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Homework Help: Active low pass filter

  1. Mar 28, 2012 #1
    1. The problem statement, all variables and given/known data

    Design this circuit to give a cut off (-3dB) frequency of 1kHz, a low frequency gain of 20, and an input impedance equal to 22kΩ

    2. Relevant equations



    3. The attempt at a solution

    So, I started off with Vout/Vin = [-R2/R1]/[1 + sCR2. I also know that the dc gain will be -R2/R1. I also know that frequency = 1/[2∏R2C]

    Im a little lost as to where to start!

    http://upload.wikimedia.org/wikipedia/commons/thumb/5/59/Active_Lowpass_Filter_RC.svg/300px-Active_Lowpass_Filter_RC.svg.png
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     

    Attached Files:

  2. jcsd
  3. Mar 28, 2012 #2

    NascentOxygen

    User Avatar

    Staff: Mentor

    Hi kimandrew20! http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    What is the input impedance of the op-amp filter you show? Start with that.

    Are you going to use just a single stage?
     
    Last edited by a moderator: May 5, 2017
  4. Mar 28, 2012 #3
    Is the input impedance just the value that the problem has specified? The 22kΩ value?

    Yes, just a single stage.
     
  5. Mar 28, 2012 #4
    I tried it again and I got a value for the capacitance and the resistors. Does this look correct?

    R1 will equal the input impedence (The two inputs, V- and V+ are at approximately the same voltage, R1 is at ground voltage, therefore R1 = input impedence = 22 kΩ

    Gain (A) = (Rf/ R1) → Rf/22k = 20
    Therefore, Rf = 440 kΩ.

    f = 1/ [2∏ * R * C] therefore
    C = 1/ [2∏ * R * f] → 1/[2∏ * (22*1000) * (1 * 1000))] = 7.234 x 10-9 F
     
  6. Mar 28, 2012 #5

    rude man

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    Homework Helper
    Gold Member

    You got the right Rf but the wrong C. Reconsider the R you used to compute C ...
     
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