No i ve seen it in one of my design
And it related to amp design
Means when i put input 0 dbm my gain is 14 but when i change my input to 30dbm my gain is 19
simulated or real?
You can get gain expansion if the gate bias is low. Increasing drive level causes gate current to flow in the device causing it to self bias at a higher current and so gain. I've never seen 5dB expansion, 1 or 2 maybe until the bias is set right, It looks like you may be biased closer to class C than you think.
Yes as FVM said, bias conditions.
Consider a class C amplifier, where you would have almost NO OUTPUT until the input power got big enough to self bias up the transistor.
other more unusual amplifiers, such as a regenerative amp, or a memory loop amp might also have nothing going on until a certain input power level is reached, and then suddenly start to work.