Re: LF: instability on MosFet amps

[Rik Strobbe <rik.strobbe@fys.kuleuven.be>]

Dear Wolf, LF-group,

to my experience most FET's are killed by over-voltage (spikes at the
drain). Exceeding current or power seems less an item.
The voltage spikes are load dependent and thus can be caused by antenna
mismatching. A R-C series network from drain to ground is a good
remedy.
I have played around with class-E PA's are they are very fascinating. But
although very simple to build you need at least a good oscilloscope to
tune them correctly.
On 500kHz I reached 92% for a 100 W PA. Most of the remaining 8% was
dissipated in the coils, rather than in the FET, so improving the coils
could even raise the efficiency.
But compared to a class D PA I found the class E PA rather sensitive
toward load. 
For a class D PA the main concern about the load (antenna) is that it is
resistive (antenna in resonance) and "more or less" 50 Ohm. My
class D PA ( 82% efficiency) seems to be happy with any resistive load in
the 40-70 Ohm range. The class E PA on the other hand really needed a 50
Ohm resistive load.

Regarding split filters: I tested them years ago on 137kHz and found out
that they work well but - at least in my case - they were quite
pernicious for the PA efficiency that dropped from about 85% to about
75%.
I didn't investigate the reason for that (as the R-C networks at the
drain solved the spike problem). A wild guess would be that the harmonic
power that is reflected by a LPF (but absorbed in a split filter) somehow
is "recuperated" in the PA.

73, Rik  ON7YD - OR7T




At 16:25 25/11/2009, you wrote:
> Hello group !
> I found that almost all amp configurations generate harmonics. In case of
> a homebrewed HF amp, with a push ?pull design, I had to build hi-pass
> filters with a resistive termination, so called split filters, for each
> band. Without dissipating the harmonic energy the stage will have to cope
> with excess voltages and thus bury the MosFets .
> My last amp stage for the DI2BJ-beacon is followed by a hi-pass that
> opens at 750 kcs . This way the first harmonic and all the higher ones go
> into thermal dissipation.
> A T-type adjustable low-pass is used to match and reduce harmonics
> further.
> Have
> fun                    
> de wolf  df2py