Output impedance of PA

Regarding PA, different output power level resulted in different output impedance.
Is this because of DC bias change so that it changes the characteristics of PA ?
Thanks.

this is a complex problem.this may be explained in two way. 1)from the nonlinear model of the device,you can simulate the output impedance by large signal S paremeter or load pull method. 2) from the I-V cureve and load line,you may find the variation of output impedance with different power level.

Will the load pull contour look the same if the PA input power is different?
For instance, 3dBm input power & 6dBm input power would get same load pull
contour ? Thanks.

Generally load pull systems are two way mesaurement setups.Therefore they are complex.
But in principles, output load pull contours are obtained at PdB compression point according to its input power.
So, the PA is pulled to P1dB point with certain level and with this level, output power is searched according to output impedance.

Normally all these mesaurments are automatic but quite expensive..

load pull contours are different when the PA input power is different.