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Question on transfer function equation

Discussion in 'Electronic Basics' started by MRW, Dec 4, 2006.

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  1. MRW

    MRW Guest

    Hi again!

    I was reading the following link:
    http://focus.ti.com/lit/ml/sloa088/sloa088.pdf

    In page 16-12 (Figure 16-12) they are showing a first order
    non-inverting low pass filter. I tried calculating the transfer
    function and came up with the following:

    Vo / Vin = (1 + R2/R3) / ( s*C1*R1 + 1 )

    Their transfer function is:

    Vo / Vin = (1 + R2/R3) / ( wc*s*C1*R1 + 1 ), wc = omega sub c (I think
    this represents the cut off frequency).

    My question is: how did they get wc?

    Thanks!
     
  2. Andrew Holme

    Andrew Holme Guest

    Your transfer function is correct. They are wrong.
     
  3. Eeyore

    Eeyore Guest

    It's a mistake. s = jwc

    Graham
     
  4. The Phantom

    The Phantom Guest

    It's not wc (omega times c); it's omega sub c. Go back and read page
    16-2, where they say:

    "For a normalized presentation of the transfer function, s is referred to
    the filter's corner frequency, or - 3dB frequency, w sub c, and has these
    relationships:"
     
  5. MRW

    MRW Guest

    Yep, I actually wrote down wc = omega sub c. Thanks for pointing out
    that page. I'm going to re-read it again and see if I can understand
    it. I'm not too comfy about s = sRC. To me, it's like saying 1 = 1*2*3.
     
  6. The Phantom

    The Phantom Guest

    I would definitely agree. I would have made up a new symbol for the
    normalized s, something like s' or s sub n or something. It's very bad
    practice to use the same symbol for two different things!

    I think I would just rewrite all the pertinent equations with a different
    symbol for normalized s before I tried to follow the analysis.
     
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