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Op Amp as AC Current Source

  1. Feb 5, 2013 #1
    1. The problem statement, all variables and given/known data
    Design a circuit based on the noninverting op-amp configuration that functions as a 1mA ac current source.

    2. Relevant equations
    Gain = Vout/Vin = 1 + R2/R1

    3. The attempt at a solution
    I really don't know how to start.
     

    Attached Files:

    Last edited: Feb 5, 2013
  2. jcsd
  3. Feb 5, 2013 #2

    berkeman

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    What is the "virtual ground" property of opamps? And how can you use that to help in this problem?
     
  4. Feb 5, 2013 #3
    Well, the current flowing into the +/- inputs is essentially 0. I understand the basics of the op amp (how to derive gain, vout, etc.), but I have never encountered a problem asking to use one as a current source. And, the book (Microelectronic Circuits & Devices, 2nd, Horenstein) is very vague, unfortunately.

    If I have a different resistance connected to Vout (not R1 or R2), then it will change the current, won't it? That's where I am getting lost. I don't know how to make the current independent of whatever load resistance is connected.
     
  5. Feb 5, 2013 #4

    berkeman

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    The property you mention is not the "virtual ground" property of opamps. Try again?
     
  6. Feb 5, 2013 #5
    V+ = v-.
     
  7. Feb 5, 2013 #6

    berkeman

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    Bingo!

    So what will the voltage at the - input always be? And so what will the current be through R1? And since the current into the - input is zero as you already pointed out, what will the output current be?
     
  8. Feb 5, 2013 #7
    iout = iR1 = iR2 = [itex]\frac{Vin}{R1}[/itex]

    So, I could choose Vin = 1V, R1 = 1kΩ, and R2 = whatever?
     
    Last edited: Feb 5, 2013
  9. Feb 5, 2013 #8

    berkeman

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    Yep!

    The Vout will depend on the load impedance. If the load impedance is too large, the output of the opamp will saturate (like if Rload = 10kOhms, and the supplies to the opamp are +/-5V). But as long as the opamp doesn't saturate, the output current will be 1mApp.
     
  10. Feb 5, 2013 #9
    Ah, okay. That makes sense.

    Thank you so much for your help! I really appreciate it.
     
  11. Feb 5, 2013 #10

    berkeman

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    Staff: Mentor

    All I did was ask questions... :smile:
     
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