This Q Factor is a measure of how “Selective” or “Un-selective” the band pass filter is towards a given spread of frequencies. In a Band Pass Filter circuit, the overall width of the actual pass band between the upper and lower -3dB corner points of the filter determines the Quality Factor or Q-point of the circuit. ƒ H is the upper -3db cut-off frequency pointĪnd in our simple example in the text above of a filters lower and upper -3dB cut-off points being at 200Hz and 600Hz respectively, then the resonant center frequency of the active band pass filter would be:.ƒ L is the lower -3dB cut-off frequency point. ƒ r is the resonant or Center Frequency.The center frequency is generally calculated as being the geometric mean of the two -3dB frequencies between the upper and the lower cut-off points with the resonant frequency (point of oscillation) being given as: An active band pass filter is a 2nd Order type filter because it has “two” reactive components (two capacitors) within its circuit design.Īs a result of these two reactive components, the filter will have a peak response or Resonant Frequency ( ƒr ) at its “center frequency”, ƒc. The actual shape of the frequency response curve for any passive or active band pass filter will depend upon the characteristics of the filter circuit with the curve above being defined as an “ideal” band pass response.
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