op amp circuit analysis help

Discussion in 'Homework Help' started by notoriusjt2, Oct 18, 2010.

  1. notoriusjt2

    Thread Starter Member

    Feb 4, 2010
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    [​IMG]
    im not looking for someone to give me the answer here

    but what is the simplest way to analyze this circuit. what steps should i take to solve this problem?

    i assume V1=9V and V3(bottom left)=5V

    how do i calculate voltage drops across those resistors without a current value?
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    The labels on the opamp stages are not clear.

    hgmjr
     
  3. notoriusjt2

    Thread Starter Member

    Feb 4, 2010
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    i think A1 and V1 are top left
    A2, V2 top right
    A3, V3 bottom left
    A4, V4 bottom right
     
  4. Engr

    Member

    Mar 17, 2010
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    I think the best way to solve the circuit is to use the nodal analysis. There is no given current in the circuit but the current in the circuit can be solve.
     
  5. hgmjr

    Moderator

    Jan 28, 2005
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    Start by acquainting yourself with the key characteristics of an "ideal" opamp.

    hgmjr
     
  6. notoriusjt2

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    Feb 4, 2010
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    Ideally Einv = Eni

    So that means that the voltage of the + and - of each op amp are equal. So the voltage at V3 = 5v
     
  7. hgmjr

    Moderator

    Jan 28, 2005
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    Due to the poor image readability, I am not sure which node is labeled V3 on the diagram.

    You can begin by determining what voltage is present on the negative input of the opamp in the upper right position.

    You then know what the positive input of the opamp in the upper right position should be.

    You know what voltage is present at the output of the opamp in the lower left position. From that you should be able to determine the voltage that you need at the output of the opamp in the lower right position.

    Once you know the output voltage needed at the output of the opamp in the lower right position you should be able to determine what the output of the opamp in the upper right position needs to be. That will be your answer.

    hgmjr
     
  8. notoriusjt2

    Thread Starter Member

    Feb 4, 2010
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    I stand corrected A3 is in the top right with its output facing to the right

    A4 is directly underneath it
     
  9. notoriusjt2

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    Feb 4, 2010
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    so am i correct in saying that the voltage @ V2 = 5V?
     
    Last edited: Oct 30, 2010
  10. notoriusjt2

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    Feb 4, 2010
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    here is a better version

    [​IMG]
     
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    No, it is not 5V.
     
  12. notoriusjt2

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    Feb 4, 2010
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    my bad, i meant V2. i edited my last post to reflect that
     
  13. Ron H

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    Yes, V2 is 5V. A1 and A2 are voltage followers (unity gain, noninverting).
     
  14. themindflayer

    Member

    Oct 29, 2010
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    Here's a very good intro to op-amps ....

    As for circuit solving ... following the golden rules in the above file .... V1 and V2 are 9V and 5V respectively ( o/p makes itself so that V diff at i/p is zero, it will be zero when the inv pin has the same volts as the non inv pins ... hence the answer)

    A3 inv pin has 4.5V, voltage across 10K A4's FB res is the same as the voltage across the 30K ... which is as you might have guessed V3 (since the junction of A4's 10k and 30k is at 0V due to the non inv being at 0V)

    so now it all boils down to what V are at A3 non-inv since if the diff btw A3's i/p is >0 V3 shoots to +ve rail (12V i suppose) if the diff is < 0V V3 goes to the -ve rail (-12V?)

    so what is the voltage at the A3's non inv i/p ... that is what i cant figure out either and call on to help ......
     
  15. Ron H

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    As you mentioned, the -input of A3 is at 4.5V. Feedback through A4 makes the +input of A3 also equal 4.5V. Since we know V2, we can calculate V4. Once we know V4, we can calculate V3.
     
  16. eblc1388

    Senior Member

    Nov 28, 2008
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    If you are still struggling, follow the steps and fill in the blanks.

    [​IMG]
     
  17. notoriusjt2

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    Feb 4, 2010
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    1. G=0
    2. V=9V
    3. G=0
    4. V=5
    5. V=4.5V
    6. V=2.5V
    7. i=0.5mA
    8. V=0V
    9. how do i find this one?
     
  18. eblc1388

    Senior Member

    Nov 28, 2008
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    You have many of them wrong.

    1. G=0 wrong
    2. V=9V
    3. G=0 wrong
    4. V=5
    5. V=4.5V
    6. V=2.5V wrong
    7. i=0.5mA wrong
    8. V=0V wrong
    9. how do i find this one?

    You need reasoning (not calculation) based on ideal Opamp model for answer to item 6 and 7.

    You also have to re-read the gain definition of common opamp configuration. Hint: gain G can be positive or negative and higher or lower than 1.
     
    Last edited: Oct 31, 2010
  19. notoriusjt2

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    Feb 4, 2010
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    so if Vin=Vout then the gain would be 1?
     
  20. eblc1388

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    Nov 28, 2008
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    If Vout=Vin then gain G=1. If Vout = -Vin then gain=-1.
     
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