# What does this (amplifying bandpass filter ?) do ?

#### 8dm7bz

Joined Jul 21, 2020
199
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

#### Attachments

• 19.3 KB Views: 36
Last edited:

#### crutschow

Joined Mar 14, 2008
33,310
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?

#### LvW

Joined Jun 13, 2013
1,683
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).

Last edited:

#### 8dm7bz

Joined Jul 21, 2020
199
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.

#### Audioguru again

Joined Oct 21, 2019
6,425
Here is a simulation using different parts values producing a different peak frequency:

#### Attachments

• 19.7 KB Views: 12

#### crutschow

Joined Mar 14, 2008
33,310
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.

#### Audioguru again

Joined Oct 21, 2019
6,425
The first stage has only a little positive feedback. The second stage has none.

#### Audioguru again

Joined Oct 21, 2019
6,425
Here is my previous simulation with some positive feedback:

#### Attachments

• 25.5 KB Views: 7

#### crutschow

Joined Mar 14, 2008
33,310
Here's my simulation with the values of the left circuit, both with and without the positive feedback:

#### Ylli

Joined Nov 13, 2015
1,079
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.