I am now designing a narrow bandpass filter. Requirements are shown below:

Center frequency is about 2kHz(10% flutuation is OK). The gain of center frequency is about 34dB(the higher the better,up to 65dB). The difference of gain between 2kHz signal and 1kHz signal(or half of the center frequency) needs to be not less than 43dB.

My current design is to use the MFB topology to build up a two-stage/four-order bandpass filter. enter image description here

Is such design OK? Or any other better topology or method?

Addition: The simulation result is

Center frequency at 1.999kHz; Gain@2kHz is 34dB; Gain@1kHz is -12dB; The phase margin of each stage is higher than 45 degree(test by middlebrook method)

  • \$\begingroup\$ It looks fine. Did you calculate the values yourself or use a filter program? Both your stages look to have about the same gain. (IIRC there is a suggestion that when cascading filters you keep the higher gain stages later in the chain for dynamic range issues.) Are you using two different opamps in each stage? If you are going to put this into production then one thing to look at is how it changes with component tolerances. (change some caps by +/- 5% and see what happens.) (Worse case, I think, is when first stage are both bigger (or smaller) and 2nd is opposite.) But play around. \$\endgroup\$ – George Herold Sep 26 '14 at 12:16
  • \$\begingroup\$ @GeorgeHerold: Thank you for your reply. I once used FilterPro of Ti, but I don`t have the required value of caps and res. So I designed it by myself. About IIRC, can you provide some links to the reference? Because I have not seen such requirement and it must be useful for my design. Because of the practical requirement, I have to use two different opamps. Would it cause some problems? All passive components are 1%tolerance and the simulation result seems ok(about 1.98kHz center frequency, gain not less than 33.5dB, the difference between 1k and 2k higher than 45dB) \$\endgroup\$ – billyzhao Sep 27 '14 at 3:49
  • \$\begingroup\$ Re: cascading sections. Do you have AoE (art of electronics)? It's just a sentence in the filter section... regarding LP filters in this case. It won't really matter for HP, and maybe not BP either. The 1% caps are going to be hard to find or expensive. I end up sorting caps into ~1% bins for filters. \$\endgroup\$ – George Herold Sep 27 '14 at 13:16

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.