Page 124 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

42IF102 transformer (both with about a 20: 1 turns ratio) instead, there will be

lower amplitude-demodulated signals (compared with the 42IF103 transformer with
a 6: 1 turns ratio) because of the higher turns ratio in these two parts, which then
results in insufficient AVC voltage for good automatic volume control.
The demodulated signals from D4 are AC-coupled to the base of the audio driver
transistor Q3 via volume control VR1. The output of Q3 is connected to an
interstage step-down transformer T4 (2.5-k

primary to 150-

secondary) to generate increased current drive into output audio transistor Q4.
Transistor Q4 is biased at about 20 mA, which results in an input resistance of
about 130

to 150

at the input of Q4. The amplified audio signal via Q4's collector is connected to
step-down transformer TS to drive a speaker or low-impedance earphone.
Although this radio runs off two cells for 2.4 volts to 3 volts, it will operate down to
1.2 volts as well. Because of the current drain of this radio, a power switch in series
with the battery is suggested.

An Eight-Transistor Radio

For designing an eight-transistor radio, a more "traditional" approach was taken.
That is, in most common descriptions of a superheterodyne radio, there is a tuned
RF circuit, a separate local OSCillator, a mixer, IF filters and amplifier(s), a detector,
and the audio amplifier (Figure 8-6A) ..