Band Pass Filters

Active Band-Pass Filter (BPF)

A band-pass filter (BPF) passes frequencies within a certain range and rejects (attenuates) frequencies outside that range. It can be made by combining the first order low pass and high pass filter. The picture below shows an example of an active BPF.


Intermediate pass frequencies fC2 < f < fC1 with a gain of -R1/R2 where fC1 = 1/(2 pi R1 C1), fC2 = 1/(2 pi R2 C2).

The bode plot of the active BPF is shown below. fPL= fpass-lower = fC2= 1/(2 pi R2 C2), fPU = fpass-upper= fC1= 1/(2 pi R1 C1)


Design Example
Design an active BPF that allows frequencies between 1.6 kHz and 16 kHz to pass through with a gain of -20.

fPL =1/(2 pi R2 C2) = 1.6 kHz, C2 = 1 μF, R2 ≈ 100 Ω
Passing intermediate frequencies with a gain of -10, i.e., R1/R2 = 10, R1 = 10 X R2 = 1 kΩ
fPU = 1/(2 pi R1 C1) = 16kHz, C1= 0.01 μF

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