# Band pass filter

Discussion in 'Homework Help' started by naspek, Mar 30, 2010.

1. ### naspek Thread Starter Member

Feb 17, 2010
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0
hey there..

i'm trying to design a bandpass filter with pass band frequency from 10 kHz to 20 kHz and gain = 1
however, my circuit didnt give me the correct graph.. my circuit is correct?

2. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,300
335

Your circuit appears to be a cascade of a high pass filter followed by a low pass filter.

Usually filters are specified by more than just saying "pass band frequency from 10 kHz to 20 kHz and gain = 1".

Is that all your problem asks for? Or have you heard phrases like "Butterworth filter", "Chebychev filter", "maximally flat), "pass band ripple", etc.

When you say "my circuit didnt give me the correct graph", are you referring to a Bode plot of the filter response? In what way did it fail to give the "correct graph"?

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3. ### naspek Thread Starter Member

Feb 17, 2010
45
0
i want to design a bandpass filter with pass band frequency from 10 kHz to 20 kHz and gain = 1 . By combining a Butterworth 2nd order low pass filter and a high pass filter to obtain the bandpass filter. The roll-off rate should be at least -40dB/ decade.

the correct graph should give me -40db/decade for the roll of rate..
but, my graph can't achieved -3dB.. that's why i said, my graph is incorrect..

Mar 6, 2009
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5. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,300
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Would you please post your graph? Set the frequency limits from 1 Hz to 500 kHz.

6. ### naspek Thread Starter Member

Feb 17, 2010
45
0
here is my graph.. i also attach my circuit file in the attachment..

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7. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,300
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Have a look at the attached image.

The blue and red plots are the responses of the individual high and low pass filters. The green plot is the overall response.

The overall response has a 40dB/decade rolloff rate, but your pole frequencies are wrong for a Butterworth bandpass response.

Your pole frequencies are on the negative real axis when they should be complex.

Explain how you determined the values for your resistors and capacitors and we can see where your mistake is.

• ###### FilPlot.png
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