Page 79 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

DC voltage at the collectors of Ql and Q2, the internal-collector-to-base

capacitances of the transistors are reduced.
Second was the choice of transistor to be very low-capacitance types, such as the
MPSH10. When working at very low voltages such as 1.5 volts or lower, the
collector-to-base capacitance is important to note because this capacitance will
affect the overall gain of the amplifying stages. At higher supply voltages (e.g., 6
volts and above), general-purpose transistors (e.g., 2N4124 or 2N3904) can be

used because the collector-to-base capacitance is lowered by the higher operating
voltage, but the higher supply voltages would mean more batteries.
When using just resistive loads on a collector, the resistive loads form a voltage
drop (collector DC current times resistance value), which, in turn, causes the
collector voltage to drop as well. This collector voltage drop then causes an

increase in collector-ta-base capacitance, which, in turn, then causes a loss in
amplification at the RF frequencies.
So in summary, the two magic ingredients are
1. Inductors connected between the supply and collector of the transistors, and
2. Using low-capacitance transistors such as the MPSH 10.

Circuit Description

Antenna coil Ll has a primary winding inductance of about 470 IJH, which requires
about 200 pF of capacitance to receive the low end of the AM band at about 530
kHz or 540 kHz. The variable capacitor used in this example is just a common
two-gang/section variable capacitor normally used for superheterodyne radios. By
paralleling the RF and oscillator sections of the variable capacitor, the 200-pF

capacitance is achieved.
of course, other antenna coil and variable capaCitor combinations can be used. See
the following table.