Page 145 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

Chapter 10

Exotic or "Off the Wall" Superheterodyne Radios

In Chapters 8 and 9, the superheterodyne radios were designed in a
straightforward manner. The radio-frequency (RF), intermediate-frequency (IF),
and audio signals were ampllified with dedicated amplifiers. This chapter presents
reflex superheterodyne radios. In one design, the mixer oscillator converter circuit
doubles as an audio power amplifier. And in another design, the oscillator converter

stage and IF amplifier circuit also operate as audio driver and audio output
amplifiers.
A One-Transistor Superheterodyne Radio

Chapter 6 introduced a single-transistor reflex radio. This one-transistor radio was a
tunable radio-frequency (TRF) circuit with the detected output recirculated back to
the RF amplifier to increase the audio level. The selectivity of this reflex radio was
adequate, and the audio output was suitable for driving generally high-impedance

earphones of 2,000 Cl or more. The selectivity of the reflex radio is limited by the
loading of the antenna coil via the input to its transistor circuit.
Selectivity can be increased by adding a second stage of RF tuning at the collector
of the one-transistor circuit. However, more often than not, undesirable osciUations
will occur with a tuned circuit (e.g., antenna coil and variable capacitor) at the input
and another tuned circuit at the output. The reason is that the antenna coil is

receiving the RF signal everywhere, including the amplified RF signal from the
collector. So is there another way to increase selectivity? Yes, one can try designing
a su perheterodyne radio with one transistor.
However, because we are using just one transistor, there is a limitation as to how
much gain is available for amplifying RF and audio-frequency (AF) signals. One

objective of this radio is to drive a low-impedance earphone (e.g., 32

). Thus, in order to provide sufficient audio drive to a IOW-impedance earphone, this
radio will drain current on the order of m1any milliamps (i.e., not a very low-power
receiver).
Design Considerations for a One-Transistor

Superheterodyne Radio

The converter oscillator circuit, which usually runs at a couple or few hundred
microamperes now will operate at about 10 times the current, anywhere from a few
to about 10 milliamperes. The higher operating current is required because the

converter oscillator circuit will pull the additional duty of amplifying audio signals.
However, the higher collector current increases the gain of the converter oscillator