# Capacitors In OpAmp Feedback Loop

#### Ohmlandia

Joined Mar 2, 2020
32
Hi all, I am writing a tutorial page for students currently not attending school. It's about opamps with capacitors in their feedback loops, used as filters. See attached circuits, the simplest of the simple! On the left, the low-pass circuit is quite commonly shown in website tutorials with in-depth analysis, equations, graphs, etc. I thought that the same would be found for the high-pass circuit at right, but not so. Why not? Is there something wrong with it? Are the two circuits not different versions of the same thing? All comments gratefully received. Thanks.

#### AlbertHall

Joined Jun 4, 2014
12,326
At high frequencies the gain of the high pass circuit approaches the open loop gain of the opamp and so I suspect it would be a better oscillator than an amplifier.

#### ronsimpson

Joined Oct 7, 2019
2,878
Left circuit. At low frequencies and DC, the gain is known and set by the bottom two resistors. At high frequencies the gain is 1. The capacitor shorts out Rfeedback. It is not a good filter because the gain will never be less than 1.

Right circuit. At low frequencies and DC the gain in known and set by the bottom two resistors. At high frequencies the gain becomes very high. It is unstable at high frequencies.

(you could move the cap to (+) input and ground and call it a low pass)

#### OBW0549

Joined Mar 2, 2015
3,566
Hi all, I am writing a tutorial page...
If you cannot instantly spot the glaring flaws in both of those circuits, you have no business writing any tutorials for anyone.

The circuit on the left is not a proper low pass filter because its gain at high frequencies levels off at 1 instead of continuing to decrease with increasing frequency.

The circuit on the right is not a high pass filter because its gain increases without bound with increasing frequency. It is almost guaranteed to be unstable, and likely oscillate.

Whatever idiot website(s) you got these garbage circuits from, don't go there again-- you'll just get yourself hopelessly confused.

#### Ohmlandia

Joined Mar 2, 2020
32
Ok! Message received and understood. Back to the drawing board ...

#### crutschow

Joined Mar 14, 2008
33,983
Here are the basics.

#### OBW0549

Joined Mar 2, 2015
3,566
Just to get you started with some material written by people who know what they're doing, here are three attachments that discuss filter design in a fairly easy-to-understand way that won't make your head spin 'round and 'round.

As a general rule, it's a good idea to be VERY cautious when getting electronic design information off the Internet; there's a lot of junk out there written by clueless idiots and unless you have enough knowledge and experience to tell sh*t from Shinola it's best to stick with application notes and articles written by professionals.

I'm not saying that non-engineers are always wrong or that they always publish garbage, just that you should be careful.

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#### Danlar81

Joined Apr 19, 2019
87
Hi OBW0459,
I know this was posted quite some time ago, however the attachments you have provided I have found to be extremely useful to design EMI reduction for CMRR in an instrumentation amplifier design I am doing for university.
Great post.

#### Papabravo

Joined Feb 24, 2006
20,985
Hi OBW0459,
I know this was posted quite some time ago, however the attachments you have provided I have found to be extremely useful to design EMI reduction for CMRR in an instrumentation amplifier design I am doing for university.
Great post.
Sorry to inform you that @OBW0459 aka Dave passed away in January 2021.

#### LowQCab

Joined Nov 6, 2012
3,929
Here are a few other PDFs that might be useful .............
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#### Attachments

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#### Danlar81

Joined Apr 19, 2019
87
Hi LowQCab,
Thank you for providing those resources. I think they would have proved useful for an assignment I just did.