far as you're concerned).
     
  12. Oct 8, 2011 #11
    How would you get the Kirchoff equations?
     
  13. Oct 8, 2011 #12

    rude man

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    Surely you can write the sum of currents = 0 at V(-). First, set V(+) = 0. Remeber Ib1, Ib2 and Ios = Ib2 - Ib1 are all dc currents. They are constants.
     
  14. Oct 8, 2011 #13
    sorry but I can't really picture where the currents are.
     
  15. Oct 8, 2011 #14

    berkeman

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    Zero effort on your part is not allowed. here. Use the hints and advice you're received so far here, and look at the datasheet for a typical opamp. If you want us to help you, you MUST show some effort on your part.
     
  16. Oct 8, 2011 #15
    Where can you find a datasheet? I really got no clue to this question.
     
  17. Oct 8, 2011 #16

    berkeman

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    Please Google LM324 datasheet.

    And the also Google TL084 datasheet.

    Compare and contrast the input bias currents between those two opamps. And are the input offset voltages different at all?

    Do the datasheets show the models for the input offset voltage, input bias currents, and input offset current?

    You might also search at the National Semiconductor website or Maxim's website for application notes on opamp offset currents and voltages. The manufacturers are very motivated to help you understand practical design considerations.
     
  18. Oct 9, 2011 #17
    yes they are different.
     
  19. Oct 9, 2011 #18

    uart

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    Take the opamp as being ideal, an then place a current source between zero volts and the -ive input terminal on the opamp. Can you analyze that circuit?
     
  20. Oct 9, 2011 #19
    Where is the zero volts?
     
  21. Oct 9, 2011 #20

    berkeman

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    In your first post in the opamp circuit figure, zero volts is the ground symbol at the bottom. The horizontal line symbol below the bottom resistor.

    Please read through this tutorial page on opamps:

    http://en.wikipedia.org/wiki/Operational_amplifier

    It covers the basics of ideal opamps, and then describes the other things that you need to consider for real opamps (like the input offset voltage and offset current that this problem is asking about).
     
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