I have a question about notch filter designs using op-amps and I hope that you can help.

Following this tutorial <https://www.electronics-tutorials.ws/filter/band-stop-filter.html>, they give examples of different notch filter designs. I'm interested in the designs using 1 and 2 op-amps:

Circuit 1:

[![enter image description here](https://i.stack.imgur.com/B4k8I.png)](https://i.stack.imgur.com/B4k8I.png)

Circuit 2:
[![enter image description here](https://i.stack.imgur.com/N9pvz.png)](https://i.stack.imgur.com/N9pvz.png)

Besides the fact that values for the resistors and capacitors have been introduced in circuit 2, *what is the difference between having one and two op-amps* connected in this fashion? What does A2 add to the circuit? It has something to do with the feedback, but I'm not sure how it's affecting the circuit.

As a note, I'd like to know more about these circuit elements and be able to figure these things out on my own. So if you have any litterature suggestions, please feel free to share :).