On Wed, 14 Sep 2005 12:21:04 -0700, Roy Lewallen
wrote:

One thing I haven't heard mentioned is that the huge capacitances of
power FETs are highly variable. They're essentially reverse-biased
junction capacitances, so they change dramatically over the period of
each cycle as the gate and drain voltages change. This tends to make
them lossy, and makes them difficult or impossible to absorb them into a
network. People have obviously found ways to live with the variable C,
but it's not a trivial problem.

Roy Lewallen, W7EL

Switching power MOSFETS are the current version of the "sweep tube"
amplifier. Looking back those 6DQ5s and 6JS6s werent pretty either.

Roger that, they are no fun to drive. The miller capacitance is also
a similar factor and you have the same effect going on from the Drain
Source path becuase of the avalanche diode. I suspect those
combined effects are why the IMD for those single ended IRF510
amps are so pitiful.

Driving them as pushpull pairs helps somewhat as you can get one
and the other 180 out of phase to help reflect a passable reflection
back to the driver.


Allison
Kb1GMX