output resistance of ota

What is the large signal output resistance of this circuit?
http://users.ece.gatech.edu/phasler/Courses/ECE4430/projects/project4/ota.jpg
If i put V+ and V- to groud, a test voltage generator Vx at the
output, the current Ix is divided between RdsM4 and RdsM2+RdsM5... The
current in RdsM4 is (Vdd-Vx)/RdsM4 (i consider Vdd because i'm
interested in large signal output voltage), the current in RdsM2+RdsM5
is Vx/(RdsM2+RdsM5)..is this right?
Thanks in advance

 

Thanks for the answer...

Now, knowing that ro of m4 and ro of m2 are in parallel, and the expression
for ro, you should be now able to calcualte the instantaenous Ro at Vx, as
the current is M2 and M2 varied.

Why are m4 and m2 in parallel? In large signal analysis i should

consider also Vdd (in small signal analysis instead Vdd is signal-
ground, so m4 and m2 are in parallel)... or not?

 

Thanks for the answer....
I think i have understand now.....if i consider large signal analysis,
all voltage and current sources that don't vary in time (as
Vdd,Vss,ecc) are ground signal (for voltage sourcer) or open circuit
(for current bias sources), is it right?