Click here to purchase the entire book in PDF format. Bandpass filters Bandpass filters are those that permit a band of frequencies to pass through the filter, while attenuating signals in frequency bands above and below the passband. These types of filters should be sub-divided into two categories: those with a wide passband, and those with a narrow passband. If the passband of the bandpass filter is wide, then we can create it using a high-pass filter in series with a low-pass filter. The result will be a signal whose low-frequency content will be attenutated by the high-pass filter and whose high-frequency content will be attenutated by the low-pass filter. The result is that the band of frequencies bewteen the two cutoff frequencies will be permitted to pass relatively unaffected. One of the nice aspects of this design is that you can have different orders of filters for your high and low pass (although if they are different, then you have a bit of a weird bandpass filter...) If the passband of the bandpass filter is narrow (if the Q is greater than 10), then you'll have to build a different circuit that relies on the resonance of the filter in order to work. Take a look [] (or any decent book on op amp applications) to see how this is done.