Filter Bandwidth help! Q factor changes with Fc

[oxygen8d]

Filter Bandwidth help!

Hi There,

I am studying a Multiple Feedback band pass filter, I have made changes to the Fc of the circuit to give an octave above 1122, to 2244 and an octave below 1122 to 561 with the same Q.

I have done this successfully but I am confused why if the Q has stayed the same Q=1.87 why does the bandwidth double with frequency, :

BW = Fc/Q

so 561/1.87 = 300
1122/1.87 = 600
2244/1.87 = 1200

Could someone explain why there is this doubling of BW with frequency while maintaining Q.

Thanks All!!!!

Jim

 

[LvW]

Re: Filter Bandwidth help!

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I have done this successfully but I am confused why if the Q has stayed the same Q=1.87 why does the bandwidth double with frequency, :

BW = Fc/Q

so 561/1.87 = 300
1122/1.87 = 600
2244/1.87 = 1200
Could someone explain why there is this doubling of BW with frequency while maintaining Q.

I am not sure if I understood you well but the quality factor Q is defined as the Fc-to-BW ratio. Thus, BW doubles with Fc - if Q is constant.

Added after 3 minutes:

And, if you tune the impedance level of parts in order to change the mid frequency, you do NOT change the Q value as it is always a ratio of parts values which remains constant.