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> Sallen Key Vs Multiple Feedback Regarding Q Factor
Blume
Posted: July 17, 2017 01:17 am
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Hi,

I'm trying to understand why Sallenkey Topology should be used for low Q's (<3) and Multiple Feedback for high Q's.

Here's what I think I understand:
The topology of a Filter is simply a means to implement it. The amplitude response is defined by the Filtercharacteristics(Bessel, Butterworth etc.)(and order) and every topology is able to satisfy the neccessary coefficients of these characteristics. And these filtercharacteristics define the Q-factor.

But I've read a couple of times in comparisons between sallen key and multiple feedback topology that salley key is used for low q's(<3) and multiple feedback is used for higher q's. And I can't find any further explanation as to why this is.

So if anybody can explain this to me or tell me where my thinking is wrong: please help!
thanks in advance smile.gif
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Sch3mat1c
Posted: July 17, 2017 09:15 am
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MFB is generally better because of lower sensitivity to component values (minimal sensitivity in fact, I think I heard?).

S-K is forgettable, but has historical precedent or something, or is easy to analyze. So, naturally, the crummier thing persists.

A good filter book should explain all of these in detail; mind that, for satisfyingly proven answers, you'll need calculus, and maybe some complex analysis too. (I don't happen to've done those proofs myself, hence my crude explanation above...)

Tim


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Answering questions is a tricky subject to practice. Not due to the difficulty of formulating or locating answers, but due to the human inability of asking the right questions; a skill that, were one to possess, would put them in the "answering" category.
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Blume
Posted: July 17, 2017 12:47 pm
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thanks for your answer, but I'm still not satisfied biggrin.gif

any recommendations for a good filter book where I can find my answers? And I really don't mind the calculus and complex analysis part smile.gif
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Sch3mat1c
Posted: July 19, 2017 01:07 pm
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Nice. smile.gif

Zverev is the best filter book that comes to mind, but, it may be /too/ technical (or too practical, i.e., lots of tables?).

Again, afraid I don't have much academic depth here so you'll have to look around a bit.

Can also look at the originals in some subjects: Zobel, Bode, and others are all available publicly in old BSTJ articles,
http://etler.com/docs/BSTJ/bstj.html

Tim


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Answering questions is a tricky subject to practice. Not due to the difficulty of formulating or locating answers, but due to the human inability of asking the right questions; a skill that, were one to possess, would put them in the "answering" category.
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