Question about active low pass filtering

Discussion in 'General Electronics Chat' started by chenron7, Mar 29, 2018.

  1. chenron7

    Thread Starter New Member

    Mar 29, 2018
    hello :) trying to analyze a circuit and couldn't understand this part of the circuit, in the description of the circuit it says its an active low pass filter. can anyone help me with analyzing this circuit? THANKS :)
  2. OBW0549

    Well-Known Member

    Mar 2, 2015
    That circuit is known as a multiple-feedback low-pass filter. This document should give you what you're looking for.
  3. MisterBill2

    Well-Known Member

    Jan 23, 2018
    Yes, it is indeed a low pass active filter. The 560Pf capacitor in the feedback loop reduces the high frequency gain, and the 5n6 capacitor from the resistor junction tends to bypass the higher frequencies. Since there is only one feedback loop I would not call it a multiple feedback filter, although it is a two-pole filter.
  4. crutschow


    Mar 14, 2008
    It has both a resistor and a capacitor going to different locations so it's a called multiple-feedback loop, as shown in the document OBW0549 referenced .
    OBW0549 likes this.
  5. OBW0549

    Well-Known Member

    Mar 2, 2015
    You can call it whatever you want to, but "multiple-feedback filter" (or sometimes just "MFB filter") is what that particular active filter topology has been called in the technical literature for at least the last half-century. The following diagram, identical to the TS's schematic in post #1, was taken from page 30 of the Burr-Brown Handbook of Operational Amplifier Active RC Networks, First Edition:


    The handbook was printed in 1966. So the name goes back a ways.