but for reasons of cost did not become successful until the 1930s.
Before the second world war other, simpler receiver technologies such as
the TRF receiver and the regenerative receiver were still widespread.
The circuit described here is based on the old technology, but
brought up-to-date a The most important part of the circuit is the
input stage, where positive feedback is used to achieve good
sensitivity and selectivity. The first stage is adjusted so that it is
not quite at the point of oscillation. This increases the gain and
the selectivity, giving a narrow bandwidth.
To achieve this,
the potentiometer connected to the drain of the FET must be adjusted
very carefully: optimal performance of the receiver depends on its
setting. In ideal conditions several strong stations should be
obtainable during the day using a 50 cm antenna. At night, several
times this number should be obtainable. The frequency range of the
receiver runs from 6 MHz to 8 MHz. This range covers the 49 m and the
41 m shortwave bands in which many European stations broadcast. Not
bad for such a simple circuit! The circuit employs six transistors. The
first stage is a selective amplifier, followed by a transistor
detector. Two low-frequency amplifier stages complete the circuit.
The
final stage is a push-pull arrangement for optimal drive of the
low-impedance loudspeaker. This circuit arrangement is sometimes called a
‘1V2 receiver’ (one preamplifier, one detector and two audio-frequency
stages). Setting-up is straightforward. Adjust P1 until the point is
reached where the circuit starts to oscillate: a whistle will be heard
from the loudspeaker. Now back off the potentiometer until the whistle
stops. The receiver can now be tuned to a broadcaster. Occasional
further adjustment of the potentiometer may be required after the
station is tuned in. The receiver operates from a supply voltage of
between 5 V and 12 V and uses very little current. A 9 V PP3 (6F22)
battery should give a very long life.
