Solid State Shortwave Receivers For Beginners - The Listeners Guide

R.F. Amplifiers
CHAPTER 6
Ancillary Equipment
If one wishes to improve the performance of a simple S.W.
receiver, it is no use, in most cases, simply adding an extra stage of
audio gain at the output. In the majority of cases doing this will not
improve performance at all, because not only does this increase the
sensitivity of the set, it also greatly increases the noise content on the
output. Thus, although a transmission may be brought up to an
audible level, it will be unintelligible as it will be lost in a high level
of background noise.
Any additional amplification should be added ahead of the
receiver so that it boosts the aerial signal. In this way a useful increase
in sensitivity can be obtained without greatly increasing the output
noise level.
There is another reason for using an R.F. amplifier, and that is
to prevent radiation from the detector when it is used beyond the
threshold of oscillation (for the reception of C.W. and S.S.B.). The
power of the R.F. signal generated by an oscillating detector is not
very great, and it is not very effectively coupled to the aerial. There
is therefore little chance of interfering with other users of the band,
but it is possible that interference to nearby T.V. or F.M. redio sets
could be caused by harmonics of the signal. Also, to be strictly within
the law, no R.F. radiation from the aerial should be tolerated. This
does not apply to the portable receivers, or any other set using a very
short aerial which limits any R.F. radiation to an insignificant level.
Untuned R.F. amplifiers can be very simple, and can make a
worthwhile improvement in a receivers performance. They do not
have a very high gain, but unless a very inefficient aerial is used, a high
gain untuned amplifier would probably just overload the receiver most
of the time anyway.
Four untuned R.F. amplifier circuits are given in Figs. 30 to 33.
The one shown in Fig.30 uses a Jugfet in the common source mode.
This has R1 as its gate bias resistor and L1 as the drain load impedance.
R2 is the normal source bias resistor and C1 is its bypass capacitor.
C2 provides output D.C. blocking.
This circuit provides quite a reasonable level of gain, but this
does drop off at bighfrequencies due to negative feedback through
various circuit capacitances. The conñnon gate circuit shown in
Fig.31 has less gain than the circuit of Fig.30 at low frequencies, but
it provides more consistant results over the entire S.W. frequency
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