operational amplifier dc input ?

Discussion in 'Homework Help' started by Xufyan, Aug 21, 2011.

  1. Xufyan

    Thread Starter Member

    Aug 3, 2010
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    I have a question regarding opamp,

    i have a non-inverting amlifier problem with Rf and Ri is given but the input voltage is not given however input DC voltage is -1.7 volt.

    according to the formula,

    Vo = (1+ Rf/Ri)x Vi(ac)

    how can i solve if the DC input voltage is given but not AC :confused:
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    But if circuit don't have any capacitor in signal path.
    The DC gain is equal to AC gain
    Av = 1+ Rf/Ri
     
  3. Xufyan

    Thread Starter Member

    Aug 3, 2010
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    why would there be a capacitor in a non-inverting amplifier :S

    will it be Av = (1+ Rf/R)x Dc input ???

    i have checked a non-inverting amp in multisim with a DC input voltage , the output is 'ZERO'
     
  4. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    It's not clear what you want to solve. Do you want to know the gain? Well you have the formula... The AC-gain will also depend on the frequency of the AC-signal.
    Or do you want to know the AC output voltage? That would be impossible, since you don't have the input voltage.

    And what's the DC- voltage doing there?:confused:
     
  5. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Maybe it is time for you to show us the diagram.
    Are we talking about the same circuit ??
    [​IMG]
     
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  6. Xufyan

    Thread Starter Member

    Aug 3, 2010
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    this is what i am thinking :S why there is no AC i/p voltage mentioned ?? :S

    and what does DC voltage is doing there :S

    this is what question says,

    Determine the output voltage of Non-inverting amplifier if Rf=120k and Ri =39k and the DC input voltage is -1.7 volts ?


    Book: Fundamentals of Operational Amplifiers and Linear Integrated Circuits by Howard M.Berlin and Frank C Getz, Jr.

    Problem 10 , Chapter 2
     
    Last edited: Aug 21, 2011
  7. Xufyan

    Thread Starter Member

    Aug 3, 2010
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    yeah, the same circuit
     
  8. Jony130

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    Feb 17, 2009
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    If so the DC gain = AC gain = 1+ R2/R1
    So for Vin =1.7V
    Vout = -1.7V * (1 + 120K/39K) = -6.93V
     
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  9. Xufyan

    Thread Starter Member

    Aug 3, 2010
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    how DC gain could be equal to AC gain :S
     
  10. Jony130

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    Feb 17, 2009
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  11. Xufyan

    Thread Starter Member

    Aug 3, 2010
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    i didn't know that , thank you :)
     
  12. praondevou

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    Jul 9, 2011
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    It doesn't say anything about AC. <they want to know the DC-output voltage.

    edit: oops I guess I'm several posts too late
     
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