What does this (amplifying bandpass filter ?) do ?

Thread Starter


Joined Jul 21, 2020
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.


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Joined Mar 14, 2008
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?


Joined Jun 13, 2013
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).
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Thread Starter


Joined Jul 21, 2020
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.