What is the condition of Gain expansion?

we often hear of Gain Compression,but how about Gain Expansion?What is the condition of Gain expansion?
Does anyone have some tutorials about it?

In general Gain Expansion is an unwanted phenomenon that appears in linear Power Amplifiers. The Gain vs Pout graph is not an ideal straight line (the gain is going higher before falling in compression).
The main reason of the Gain Expansion is the power class (and the bias) of the amplifier.

I just know that as you mentioned above,Would you please explain it more detailed in physical?
Thanks in advance!

for example, at low input power 0dbm, the gain is 10,
when the input power increased to 10dbm, the gain is decreased to 9
then is point is called 1dB compress point

You can find in this attachment an explanation about gain compression or gain expansion but in French language, if you didn?t understand the language you can ask me

Many thanks to Foufou,I don't understand french,but i know the general meaning of the formulas,from the expression, Gain=20log(a1+3/4(a3)A^3)
If the a3>0,we can say it is gain expansion, isn't it?
But a3 is often <0?

from some paper,it is said that when Iq is low,gain expansion would happen

Class C is an extreme example of gain expansion. Without drive you get no gain. Gain then expands with drive until you reach compression.