as my question state, I am having trouble understanding the circuit using a NMOS/PMOS as a current source or voltage source according to the pins they are connected and also the input and output impedance present when looking at only one pin of the MOS. my lack of understanding comes in the application when they explain that the NMOS for example behaves a source current as it has small input resistance or output resistance by just looking at one pin or when the connect it the gate of PMOS to opamp and think of it as just impedance!!!
what I know:
we have CS, CG and CD amplifier mode
the MOSFET is alawys used in the saturation region
a voltage source has a very small output resistance
a current source has a very big output resistance in parallel to the current source.
r0>>Rd
NMOS/PMOS in saturation act as follows ID is proportional to Vgs as shows on the graph


and here is a summary of the MOSFET impedances:

 

The gate of a MOSFET has a near infinite input resistance.
The drain of the MOSFET has a high impedance (near constant-current with change in load resistance) when in the saturation region.
The source of the MOSFET has a low output impedance (near constant-voltage with change in load resistance).

So what don't you understand?

 

the first one is clear for me. the gate of a MOSFET Rin=infinite
but, The Drain and the Source of the MOSFET have the same impedance as when I look at both inputs I see I/gm//ro

please correct me, all these three represent the common source, common gate and common drain but we just ,respectively, consider also the connection of the drain to ac ground, source to ac ground , and gate to ac ground in these three configuration?