Page 150 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

radio. In the next design using two transistors, the performance is improved

significantly, although this design is not considered to be low power.
A Two-Transistor Superheterodyne Radio

Although neither one- nor two-transistor superheterodyne radios generallly were
made commercially, there were three-transistor designs. For example, in 1960, the
Truetone Model DC3090 was sold. This was a three-transistor superheterodyne
design with a converter circuit, an IF/AF amplifier, and an audio power amplifier..
The detected AM signal from the IF amplifier was fed back to the IF amplifier for

further AF amplification, and the audio signal from the IF amplifier then was
connected to the audio power amplifier for driving a loudspeaker. Thus the
Truetone radio was a reflex radio using the IF amplifier for amplifying audio signals
and 455-kHz IF signals from the converter circuit.
Thus the Truetone radio had the following characteristics:
1. A converter oscillator circuit for translating the RF signal from an antenna coil to

an amplified IF signal using a first transistor
2. An IF amplifier with two IF transformers using a second transistor
3. Audio amplification with two stages, a first stage via the IF amplifier and a
second stage via an audio output amplifier using a third transistor to drive a

speaker
However, the design for a two-transistor radio can have the same characteristics if
the converter oscillator circuit works as the first-stage audio amplifier. Thus the
two-transistor superheterodyne radio has the following characteristics:
1. A converter oscillator circuit for translating the RF signal from an antenna coil to

an amplified IF signal and for amplifying audio signals from the detector circuit
using the first transistor
2. An IF amplifier with two IF transformers that also provides large audio signal
amplification to drive a speaker using the second transistor

In both cases, the three- and two-transistor designs have a converter stage, an IF
stage, and two stages of audio amplification. But the two -transistor
superheterodyne radio design has sort of a two-stage reflex circuit, whereas the
three-transistor radio has a one-stage reflex circuit. Figure 10-3 presents the block
diagram for a two-transistor superheterodyne radio.