![574B9A65-5C39-4204-B2A5-3BD50973F609.png 574B9A65-5C39-4204-B2A5-3BD50973F609.png](https://cdn.imagearchive.com/groupdiy/data/attachments/54/54303-974f2cf32eb27a98438967e1c62b34fd.jpg)
https://www.edn.com/three-op-amp-state-variable-filter-perfects-the-notch/
The author notes that either the V1 or V2 node can be used, but that V2 is lower noise. I’m impressed by the virtual earth being used as an output - never seen that before.
Also there’s this version, I’m not sure how you can change just one pole of a two pole filter and get the same bandwidth, must be particular to the band-stop form. Any chance this could work over a wider frequency range?
![BFD3DC6A-77C1-42A9-A35A-BA1D2B3F5106.png BFD3DC6A-77C1-42A9-A35A-BA1D2B3F5106.png](https://cdn.imagearchive.com/groupdiy/data/attachments/54/54304-460768e91a21cb3d58227ff44f64a31d.jpg)
^ from Notch Filter Allows Single Pot Adjustment of Notch Frequency With Little Change In Its High Q Value
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