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

Thread Starter

qwerty11

Joined Apr 9, 2013
9
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:
upload_2016-1-7_1-21-31.png
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?
 

MikeML

Joined Oct 2, 2009
5,444
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...

335.gif
 
Last edited:

Thread Starter

qwerty11

Joined Apr 9, 2013
9
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...

View attachment 98041
Thank you so much!!!
 

atferrari

Joined Jan 6, 2004
3,584
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...

View attachment 98041
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.
 
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