RC =1 at phase shift of 45°; hence, the gain for an RC section is 1/0.707. The op amp must have a gain greater than four because the gain of four sections is 0.25. The buffered RC sections in this ...

Looking at the circuit diagram it follows the classic op-amp with a long-tailed pair of NPN transistors driving a PNP gain stage and finally a complimentary emitter follower as an output buffer.

The ideal op amp — which we can’t get — has infinite gain and infinite input impedance. While we can’t get that in real life, modern devices are good enough that we can pretend like it is ...