[AMPS] 50-500 MHz in solid-state?

[George K. Watson]

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.
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.

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). The drain-source varactor capacitance can also make life interesting especially 
at higher frequencies or in switch mode amplifiers.

You can reasonably do a KW at 50Mhz from a set of conventional 50V FETS. The active parts will
cost around $500.00 and the power supply will cost more than that. Add reasonable protection
circuitry and what not and the costs will equal those of the high end commercial 6m amplifiers.

Funny thing is... I like semiconductor amplifiers and have two under construction at the moment for
amateur application and have designed and built a couple for plasma generation professionally.
But for lots of power at those frequencies, the high output impedance and forgiving nature of a tube
makes it hard for amateurs to justify working with semiconductors.

A breakthrough will occur when the MOSFET amplifier can be supplied directly from a rectified and filtered
line voltage (with a buck converter perhaps) and is capable of dissipating a couple of hundred watts.
Of course, sucking that heat out from such a small area and flushing it will still be difficult.

George K. Watson
K0IW

--
FAQ on WWW:               http://www.contesting.com/ampfaq.html
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm