Question about active, second-order filter

Discussion in 'General Electronics Chat' started by ohio_rifleman, Jul 14, 2011.

  1. ohio_rifleman

    Thread Starter New Member

    Jul 14, 2011
    I am looking at someone's design of an active, second-order filter. Here it is:

    I find this design unusual, in that it is simply two, passive, first-order filters connected in series followed by an op amp buffer. I have never seen this before; in my experience, active, second-order filters always have a Sallen-Key or multiple feedback topology.

    Why would someone use this filter in lieu of a Sallen-Key or multiple feedback topology? Are there any advantages to this design? Are there disadvantages to this design? If so, what are they?

    Thank you
  2. seecumulus


    Jul 13, 2011
    Just a quick on top.
    I would believe that the designer wanted or unwanted significant nodes
    in their response characteristics.
  3. billbehen

    Active Member

    May 10, 2006
    The circuit you show is not really an active filter, but rather a two stage low-pass RC filter with a voltage follower to buffer the output. In other words, the op - amp doesn't enter into the frequency response of the filter, it just keeps whatever stage follows the filter from loading it down, and ruining the expected response!