what order is a filter -20dB/dec but total phase shift of 270 degrees?

Discussion in 'Wireless & RF Design' started by qwerty11, Jan 6, 2016.

  1. qwerty11

    Thread Starter New Member

    Apr 9, 2013
    Hi everyone,

    I have just designed the internal circuitry of an opamp, I incorporated this into a design for a low pass filter (in the Sallen-Key configuration), like this:
    I thought I would be making a second order filter, but instead it looks like it is first order from the roll-off. Also there is a -270 degrees total phase shift - I thought that first order circuits had a total phase shift of -90degrees? Can anyone explain what is happening?
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    Yours is neither a Chebycheff, nor a Butterworth. It is somewhere in between.

    When modeled with a ideal (but limited bandwidth) opamp, both are definitely second-order, and in the upper frequency limit, have a 180deg phase shift...

    btw-the resistor values are about an order of magnitude (maybe two) too low for a practical circuit...

    Last edited: Jan 7, 2016
    qwerty11 likes this.
  3. qwerty11

    Thread Starter New Member

    Apr 9, 2013
    Thank you so much!!!
  4. atferrari

    AAC Fanatic!

    Jan 6, 2004
    Hola Mike,

    What do you look at to qualify the shown "chebycheff" as such? Is that bump in way of fc, enough to qualify in the class? No evident ripple in the pass band (other than the above) that I can see.

    Not experienced in this myself. Intrigued.