associated gain

Dear all,

I have tried to simulate the LNA. I use S21 to see my LNA power gain. However, I see the simulator can give associated gain to me. I have tried to plot this. The value is different compared with S21. Acutally, what is the definition of the associated gain? And why is it different compared with S21? Moreover, I should believe which one?

Thanks

wccheng

S21 is small-signal gain.it is believable.

S21 is equivalent with Low Signal Gain.
Power Gain is the difference between Output Power and Input Power.
When the input power is high, LNA is compressing and the Power Gain is not anymore equal with low signal Gain (or S21).

Dear all,

Firstly, very thanks for your reply. Then what is associated gain?

Thanks

wccheng

I guess associated gain is the gain at some special condition.
For example, the ATF-551M4 at 2GHz (2.7V, 10mA) includes: 0.5dB noise figure, +24.1dBm third-order output intercept point (OIP3), +14.6 dBm output power at 1dB gain compression (P1dB) and 17.5dB associated gain..
I guess the associated gain here is the gain at P1dB.