Page 90 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

Chapter 6

Transistor Reflex Radios

This chapter introduces a method of using a form of signal recirculation to reduce
the number of active components for amplification in radios. Unlike the tunable
radio-frequency (TRF) radio circuits shown in Chapter 5, where each amplification
stage amplified only one type of signal (e.g., an RF signal), reflex radio circuits
allow a single transistor stage to amplify both radio-frequency (RF) and

audio-frequency (AF) signals.
Motivat·on Behind Amplifying Both Radio-Frequency and

Audio-Frequency Signals

When the first transistor radios were designed and sold commercially in the
mid-1950s (e.g., the four-transistor radio Regency TR-1 in 1955), transistors were
very expensive. In the mid-twentieth century, audio small-signal transistors were
about $1 to $2 each. For example, in the 1956 Allied Radio Catalog, the very
famous Raytheon CK722 (PNP audio germanium) transistor sold for $2.20. Back in
1956, a loaf of bread cost less than 25 cents. If you wanted a "high-frequency"
transistor in 1956, Raytheon sold its CK760 (aka 2Nl12) for $6.35. Back in those

days, anything in the 2-MHz to 4-MHz (or more) range was considered a
high-frequency transistor.
Of course, by the 1960s, transistor prices dropped, but they were still somewhat
expensive. A quick look at the 1966 Allied Radio Catalog shows that a PNP audio
transistor such as the ReA 2N408 was only 38 cents, and its "high-frequency"

transistor 2N412 (16.5MHz) went for only 43 cents. Remember, though, that in
1966 a gallon of gasoline was less than 40 cents.
So the motivation to design a radio that amplified both radio frequencies and audio
frequencies was economics. Back in those days of building transistor radios, the
transistors were considered costly. Thus, minimizing the number of transistors in a

design allowed the radio to be sold at a cheaper price.
And throughout the 1950s and 1960s there were basically two types of transistor
radios one could buy. The most common was the superheterodyne radio, usually a
six-transistor radio, and the other (less common) was a reflex design using only
two transistors.

It should be noted that there were some superheterodyne radios that included a
reflex circuit as well. But these radios used one of the intermediate-frequency (IF)
transistors to amplify both the IF signals (e.g., a 455-kHz amplitude-modulated
[AM] signal) and the low-level audio signal. One of these radios, the Sylvania
four-transistor Model 4P19W used the IF stage as an emitter follower amplifier