I suspected that might be the case.
So the MOSFET RF amp (with 5mA Ids) finished with the Omni design.
Not too sure why. A MOSFET RF preamp with decent drain current/
correct biasing can have intercepts above 20dBm. A perceived
trendyness and fadishness has a lot more to do with RF design that
some might admit. I'm sure MOSFETs looked as cool as flares in the
early 1980s.
Though broadband impedance matching in and out with BJT feedback amps
is pretty easy to do and gives very good results.
The 2N3866 seems to be an intermediate step in TT RF amp design. The
Argosy 2 had moved to the 2N5109 after trying the 2N3866 in the
second revision Argosy. I presume a higher current for better
intercept and perhaps lower noise figure.
Personally, I like grounded gate JFETs for RF amps -- simple design
broadband or tuned, low Z input, high reverse isolation, low noise,
high IP3, easy to add RF gain control (including attenuation!).
I presume the next design, the Paragon (1) in 1988 which was the
first up-convert radio from TT, used a 2N5109 for the RF amp (anyone
care to check?)
73
Kevin
N7WIM
On May 20, 2007, at 2:16 PM, Duane - N9DG wrote:
> In both the Corsair and Corsair II schematics I see a 2N3866
> used for the RF amp stage. No MOSFETs anywhere in the front
> ends.
>
> Duane
> N9DG
--
N7WIM / G8UDP
Kevin Purcell
kevinpurcell@pobox.com
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