how would you determine Rin of this part of the circuit. I thought we could approximate this by β/gm. This gm is approx. by 40*Ic, but these numbers do not match my simulations. What am i doing wrong.
Still don't understand, Did it make Q36 also in saturation? Otherwise if Q36 was in active then the input resistance should still be β/gm and if you calculated gm from the simulated current through Q36 it should still have added up.
you are correct, but isn't the current through Q36 determined by the current through Q37 which acts as a current source. If this Q37 is saturated it isn't able to provide the current i want it too, and so calculation and simulation don't add up.