All About Circuits Forum cutoff frequency of a bandpass filter
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#1
01-13-2010, 03:56 PM
 QMcat New Member Join Date: Jan 2010 Posts: 2
cutoff frequency of a bandpass filter

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
01-13-2010, 04:43 PM
 Audioguru Banned Join Date: Dec 2007 Location: Ontario, Canada Posts: 9,411

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
01-13-2010, 04:50 PM
 t06afre Senior Member Join Date: May 2009 Posts: 5,939

You should also use a operational amplifier between the two sections. The two sections need to be "isolated" from each other
#4
01-13-2010, 05:02 PM
 The Electrician Senior Member Join Date: Oct 2007 Posts: 1,743

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).

 Tags bandpass, cutoff, filter, frequency

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