[Amps] QST, and class AB/F MOSFET amps

[Peter Chadwick]

Manfred said:
>A class AB/F amp with ARF1500 MOSFETs would be free of 
hotspotting, because in the AB class it would run at very low voltage 
(20 or 30V at most) and above that it would be running class F. At the 
same time, the reduced dissipation goes a long way in helping with the 
cooling problem. The added complexity of the modulated switching power 
supply would be rewarded by the high efficiency. And the whole thing 
would still be small, lightweight and reasonably cheap.<

It seems to me that one parameter which could cause major problems would be AM to PM conversion in the MOSFETs as you varied the voltage. You could get around that by using either Cartesian or Polar Loop feedback techniques, although you would have to take into account the poles in the loop response determined by the modulated power supply characteristics. I don't know if the MOSFETs are characterised for all 4 S parameters over a wide supply voltage range: that's the info you'd need to determine (by simulation or otherwise) the amount of AM to PM. The rest of the loop could be simulated, although simulating the PSU response might take a long time unless (or maybe even if!) you have a Sun or similar workstation - depending what software you have available.

73
Peter G3RZP