cutoff frequency of a bandpass filter

Discussion in 'General Electronics Chat' started by QMcat, Jan 13, 2010.

  1. QMcat

    QMcat Thread Starter New Member

    Jan 13, 2010
    a bandpass filter that build up by two simple a RC-lowpass and a CR-highpass in series. When combines those two, the highpass affect the lowpass and vice versa. how to calculate the both cutoff frequency?
  2. Audioguru

    Audioguru New Member

    Dec 20, 2007
    Ontario, Canada
    They combine to make a very poor filter because they are too simple. If the bandwidth is as narrow as is possible (the RC values are the same) then they attenuate everything including the frequency you want.
  3. t06afre

    t06afre AAC Fanatic!

    May 11, 2009
    You should also use a operational amplifier between the two sections. The two sections need to be "isolated" from each other
  4. The Electrician

    The Electrician Senior Member

    Oct 9, 2007
    You can do almost as well as using a buffer between the sections by scaling the impedances so that the second section doesn't load the first.

    For example, if the first section consists of a 10k resistor and .01F cap, then make the second stage from a 100k resistor and .001μF cap.

    Even then, it's not a very good filter. With only Rs and Cs, it's not possible to build a circuit with complex poles; all the poles are on the negative real axis, and good filters need complex poles (and zeros).
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