# BODE plot for band-pass filter???

Discussion in 'General Electronics Chat' started by Lainya, May 24, 2008.

1. ### Lainya Thread Starter New Member

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What would cause the cutoff frequencies not to be -3 dB down from the center frequency on a BODE plot for a resistive-capacitive band-pass filter?
The filter was simply a RC low-pass filter, followed by an RC high-pass filter. The calculated cutoff frequencies of the high-pass and low-pass filters were 8kHz and 15kHz.

Last edited: May 24, 2008
2. ### thingmaker3 Retired Moderator

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May 16, 2005
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"Cutoff frequency" is 3dB down from "center frequency" by definition. Are you perhaps asking why the plot is not symmetrical?

3. ### Ron H AAC Fanatic!

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They will load each other unless you have a buffer (voltage follower) between them.

4. ### Caveman Active Member

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Apr 15, 2008
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Remember that bode plots are approximations. This is especially true near the cutoff frequencies.

The filters will affect each other even if the buffer is there because your cutoff frequencies are so close to each other. For example if the low pass filter is first, at 8kHz (the cutoff frequency of your high pass filter) will already be -1dB down before it goes into the high pass filter.

Your center frequency won't even be exactly halfway between them linearly. It will be logarithmic. So it will be at SQRT(15/8)*8kHz = 10.95kHz.