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 filters. 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.

Solution

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