1. Active Low-Pass Filters
The 3-dB frequency c is related to o by a factor known as the FREQUENCY CORRECTION FACTOR (kLP), thus;
Parameter table for Sellen-Key 2nd order LPF
Type of response 
kLP
Bessel
1.732
0.785
Butterworth
1.414 1
Chebyshev (1 dB)
1.054
1.238
Chebyshev (2 dB)
0.886
1.333
Chebyshev (3 dB)
0.766
1.390

2. Example
Design a Bessel 2nd order low-pass filter with a 3-dB frequency of 5 kHz. Use C = 22 nF.

3. Solution
From the table:
Hence:

4. Solution
In order to minimize the offset error, we have to meet the following condition:
Or;
And:
From the parameter table;

5. Solution

6. Solution
A Bessel 2nd order LPF

7. Frequency response curve of a 2nd order Bessel LPF
Solution fc
Frequency response curve of a 2nd order Bessel LPF