A bipolar regenerative receiver
Circuit : Ramon Vargas - Lima, Peru.
Contrary to what some
radio experimenters think, a bipolar regenerative design can be made
to work efficiently. The major concern is the low input impedance of
the detector-amplifier bipolar stage. Nevertheless, it can be easily
compensated with positive feedback or regeneration. A sufficient
amount of regeneration can make tuning astonishingly sharp. Another
concern is the quality of the detected audio. This, to my knowledge,
is subjective. The quality of sound coming out from an earphone can
be rated good or fair by two different people. I would suggest that
you decide by yourself. So, come on and try the following schematic
for the 530 kHz to 1650 kHz AM Broadcasting Band.
notice that the 475 pF variable capacitor tunes in the stations
whereas the 200 pF variable capacitor controls regeneration. The
latter is known as the throttle capacitor. L2 is the tickler coil. In
order to regeneration to take place, L1 and L2 must be correctly
phased ( very important!
The power consumption is
very low. The 2N3904 drains some 60 uA from the 9 volt battery and
the AC126, about 0.5 mA.As a benchmark, medium
powered ( 5 to 10 kw ) local stations within 25 km from my site are
heard as fair to loud audio signals.
The audio output stage has no external bias, and doesn't need any. This is because Iceo, the leakage collector current
( about 0.5 mA in my prototype ), is
sufficient to build up a usable Beta ( or current amplifying factor ) in the
germanium AC126 transistor. This is a bit unusual but it works fine. Also, the
signal detection is carried out by the 2N3904 transistor, as it is driven, thanks
to regeneration, into its non-linear region. In other words, it works as an
Photographs of Ramon's Prototype