Re: Band pass filter
naspek said:
1st, the pass band is from 10kHz to 20kHz..
so.. my mid frequency, fc must be 15kHz right?
my fl=10kHz and my fh = 20kHz..
fl and fh must cut -3dB.. which mean, at -3dB is my fl and fh
my roll off rate is at least -40dB/decade..
OK, some comments seems to be appropriate:
1.) In general, you have two choices: (a) Series connection of two bandpass stages or (b) series connection of a lowpass and a highpass (as you did).
2.) Because of the rather large bandwidth (10 kHz with mid frequency around 15 kHz) alternative (b) seems to be the best one.
3.) However, the exact design is not very easy:
You cannot design the corner frequencies for both stages separately to 10kHz resp. 20 kHz, since both stages influence each other. More than that, if the mid band gain has to be "1", you cannot use unity gain Sallen-Key stages (each having a gain of only unity).
4.)
Exact Design: Apply the bandpass-lowpass transformation, find the low pass poles of a 4th order transfer function, transform back to bandpass and design both stages of second order using multi-feedback or Sallen-Key (with gain) topology.
This is a rather challenging process.
5.) Or use one of the filter design programs (filter-solutions.com offer a demo version which suits to your requirements)
Good luck
Added after 2 hours 45 minutes:
Addendum to 2): Series connection of two different (!) bandpass stages is also possible, however, the design procedure is not much less if compared with the LP-HP approach (BP-LP transformation before).