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 ...

The K2-W vacuum-tube op amp has an LTspice-simulated open loop gain of 27,000. The modeled K2-W input impedance is >100 MΩ. The LTspice modeled result shows 1.00 nA of input current. Early SPICE ...