Page 108 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

connected to Q1, a near-unity gain amplifier with high-resistance input, so as not to

load down the Q of Ll's primary winding.
Q2 serves two purposes: One is to amplify the RF signal from Ll's primary winding
and send back the signal to Ll via the one-turn secondary winding for regeneration
or positive feedback. The second purpose of Q2 is to provide demodulation of the
AM signal via power detection. When AC signals of greater than 10 mV peak are
coupled to a common emitter amplifier such as Q2, distortion occurs in a manner

that demodulates AM signals.
Gain (or regeneration) control variable resistor VR1 is used to set the amount of
regeneration such that the positive feedback increases the RF gain and increases
the Q of Ll. The audio signals are extracted from Q2 via audio transformer Tl's
primary. Capacitor C4, which is across the primary of Tl, filters out the RF signal

from Q2, and C4 also provides an RF signal path to the secondary winding of Ll.
Audio amplifier Q3 is a common emitter amplifier that further amplifies the
demodulated AM signal. This audio amplifier was needed because of the very low
levels of audio signals from power detector Q2.
The radio design in Figure 7-4 was one of the first transistorized regenerative

radios that finally worked for me-after a few other tries.

,I... L 1 Secondary

RF Amplifier 01/02 -
RF Antenna Filter
Power Det Q2
L 1·VC1
Audio Amp 03
Audio Out
/ I'
+3

Gain Control V RI R7 C7
56K '"'
CS 1 ut +3 R6
j
L 1 Secondary m-l~ 1-n Cl Audio Out
mi- 003~4~ lie " 2N4124
1 Turn a3

R31K Audio Transformer m
al
• VC 1A VC 1S MPSH10
L 1 Primary R2 a2
m MPSH1Q
470 uh lMeg
140 pt 60 pt lC;
R4
56K
R5
C3 lOK
Dl D2 D3 04
Rl VRl
+3 "1 lOOK
l OOK
lN914 lN914 lN914 lN914

Cllut