rfmagic wrote on Dec 30th, 2010, 1:19pm:Hi,
I am designing a 5th order Butterworth filter using casceded biquads. I have seen many topologies to implement a biquad but I am not sure I got the main advantages of each type.
I appreciate is someone could tell me the main advantages and disadvantages of the following biquad types:
1. Sallen_Key
2. MFB
3. Tow-Thomas
Thanks
At first, I wouln't call S+K and MFB a "Biquad" since both cannot create a biquadratic transfer function - but that's not too important.
In particular:
* S+K is rather sensitive to parts tolerances, but less sensitive to opamps GBW.
* Just the opposite is true for MFB
* The TT-biquad also is not suited for rather high frequencies (because of sensitivity to the GBW). More than that, it uses more than one opamp (as both other alternatives, S+K, MFB).
*One additional hint: The cascade approach (several stages in series) is not the best from the sensitivity point of view. What about active realization of a passive ladder structure (FDNR technique)?