Multiple Feedback Bandpass Filter vs Inverting Bandpass Filter

Thread Starter

Dritech

Joined Sep 21, 2011
847
Hi all,

I want to filter a band of frequencies between 50Hz to 300Hz. I am currently using the Inverting Bandpass Filter design as shown below:



I am now planning to increase to roll-off of the filter. Will a Multiple Feedback Bandpass Filter work since I have a narrow pass-band? I was considering using the schematic below:
 

Veracohr

Joined Jan 3, 2011
695
You could quickly run a simulation to find out. But I doubt the MFB would have a big difference of rolloff. Both circuits are 2nd order so should have about 12dB/octave slope. Increase the filter order to increase the rolloff slope.
 

Thread Starter

Dritech

Joined Sep 21, 2011
847
What is the difference between roll-off and the Q-factor?
I found that MFBs have high Q-factor. Is this the same as roll-off?
 

crutschow

Joined Mar 14, 2008
24,317
Your first design is a first-order filter with a 6dB/octave rolloff, not second-order.
Q determines the bandpass, not the rolloff rate.
Here's a website that will help calculate the part values for a given desired frequency response.
 
Last edited:

Thread Starter

Dritech

Joined Sep 21, 2011
847
Your first design is a first-order filter with a 6dB/octave rolloff, not second-order.
Q determines the bandpass, not the rolloff rate.
Here's a website that will help calculate the part values for a given desired frequency response.
Is the a first order or a sendond order? because the site states that "An active band pass filter is a 2nd Order type filter because it has two reactive components (two capacitors) within its circuit design."
 

crutschow

Joined Mar 14, 2008
24,317
Is the a first order or a sendond order? because the site states that "An active band pass filter is a 2nd Order type filter because it has two reactive components (two capacitors) within its circuit design."
That's generally true if both reactive components control both of the roll-offs, such as the MFB circuit, but that's not true in the first circuit.
In that circuit, one capacitor controls the low-pass roll-off (first-order) and the other controls the high-pass roll-off (first-order).
 
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