Page 96 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

The forward voltage drop across the base and emitter of Q1 is about 0.6 volt, so

thus the operating collector current of Q1 is about 200 ~A [= (0.8 volt - 0.6 volt)/l
k

] .

Amplified RF signals are passed through the primary of Tl and C3, an RF bypass
capacitor to the RF load inductor L2, and resistor R1. As stated previously, L2
should be mounted 90 degrees from the antenna coil L1 to avoid recirculation or

feedback of the amplified RF signal (back) to the antenna coil.
Rl at 47 k

is actually optional. R1 is paralleled across the inductor to avoid possible
oscillation. In the prototype radiO, R1 was not used, and there were no oscillation

problems. It may not be needed, for example, if Rl is removed and oscillations do
not occur.
The amplified RF signal then is demodulated via D3, C7, and R5, which provides a
low-level audio signal (along with a residual amplified RF signal). This low-level
audio signal is fed back to the base of Q1 via C6 and R4 and the secondary winding
of Ll. Normally, one would not think that R4 is necessary and that the signal from

C6 can be connected directly to Cl. In many cases, connecting C6 to Cl would be
fine in terms of not having oscillations. But to further avoid recirculating any
amplified RF signals back to Ql, R4 at 1,000

forms RF filtering with Cl. To reiterate, there are actually some residual RF signals
at the anode of D3, which when coupled back to the base of Ql will cause some

oscillations. So R4 is needed to reduce or eliminate oscillations.
The amplified audio signals then are extracted via T1's primary winding (10 k

), and the amplified audio signal is provided via its secondary winding (e.g., 2 k

). The secondary winding may be connected to a high-impedance earphone (e.g.,
crystal earphone or 2000-

headset).
The approximate current drain at 1.5 volts is less than 200 ~A, and at 3 volts, the
drain is about 250 ~A. To extend the life of the battery, an On-Off switch may be
used in series with the battery. Voltage reference circuits Dl and D2 provide a
somewhat stable operating DC collector current for Q1 as the supply voltage is

varied.
A couple of extra notes: