Large signal output capacitance measurements

Hi all

I'm working on a power amplifier and I want to measure the ability to output under large signal condition. I use a non-linear model of the GaN power transistor.

Power output is about 36 dBm, f = 8.2 GHz

I tried both methods:

(1) I have spent under bias Vgs - 6V to 1V, Vds = 28V. I use solver C_PRC linear tab to find the parallel to the output capacity. I got a chart that changes from 0.3pF to 0.5pF. In this simulation I don't use tuner to impedance, just ports (50 ohm) and sources of bias.
(2) amplifier is biased to 28V and 125mA. Then, I drive amplifier with of 20dBm to 30dBm input power. In this simulation, I used HBTUNERs to maximum power output (power input 25dBm at the entrance) and the output. I can see there is a change in capacity at the entrance vs input power change. However, in the output, production capacity is constant and 0.52pF. I am able the ability of the drain of transistors, tuner. I use great Solver for the parameter measuring signal Y and calculate the capacity.

The interesting thing, I can meause entry changes easly to large signal capacity, but output capacity is constant!
I have two questions:

(1) why the three measures are different?

(2) for large signal measurements, for example; 30dBm output, 30 v peak-to-peak output average 40dBm 50 v peak-to-peak. So, there is a change in capacity over a period of time. When I measure the Solver big signal, Solver uses the high peak values? If so, can I change low peak value?

(3) can be used for important signals gamma probes?

Thank you

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