Is this buffer just a high or low side follower or a complementary pair?
The application is a simple vacuum tube curve tracer. I used it mainly for low power tubes, and the grid voltage driver didn't have to supply any grid current, since I always kept the grid supply negative with respect to the cathode. More recently, I needed to characterize some power tubes, and had to get data operating them Class A2 (positive grid current), and so I had to add a buffer that could source current to the grid. I used an NDP603 N channel MOSFET, with D connected to V
CC, G connected to the original grid drive output, and S connected to –V
CC via a 5.6k resistor, with S being the new grid drive output. So, the sink current is limited by the 5.6k resistor, but it's insignificant, while the source current is whatever the MOSFET can supply, which is more than high enough. The high V
GS isn't a problem, because I have a zero adjust pot that is adjusted during setup, and I haven't needed to go anywhere near the positive rail. Actually, the high V
GS is probably an advantage, since it gives a bit more of a bottom end which is often more useful.
The high gate capacitance shouldn't be an issue, since the frequencies involved are very low.