FDNR in active filters
Hi!
Has anyone used FDNR (frequency dependent negative resistances) in an anctive filter? I started long time ago to design such a filter but other things got in the way... still FNDR looks very cool. Look at page 10 http://search1.analog.com/searchProx...3DFNDR%3Bfs%3D 
easier
search part number AD8541

Check out BurrBrown (TI) ap note AB026A
by Rick Downs, circa 1991: http://wwws.ti.com/sc/psheets/sbaa001/sbaa001.pdf They are very flexible though my experience is that they must be carefully laid out and care given to scaling impedances and picking the opamps so that the opamps don't oscillate. PC board with ground plane is a VERY good idea. Also, don't believe any of the B.S. about the "opamps not being in the signal path". One can make the argument is that this is true in some literal way, but it's like saying that a shunt resistor on an attenuator isn't in the signal path. Still, a very interesting and flexible topology. 
FDNR
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Sony has these FDNR's in there top of line models. But you can very clearly hear the difference if you replaced the NE5532's by AD712's or AD827's ! :cool: 
Maybe the truth lies inbetween.
If you build a notch filter for instance, the OPAMP will definitely have as much influence as if it was IN the signal path (if not even more, due to the high Q circuit). But there is also the possibility that the influence is reduced (I don't say zero) compared to "ordinary" filters at frequencies where the FDNR circuit is having high impedance. Regards Charles /having used simple non FDNR shunt circuits for VCFs 
I have also read somewhere that it's neccessary to have matched opamps (duals).

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analog filter humor
from Maxim's website:<em>
Analog Filter Design Demystified This article shows the reader how to design analog filters. It starts by covering the fundamentals of filters, it then goes on to introduce the basic types like Butterworth, Chebyshev, and Bessel, and then guides the reader through the design process for lowpass and highpass filters. Includes the derivation of the equations and the circuit implementation. It's a jungle out there. A small tribe, in the dense wilderness, is much sought after by head hunters from the surrounding plains. The tribe knows it is threatened, because its numbers—killed off by the accelerating advance of modern technology—are dwindling at an alarming rate. This is the tribe of the Analog Engineers. <b>The guru of Analog Engineers is the Analog Filter Designer, who sits on the throne of his kingdom and imparts wisdom while reminiscing of better days. You never get to see him even with an appointment, and you call him "Sir." The countless pages of equations found in most books on filter design can frighten small dogs and children. This article unravels the mystery of filter design, enabling you to design continuoustime analog filters quickly and with a minimum of mathematics. The throne will soon be vacant. The Theory of Analog Electronics Analog electronics has two distinct sides: the theory taught by academic institutions (equations of stability, phaseshift calculations, etc.), and the practical side familiar to most engineers (avoid oscillation by tweaking the gain with a capacitor, etc.). Unfortunately, filter design is based firmly on longestablished equations and tables of theoretical results. Filter design from theoretical equations can prove arduous. Consequently, this discussion employs a minimum of math—either in translating the theoretical tables into practical component values, or in deriving the response of a generalpurpose filter. 
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