Solid State Shortwave Receivers For Beginners - The Listeners Guide

Components List for Fig.19
Resistors (¼ watt 5%).
R1 390 ohms R3 1.8 Meg.
R2 1k R4 4.7k
Capacitors.
C1 5.6nf ceramic.
C2 l5nf plastic foil.
C3 470nf plastic foil.
C4 4mfd. 10v.w.
C5 100mfd. 10v.w.
VC1 50pf air spaced (type C804).
VC2 365pf air spaced (type 0).
Semiconductors.
Tr1 2N3819.
Tr2 BC109.
Inductors.
T1 Denco Green D.P. coils Ranges 3, 4, and 5.
L1 10mH. (Repanco CH4).
Miscellaneous.
S1 S.P.S.T. toggle switch.
Sockets, chassis, panel, etc.
When used in these modes, none of the devices provide any
voltage gain, and, in fact, they have a little less than unity voltage gain.
They act as a sort of impedance transformer, converting a high input
impedance to a low output impedance.
The advantage of using this mode is that it is supposed to provide
a higher quality output than when using the more usual common gate
or common source configurations (or the bipolar or valve equivalents).
It is also supposed to provide a detector that is not easily ovenloaded.
It is unlikely that in use a noticeably higher output quality will
be obtained with this type of circuit, but it does have a better performance
on strong signals, and it has an apparent increase in selectivity
because of this. The actual selectivity is not really any higher, it is
simply that a strong signal on the band being tuned can sometimes
make it impossible to use a tight aerial coupling to the input coil without
the set being overloaded, with weak signals being unintelligible as a
result. An infinite impedance detector allows a much stronger input signal
to be used, and in consequence weak signals can be received even if there
is a very strong signal close by.
On the other hand, its slightly lower gain is something of a
disadvantage when propagation conditions are poor, and only weak
signals can be received. Anyway, this type of detector does provide an
interesting alternative to the more usual circuits, and the circuit described
here is capable of excellent results.
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