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Band-Pass Active Filter

A band-pass filter has a pass-band between two cut-off frequencies fce (lower cut-off frequency) and fcu (upper cut-off frequency) such that fcu > fclAny input frequency outside this pass-band is attenuated.

Bandwidth (BW)

The range of frequency between fCL and fCU is called the bandwidth.
Description: 5140.png
The bandwidth is not exactly centered on the resonant frequency (fr).
If fCU and fCL are known, the resonant frequency can be found from,
fr = Description: 5146.png
Description: Description: 1691.png
Band pass filter characteristics

Quality Factor (Q)

It is defined as the ratio of resonant frequency to bandwidth, i.e., Q = Description: 5237.png
Q is a measure of the selectivity. Higher the value of Q, the more selective is the filter, i.e., narrower is the bandwidth.

Types of Band-Pass Filters

  1. Wide Band-Pass Filter wide-band filter has a bandwidth that is two or more times the resonant frequency; i.e., Q  0.5.
    It is made by cascading a low-pass and a high-pass filter circuit.
  2. Narrow Band-Pass Filter A narrow band filter has a quality factor, Q > 0.5.
    It is made by using a single op-amp and multiple feed back circuits.

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