I'm designing a power amplifier using some Exicon lateral MOSFETs as the final common-source stage. These will be used in various experimental systems. Each only needs to run around +/-50V out, up to 4A, and driving sinusoidally between 50 and 400kHz. The load can be approximated as a lossy series LC circuit, driven near resonance with R generally 10-20 ohms. There's an additional >200pF of capacitance on each output source to ground due to the capacitance to the heat sink.
These MOSFETs have huge capacitances: N ch: Ciss 900pF, Coss 500pF P ch: Ciss 1.8nF, Coss 850pF
MOSFET "lore" (particularly from audio power land) says that you need fairly substantial series gate resistors to protect these FETs. Unfortunately the dominant pole in the amplifier is in the preceding voltage-gain stage, so I'm having some HF stability issues when I use resistors of the recommended value (>300 ohms). I'd like these amplifiers to tolerate a variety of loads, not require tweaking for particular systems.
QUESTION : are these resistors necessary? How can a proper value be determined?
Thanks for any insights! -F