2nd order Butterworth LPF cascade with 2nd Butterworth HPF

[Dion_Aus]

Hi guys, my task is to cascade a 2nd order Butterworth LPF with a 2nd Butterworth HPF, individually my 2 filters function exactly as I want, however when I connect them together (ie LPF -> HPF) I do not get any passband, when my LPF -3dB frequency is 10kHz and my HPF's -3dB frequency is 5kHz... is it the way im connecting them (series)??? ive uploaded the bode plots for the individual filters.

Would appreciat the help, thanks.





Am I wrong in assuming there should be a passband between 5kHz and 10kHz???

 

[LvW]

Re: 2nd order Butterworth LPF cascade with 2nd Butterworth H

Hi guys, my task is to cascade a 2nd order Butterworth LPF with a 2nd Butterworth HPF, individually my 2 filters function exactly as I want, however when I connect them together (ie LPF -> HPF) I do not get any passband, when my LPF -3dB frequency is 10kHz and my HPF's -3dB frequency is 5kHz... is it the way im connecting them (series)??? ive uploaded the bode plots for the individual filters.

Would appreciat the help, thanks.

Am I wrong in assuming there should be a passband between 5kHz and 10kHz???

No, in principle, you are not wrong. You should get "any passband".
However, have you no specification for the passband ? Because I don`t expect that the pass band will be very useful. But it should be possible to identify it.
But watch the opamp behaviour. For a 10 kHz band edge the opamp should have higher frequency capabilities - as you can see in the disturbed high pass characteristic.

 

[Dion_Aus]

Re: 2nd order Butterworth LPF cascade with 2nd Butterworth H

The resultant Bode plote never gets any higher than like -545dB??? I am really confused as to why this is

 

[FvM]

Sounds like your circuit is wrong somehow. Start with checking the bias points.