"Rick Karlquist N6RK" wrote:

Why bother with MOSFETs? You can get varactor diodes used for tuning
AM broadcast receivers with that much capacitance.


You can get several thousand pF in one diode from Toko. 1500 pF is
by no means the limit.

I really doubt these varactor diodes are much good for tuning a
transmitters PA? I don't want to go into retyping all the pages from
G3VA, but the circuits are still shown as noted in the first place at
http://home.online.no/~la8ak/c97.htm , you need some amount of mosfets
to build a 10nF variable capacitor, but it is realisable. Don't know
about other types mosfets than the mentioned ARF446/7 and APT10050, I
got some hundreds IRF630 for nothing on a Scandinavian VHF
meeting......
Technical topics, Radcom December 2003 pg 71 (G3VA): The note is based
on "Frequency-Agile Class-D Power amplifier, by Frederick H Raab and
Daniel Ruppe of Green Mountain Radio Research Company, and was
presented at recent IEE HF Conference at Bath University. It describes
(IEE Conference Publication No.493, pp81-85) an electronically-tuned
class-D power amplifier that addresses the simultaneous needs for
efficiency and frequency agility.

Since the yanks probably wouldn't accept any ideas from the ozzer
world, I can enlighten you that Dr Raab is WA1WLW

73
from LA8AK in Europe
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