Capacitors In OpAmp Feedback Loop

Thread Starter

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.filters.jpg
 

AlbertHall

Joined Jun 4, 2014
9,899
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
684
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
3,411
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.
 

OBW0549

Joined Mar 2, 2015
3,411
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.
 

Attachments

Top