Most VHF/UHF amplifiers for
use with a symmetrtcal antenna
(such as an open or closed dipole)
have a balun at the input. A balun
(short for 'balanced to unbalanced')
is an inductive device that converts
the symmetrtcal (balanced) RF signal
into an asymmetrical (unbalanced)
signal that can be applied to
the base ofa transistor. Unfortunately,
baluns have an inherent loss of 2-3 dB,
while most input transistors (typically
a BFR91 or similar) have noise figures
not better than about 2 dB.This
explains the rather poor overall noise
figure of 4-5 dB of this type of input
stage.
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