Hello,
recently I came across an opamp circuit I don't understand. The input is a 100kHz 3Vpp square wave.

I would like to know the general working of that circuit, but also how it works. And since it looks like a copy of 2 similar circuits, the working of one of them is sufficient (?).

One thing I already found strange is that the AD8616 has a maximum supply voltage of 6V, and the circuit supplies 8V.

EDIT: Any articles explaining this type of circuit are also good.
-8

 

Look up Sallen-Key Filter.

Two of them are used in series to increase the rolloff in the stop-band.

Yes, that op amp cannot tolerate a supply voltage above 6V, so applying 8V is an error.

Where did you "come across" that circuit?

 

No - both are NOT Sallen-Key filters. S&K structures have only one single (positive) feedback path and a fixed-gain amplifier.
Instead, your circuit resembles two so-called "Multi-Feedback (MF)" topologies - however, in specific form with a small amount of positive resistive feedback. This feedback enhances the Q of the bandpass (better selectivity).
This modification of the classical MF-structure was introduced by A. Antoniou (1969).
It is not a simple task to describe the working principle in this answer. For this purpose, there are many good books on filter theory...
* Just one sentence: The basic principle of the filter exploits the fact that a "bridged T-network" (which is in the feedback path) can produce a pair of conjugate-complex zeros which - due to the feedback principle - are transferred into a conjugate-complex pole pair (as required for high-Q filters).

 

Ok, thanks. I will read up on how those "Multi-Feedback (MF)" topologies work then. At least now I know the name of the child.

 

Here is a simulation using different parts values producing a different peak frequency:

 

Didn't look like a S-K to me either, so I did a little looking. Found this: https://k0wfs.com/2015/08/15/op-amp-based-cw-audio-filter-kit/ and this: http://www.bowdenshobbycircuits.info/opamp.htm (see the Single Op-amp Bandpass Filter section).

 

both are NOT Sallen-Key filters

Yup, I didn't look closely enough at the circuit.
The positive feedback (which I thought was negative) sort of threw me.

 

The first stage has only a little positive feedback. The second stage has none.

 

Here is my previous simulation with some positive feedback:

 

Didn't look like a S-K to me either, so I did a little looking. Found this: https://k0wfs.com/2015/08/15/op-amp-based-cw-audio-filter-kit/ and this: http://www.bowdenshobbycircuits.info/opamp.htm (see the Single Op-amp Bandpass Filter section).

Be careful with internet contributions. The circuit in the first link is wrong and will not work.

 

Here's my simulation with the values of the left circuit, both with and without the positive feedback:

 

Be careful with internet contributions. The circuit in the first link is wrong and will not work.

You're right! I didn't look close enough.