Multiple Feedback Bandpass Filter vs Inverting Bandpass Filter

Discussion in 'General Electronics Chat' started by Dritech, Apr 4, 2016.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    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:

    [​IMG]

    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:
    [​IMG]
     
  2. Veracohr

    Well-Known Member

    Jan 3, 2011
    550
    75
    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.
     
  3. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    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?
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,001
    3,229
    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: Apr 5, 2016
  5. Veracohr

    Well-Known Member

    Jan 3, 2011
    550
    75
    Whoops.
     
  6. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    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."
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,001
    3,229
    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).
     
    Dritech likes this.
Loading...