Page 98 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

• cs: 220 ~F, 16 volts

• Ri: 47 k
n

• R2, R4, R7: 1 k
n

• R3, R5, R6: 100 k
n

• RS: 47 k
n

• Ll antenna coil: 470 ~H primary, 23 ~H secondary

• L2: 3.9 mH or 4.7 mH
• VCl two-gang variable capacitor: 140 pF, 60 pF
• Ti audio transformer: 10 k
n

primary, 2 k
n

secondary
• T2 audio transformer: 120
n

to 500
n

primary, B
n

secondary
• 01, 02: 1N914
·03: 1N34
• Qi: MPSH10

• Q2: 2N50BB, 2N50B9, or 2N3904
For the two-transistor reflex radio, an audio power amplifier is added via Q2, RB,
C8, T2, and C7. The secondary winding of T1 is biased to about 0.9 volt DC such
that the emitter of Q2 is about 0.3 volt. This results in a DC collector current of
about 6 mA for Q2. At this point we are not as concerned about building a
low-power radio.

Depending on the operating voltage, the optimal primary impedance of T2 is
determined. The secondary impedance of T2 is 8
n

for driving a loudspeaker or 8-
n

to 32-11 headset. For a given DC current and supply voltage, the optimal primary

impedance Z of T2 is roughly equal to Vsupply/Q2 collector DC current. So, if we
use two cells for 3 volts and the DC collector current is 6 mA, T2's primary
impedance is about 3/ 0.006 = 500