1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Op amp filter

  1. Sep 16, 2007 #1
    1. The problem statement, all variables and given/known data

    We have a filtered amplifier (diagram is attached). We do not know Cf or Rf.
    We know that:
    1 volt input at 5 Hz = 9.25 volt output
    1 volt input at 200 Hz = 6.77 volt output
    1 volt input at 500 Hz = 3.66 volt output

    2. Relevant equations

    The transfer function for the first amplifier is H(w) = -Zf/Zi = -10000/2000=-5
    The transfer function for the second amplifier is H(w) = -Zf/Zi, where Zi = 5000 and
    Zf = Rf/(Rf j w Cf + 1).

    3. The attempt at a solution
    If we multiply the two transfer functions above, we obtain
    H(w) = V0/Vi = Rf/(1000(Rf j w Cf + 1))

    I multiplied the frequencies by 2*pi and made appropriate substitutions in the above equation to get the following three expressions:
    9.25 = Rf/(1000(31.4159 Rf j Cf + 1))
    6.77 = Rf/(1000(1256.64 Rf j Cf + 1))
    3.66 = Rf/(1000(3141.59 Rf j Cf + 1))

    Three equations for only two unknowns?

    I get that Cf = -5.3E-8 i and Rf = 9395.

    What am I doing wrong?

    Attached Files:

  2. jcsd
  3. Sep 17, 2007 #2
    Since you are only given the magnitudes (and not the phases) of the input and output signals you must work with the absolute value of the transfer function.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Op amp filter
  1. Op-Amp Bandpass Filter (Replies: 13)

  2. Op amp (Replies: 1)

  3. Op-Amp question (Replies: 13)