Solid State Shortwave Receivers For Beginners - The Listeners Guide

The gain of the I.C. is fixed at approximately 50 times (34dB.)
by an integral feedback network. This is rather more than is required
in this particular application, and in fact very little voltage gain is
required here as the output from the basic receiver circuit has quite a
high amplitude. The output stage is really only required to provide
the high signal currents required to drive the speaker. R9 is used to
attenuate the output from the basic receiver to some degree, and so
provide a more realistic level of audio voltage gain.
It is extremely important to ensure that no R.F. signal finds its
way into the output stage, as this would result in violent instability.
Additional R.F. filtering is provided by C11.
Note that if the output stage is added, C7 of the basic receiver
plays no useful part in the operation of the set, and is omitted.
Apart from feeding a speaker, the output stage will also work
satisfactorily into any type of headphone or earpiece.
CMOS Receiver (Fig.22)
CMOS devices seem to appear in circuits for the amateur with
increasing frequency. These are a range of logic circuits, but they are
also suitable for many linear applications, inclyding S.W. receivers.
This receiver is based on two of the four gates that are contained in an
RCA. CD4001AE I.C.
It should perhaps be pointed out that this is not intended to be
a joke, or a gimmick. This receiver is capable of a very good performance,
and with the cost of the I.C. being less than that of many R.F.
transistors and f.e.t.s, it provides a very novel and practical alternative
to more conventional sets.
The CD4001AE contains four 2 input NOR gates. By connecting
the two inputs of a gate in parallel, an inverter can be formed. This can
be biased to operate as a linear amplifier by connecting a resistor between
the input and output. CMOS devices are based on complementary
MOSFETS, and they thus have extremely high input impedances. When
used in this way as a linear amplifier, the input impedance of the circuit
is approximately equal to the value given to the biasing resistor. In this
case a high input impedance is required so that a low level of loading is
placed on the tuned circuit.
As will be seen by refering to Fig.22, the tuned circuit is coupled
to the input of one amplifier by way of D.C. blocking capacitor, C1.
R1 biases the inverter.
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