Page 81 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

with a larger valued inductor CL3) and load resistor CRS). Thus the gain of the Q2

amplifier at its collector is about 30 or 40.
It should be noted that there are resistors connected in parallel to the load
inductors. This is to ensure that the amplifiers do not oscillate by limiting the gain
at high frequencies. An inductor's impedance increases with frequency. For
example, removing the 20-ko resistor across L2 will result in an oscillating circuit
instead of an amplifying circuit.

With amplifiers Ql and Q2, the total gain is about 300 to 400. To demodulate the
amplified AM signal, a germanium diode is used as an envelope detector with a
peak hold capacitor C7. Note that the peak hold capacitor provides a
larger-amplitude audio signal than without C7. Resistor R6 allows for a discharge
path to C7 so that the audio signal is demodulated without gross distortion when

the AM signal's envelope is decreasing. Without a discharge resistor on the peak
hold capacitor, the demodulated signall will be "stuck" at a voltage related to the
peak of the AM signal. DC blocking capacitor C8 then is connected to a crystal
earphone for listening. Note: One lead of the crystal earphone is connected to
ground.

Variation of the Design (Alternate Design of the TRF
Radio)

A variant of this design was tried with a loop antenna with antenna transformer.
Because there is less RF signal from this combination, loop antenna/transformer as
compared to a ferrite antenna coil, an extra stage of amplification was added

(Figure 5-2).