Hi all,

I was wondering how to calculate the MOSFET output impedance. I've come across two ways. Both seem correct.

1) based on channel length modulation, Rout ≈1/(lambda.IDS).


2) Based on reverse leakage through cgd, Gout ≈gm.Cgd/(Cgd+Cgs)=wT.Cgd. → Rout=1/(wT.Cgd).
- This one assumes that the gate is left in open circuit.

AS we can see neither of them are frequency dependent. Do I just add them in parallel? And which one tends to dominate (seems like the second one is quite a bit smaller for 0.18 um technology)?

thanks,
Aaron