Hi Daniele,
You wrote:
... As for the BD139, I didn't even suspect it could be used in a RF preamp, I
tought of it as good for power stages...
Yes, it's the low Rbb what matters. See:
http://www.janascard.cz/PDF/Design%20of%20ultra%20low%20noise%20amplifiers.pdf
And for ferrite loops I would recommend the
antenna section of those doctoral thesis available there:
www.vhl.tudelft.nl/pelgrum/DissertationPelgrum.tcl
I'm running a "counter wound" ferrite loop along with
3 x BC550 in parallel since many years.
Another option could be using an instrumentation amplifier
with low noise at low input impedance like INA217
(G=100 up to 800 kHz). So you don't need any kind of
push-pull amplifier or balancing input transformator.
See: http://www.ti.com/lit/ds/symlink/ina217.pdf
BTW: even that old low noise version of the video
amplifier NE592 works fine. It's called "TL592".
Since it's input impedance is not very low using
somewhat higher inductance is recommended (~2-5 mH).
73,
Peter, df3lp
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