Paul Burridge wrote:

I was thinking about having a go at contriving (I won't say
"designing" for obvious reasons) a class 'C' RF amp using MOSFETs
instead of the usual BJTs/toobz. They seem - on the face of it at

Paul,
assuming that I understand your question and assuming you want a
50ohm-50ohm HF amplifier for CW, I have seen several designs using an
IRF510 MOSFET for amplifiers up to 30 MHz.
On a QST article using 2 IRF510 in a push-pull fashion some 30W could be
achieved over the entire HF range. For a simpler design check the NB6M
Miniboots (http://www.amqrp.org/kits/miniboots/miniboot.htm).

To my understanding, switching MOSFETs can be used as HF C-class
amplifiers as long as their gate capacitance is low (few pF).

As an example, IRF530 has a way too high gate capacitance to allow
acceptable HF results. On 80m it might be good, thought.

HTH,
Paolo IK1ZYW