I'm trying to understand how MOSFET resistances work, but I'm seeing a lot of things that don't always fit together (probably due to my lack of understanding). Specifically the amplifier configurations (CS, CG, CD). Is this correct:
- Looking into the gate, resistance is infinity.
- Looking into the source, resistance is r0
- Looking into the drain, resistance is r_ds = 1/g_m
Is this view flawed somehow? Thanks!
Here is an example from Sedra Smith (which is a bit more involved but really, I have no idea what's going on here).
For the purpose of determining the close-loop gain of this amplifier with feedback, the A circuit is shown:
I understand why r02 is in parallel with Rf, but what is the 1/gm2 resistor doing there? Why is the first transistor a current source and a resistor, and the second a pair of resistors?
Edit: on second thought, I don't understand also why the first r0 is at the drain of the first transistor, and the second r0 is at the source of the second transistor.