[sdiy] Filter stability and self oscillation

[Aaron Lanterman]

On Mon, 29 May 2006, Nicholas Gregorich wrote:

> Just finished my second electronics course and feedback (negative) was 
> one of the last subjects we studied.

> I found it very interesting to learn about negative feedback and finally 
> get some educational insight into what makes a resonant 
> filter...resonate, but some things just aren't adding up and I'd like to 
> request some clarification.

Nicholas - check out my lectures on the two-pole structures, Sallen-Key, 
state-variable, etc. (one of these days I'll get the four pole cascade 
with feedback analysis up - I showed some commercial videos during 
lecture, so I want to edit those out before posting it)

http://users.ece.gatech.edu/~lanterma/ece4803

I go into a lot of your questions in some detail, in varying degrees of 
coherence. My initial Sallen-Key lecture is pretty buggered since I 
randomly and unknowingly switched notations around part way through the 
lecture for no apparent reason.

The interesting thing about the Sallen-Key and state variable is that 
feedback has opposite effects - in the S-K, increasing feedback increases 
resonance, but in the state variable, it tames the resonance. One way to 
think about what the state variable is doing is imaging that you dumped 
the feedback loop all together, and see what you would you get.

- Aaron

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