Op amps often use the "slave" side of a current mirror to provide the tail current for the input transistor pair, such as in this simple op amp voltage follower schematic:
The "gates" node in the current mirror is constant. Therefore, the gate-source voltage of the two PMOS transistors is constant as shown in the first plot. This should result in a constant channel resistance of both M1 and M2.
Now the second graphic shows how the drain-source voltage over M2 varies by a tremendous amount when sweeping the common-mode voltage point.
But still, the current through M2 is constant (third plot). This demands that M2's resistance strongly varies, which seems to contradict its constant gate-source voltage.
Question 1: How is the current through M2 constant, despite a strongly varying drain-source voltage and constant gate-source bias?
Questions 2: When I replace the mirror transistors with PNPs, the current mirror performance becomes abysmal (see plot below). Why are BJTs so much worse in this regard?