buddypoor
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Hi Berti,
I agree with HCWWong, that the required BW depends on the structuire chosen as well as on the pole parameters (Q resp. pole frequency). However, of course it depends also on the allowed deviation from the nominal response. More than that, you should not forget the influence of the slew rate, because it makes no sense to look only at the small signal BW. In addition. the topologies as mentioned by HCWWong (Tow-Thomas resp. Akerberg-Mossberg) are multiple opamp-structures and, therefore, are economical only if their specific advantages (lowpass, highpass and bandpass simultaneously, all of second order) are to be used. According to my experience and in agreement with nearly all recommendations in filter books, the "best" second order structure from the viewpoint of economical use of the small signal BW is the GIC-based topology. The influence of the limited opamp BW on the filter response will be reduced if both opamps are matched (dual amp chip). The situation is even more complicated since there are other active candidates than opamps, each with their own structure (OTAs, CFAs and Current Conveyors). In general, your question touches a very important and - at the same time - a very difficult question: What is the "best" filter topology for a certain application ? I am involved in filter design since 25 years (and I have written two books in german on filters), but up to now I did not come to a final conclusion. The answer resp. the choice of a specific topology depends on several items (accuracy requirements, filter characteristics, filter order, frequency range, pole data, application conditions and economical as well as operational aspects like power requirements). But, in any case, I am sure that circuit simulation gives much more information than a simple formula because it can take into account more important opamp nonidealities like input/output impedance, slew rate, load influence, parasitic signal pathes (i. e. through the passive network directly to the opamp output; very important for Sallen-Key-structures). Sorry, too much came into my mind....... Regards Lutz vW
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