op amp resistor sizing question

Discussion in 'Homework Help' started by notoriusjt2, Oct 31, 2010.

  1. notoriusjt2

    Thread Starter Member

    Feb 4, 2010
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    [​IMG]

    Vin = -Vout(R1/R2)
    1=-12(1000/R2)
    solve for R2
    R2=83.3Ω

    correct?
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    You have made an incorrect assumption.

    Notice that the feedback is positive not negative.

    hgmjr
     
  3. notoriusjt2

    Thread Starter Member

    Feb 4, 2010
    209
    0
  4. hgmjr

    Moderator

    Jan 28, 2005
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    Section 4 of the article that you have linked to covers the topic fairly well.

    hgmjr
     
  5. notoriusjt2

    Thread Starter Member

    Feb 4, 2010
    209
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    yes it does, its a great article, and I followed it to a T.

    "Since V+ is a voltage divider formed by R1 and R2between Vin and Vout it follows that:
    • V+ = (Vin R2 + Vout R1 )/(R1 + R2)

    Combining these equations and solving for V+ = 0 yields Vin = -Vout R1/R2.

    I get 83.3ohms, but that answer is listed as incorrect
     
  6. hgmjr

    Moderator

    Jan 28, 2005
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    Keep in mind that the problem statement states that the input switches when the +input of the 1K resistor reaches 1V. If you assume that the negative input of the opamp is at 0V then you can calculate the current that is flowing in the 1K resistor at the time the output switches.

    hgmjr
     
  7. notoriusjt2

    Thread Starter Member

    Feb 4, 2010
    209
    0
    it has Vin across the two input wires. does this mean that Vin+ is actually +.5v and that Vin- is actually -.5V?
     
  8. sharma@vivek

    New Member

    May 2, 2010
    9
    0
    since the input is between the two terminals of the opamp, you need to solve the value for R by superposition theorem.
     
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