Re: LF: instability on MosFet amps

To:	"[email protected]" <[email protected]>
Subject:	Re: LF: instability on MosFet amps
From:	Rik Strobbe <[email protected]>
Date:	Thu, 26 Nov 2009 09:57:29 +0100
In-reply-to:	<C1D951F883084B1C9F845B4EDA782028@df2py>
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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

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