Solid State Shortwave Receivers For Beginners - The Listeners Guide

Components List for Fig.17
Resistors (¼ watt 5%).
R1 1k R5 39k
R2 680 ohms R6 100k
R3 2.2k R1 100k
R4 39k R8 1k
Capacitors.
C1 10nf plastic foil or ceramic.
C2 100mfd. 10v.w.
C3 15nf plastic foil.
C4 100mfd. 10v.w.
C5 470nf plastic foil.
C6 3.3pf ceramic.
C7 100nf plastic foil.
C8 10mfd. l0v.w.
VC1 75pf air spaced (Jackson type C804).
VC2 365pf air spaced (Jackson type 0).
Semiconductors.
Tr1 2N3819.
I.C. 748C.
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.
R1, the drain to source resistance of Tr1, and R2 form a potential
divider, and about 1V is present at Tr1 source. Thus the gate of Tr1 is
held about 1V negative of the source terminal, and the required reverse
bias is produced.
Probably the major advantage of FETs over bipolar devices in this
type of receiver is the fact that FETs have input impedances in the
region of 1000 Megohms, whereas bipolar devices have input impedances
of only a few kohms or even less. As mentioned earlier, a very low level
of loading must be placed on the tuned circuit as otherwise selectivity
will be seriously degraded. This is obviously achieved here, and the
circuit provides very good selectivity.
C1 and C2 are source bypass capacitors, and these are needed to
prevent R2 from introducing a degree of negative feedback to the circuit
with a consequent loss of gain and efficiency. C1 provides R.F. decoupling
and C2 is used to give A.F. decoupling. At first sight it might appear
as though C1 is superfluous, with C2 providing both A.F. and R.F.
decoupling. Theoretically this is indeed the case, but in practice
electrolytic capacitors are not very efficient at high frequencies, and
this makes necessary the inclusion of C1.
43