Question about active, second-order filter

Thread Starter


Joined 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


Joined 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!