I have simulated simple cascode LNA, where the only difference is I am taking voltage sample across D-S of M1 (figure attached) for some purpose. I ran PSS simulation for IIP3 and 1-dB point calculation. The problem is, it is showing both points to be almost the same point that is around -19dBm. Why these values (IIP3 and 1-dB compression points) are coming same?

I have tried PSS alone and PSS+PAC together but both give the same result.
I followed these procedures.
1>f0 is 2.14GHz, second tone is selected as 20MHz away that is 2.16GHz and the power applied is -40dBm. So first order and 3rd order harmonics are taken to be 2.14GHz and 2.12GHz, or 2.14GHz and 2.18GHz, or 2.16GHz and 2.12GHz or 2.16GHz and 2.18GHz for plotting IIP3. Anything it yields the same.


Thanks and regards,
Vinay.