-----Original Message-----
From: George K. Watson <watson@epiloglaser.com>
To: 'Amps Reflector' <amps@contesting.com>
To: <amps@contesting.com>
Date: 14 June 1999 17:38
Subject: RE: [AMPS] 50-500 MHz in solid-state?
>
>Combining over the frequency range is a pain, so the upper
power limit
>is that of a single device in most cases.
>Impedance, impedance, impedance. The electrical problems
are actually
>pretty nasty with parasitic inductance starting to dominate
as the frequency rises.
It depends on the device - I use PointNine Technologies
devices at this power and over the frequency range without
any bother. Motorola and Philips UHF parts are dreadful in
comparison. The real problem I find is material to load the
transformers to get the inductance at 50MHz without loss at
500MHz.
>Higher impedance MOSFET devices are emerging (1000V VDS),
but they are frequency
>limited to about 100MHZ, and not capable of generating more
than 150W or so. It might
>be possible to combine these devices as the impedances are
high enough, but I have not
>tried this.
High voltage is not a universal cureall. True, the load
resistance goes up, but if the output capacitive reactance
doesn't go up at least as fast, the upper frequency limit
is reduced. In general, that tends to be the way things work
out.
>
>Even in single device designs at those frequencies,
construction details and an intimate
>understanding of the sources of parasitic inductance are
essential. "Cut and try" methods
>often result in a brief, pretty blue flash inside the FET
and a slagged die or bond wire ( I have a small
>bag of slagged FETS).
I bet mine's bigger than yours :-) (Not necessarily
something to boast about)
Steve
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