Q-Factor and passband for butterworth/Sallen-Key Topology

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karthick1987

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Hi all,

I was doing some reading and found a lot of terms which I didnt quite understand fully For eg: Flat Passband frequency response, This was a 2nd order Sallen key filter. I was under the impression that it was a LPF, what is the term Passband refer to in this case.

Any advise?

Cheers
K

- - - Updated - - -

Also regarding Q-Factor, I know its been discussed before but I am a very mathematical person. I see the quadratic and see different poles in a quadratic. So for me when I analyse a sallen key filter I would split it into 2 poles and plot the bode plot accordingly. I want to know what the Q-factor signifies physically and practically?

thanks guys
 

Also regarding Q-Factor, I know its been discussed before but I am a very mathematical person.
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Then you'll surely came to know that Q is a property of a complex pole pair, respectively a second order filter. Higher order filters can't be described by a single "Q-factor" rather than the location of all poles (and possibly zeros) in the complex plane.

Common filter prototypes like Bessel, Butterworth, Chebyshev (the latter with additional pass-band ripple parameter) have characteristic pole zero plots each. Higher Q of individual second order building blocks corresponds to steeper transition band, larger pass band ripple in frequency characteristic and more overshoot in pulse response.
 
Supplementing FvM's explanations I want to inform you that a lowpass filter has a passband reaching from dc (0 Hz) to the respective cut-off frequency, which is specified by the user.
However, for lowpass transfer functions of 2nd order (or higher) there are several alternatives HOW the passband looks like (with or without some peaking).
A so called "maximally flat response" is identical to the well-known Butterworth response.
Other alternatives are connected with the names Bessel and Chebyshev.
 
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