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Opamp follower and Sallen-Key filter behaving strange

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Benta:

--- Quote from: CircuitChaos on June 13, 2023, 11:05:53 pm ---
--- Quote from: Circlotron on June 10, 2023, 11:17:27 pm ---Just a quick question regarding component values. For a Butterworth LPF the feedback capacitor is double the value of the shunt capacitor. Same deal with the two resistors in the HPF. That’s assuming you were after a Butterworth response of course. Not sure what response equal value parts would give using a gain of 1. There are equal value Sallen-Keys but they have a gain of greater than 1.

--- End quote ---
I'm not sure if I correctly understand, but it's not a Butterworth topology. It's a Sallen-Key, with gain = 1. Are you saying that Sallen-Key with gain = 1 shouldn't have components of equal value?

--- End quote ---
Sallen-Key is a filter topology, Butterworth is a filter transfer function. Sure, you can have an equal value Sallen-Key filter.

Circlotron:

--- Quote from: Benta on June 13, 2023, 11:37:55 pm ---
--- Quote from: CircuitChaos on June 13, 2023, 11:05:53 pm ---
--- Quote from: Circlotron on June 10, 2023, 11:17:27 pm ---Just a quick question regarding component values. For a Butterworth LPF the feedback capacitor is double the value of the shunt capacitor. Same deal with the two resistors in the HPF. That’s assuming you were after a Butterworth response of course. Not sure what response equal value parts would give using a gain of 1. There are equal value Sallen-Keys but they have a gain of greater than 1.

--- End quote ---
I'm not sure if I correctly understand, but it's not a Butterworth topology. It's a Sallen-Key, with gain = 1. Are you saying that Sallen-Key with gain = 1 shouldn't have components of equal value?

--- End quote ---
Sallen-Key is a filter topology, Butterworth is a filter transfer function. Sure, you can have an equal value Sallen-Key filter.

--- End quote ---
Yes, but AFAIK you can’t have an equal value SK with a gain of one. See Don Lancaster’s Active Filter Cookbook starting at page 121.

Benta:

--- Quote from: Circlotron on June 13, 2023, 11:52:56 pm ---Yes, but AFAIK you can’t have an equal value SK with a gain of one. See Don Lancaster’s Active Filter Cookbook starting at page 121.

--- End quote ---
Of course you can. But it'll only have a Q=0.5 which is not very good.
Don Lancaster likes to simplify things. Good for beginners and amateurs, not very useful for engineers.

mawyatt:
There's a 3rd Order Sallen Key Type Butterworth filter with a gain of 1 that uses equal valued components. Here's a thread that discusses such, we published this in EDN long ago!!

https://www.eevblog.com/forum/projects/7th-order-butterworth-filter-help/25/

Found this in our notebook, you can swap the R and Cs for a LP to a HP.

Edit: You can have just about any practical filter gain by scaling the first unity gain buffer to have the overall desired filter gain. We've utilized this unique filter structure many times over the years, and it's so easy with just a quad op amp and a few equal valued components.

Best,

Circlotron:

--- Quote from: Benta on June 14, 2023, 12:17:07 am ---
--- Quote from: Circlotron on June 13, 2023, 11:52:56 pm ---Yes, but AFAIK you can’t have an equal value SK with a gain of one. See Don Lancaster’s Active Filter Cookbook starting at page 121.

--- End quote ---
Of course you can. But it'll only have a Q=0.5 which is not very good.
Don Lancaster likes to simplify things. Good for beginners and amateurs, not very useful for engineers.

--- End quote ---
Well anyway, this is the response of the final filter in the OP's schematic (using a TL072) and with the feedback capacitor the both same value (dotted line) and double the value (solid line) of the cap that goes to ground. With double value the response is possibly more useful.

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