how to improve an amplifier

I design a amplifier
f:(30-400)MHz
Pout:>25W(CW) @IM3<-25dBc
The circuit and picture as following:
the Mosfets are two MRF9045.
the amplifier's IM3=-26dBc@270Mhz@25W(CW)
but the amplifier's IM3=-18dBc@400Mhz@25W(CW)
How can I improve the IM3?
thanks.

I think you might want to improve its P1dB. For my experience on this device, if you get P1dB of 50W, then you can reach -30dbc for IM3. Also I suggest to change the IdQ to see what will happen.

You can add pre-distortion circuit and tweak the matching circuit.

For a so wide band application, it's not easy to add predistortion. I think you might want OBO to reach your goal.

Hi chaoFan
what is OBO?
Thanks.

Added after 3 minutes:

Hi kspalla
Which pre-distortion circuit is suitable for my amplifier?
May you give me a circuit?
Thanks.

Added after 31 seconds:

Hi kspalla
Which pre-distortion circuit is suitable for my amplifier?
May you give me a circuit?
Thanks.

OBO=Output power Back Off