Page 80 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits
P. 80
Variable 'Capacitor Ante na Coil
140 p,F 680IJ H
180 pF to 200 pF 470~H
270 P' 330IJ H
330 p,F to 3,65 pF 25'O(J H
540 p,F (270 plF x 2) I 165 11 H
For higher Q performance, it is generally better to use a combination with higher
capacitance for the variable capacitor. In other types of radios, however, such as a
superheterodyne radio, using a higher-capacitance variable capacitor does not offer
too much of an advantage because the selectivity will be determined mainly by the
IF coils or IF filters (e.g., 455-kHz ceramic filters).
The secondary of the antenna coil generally is less than 10 IJH, so a smaller signal
is delivered to the base of Ql. But this is okay because the high Q of the antenna
coil is preserved by the step-down transformer action from primary to secondary
windings. The high Q performance of the antenna coil is essential for sensitivity
and, more important, for selectivity of the radio.
Signals from the stepped-down secondary winding of Ll are connected to the input
of amplifier Ql, which is biased by a DC voltage reference circuit consisting of Dl
and D2 to form about 1 volt. The 1 volt DC biases the base of Ql, and the D.6-volt
drop from the base to emitter of Ql produces about 0.4 volt (to 0.3 volt) at the
emitter of Ql. Thus the l-volt reference sets up the collector current of Ql (and
Q2) to be about 0.4/2,200 5 = 72 ~A.
The voltage gain of a comm.on emitter amplifier shown for Q1 or Q2 is the ratio of
the AC voltage m,easured at the collector and the base. For example, the magnitude
of the voltage gain = Vout@collector/Vin@base, where Vout and Vin are AC
signa!ls.
With the inductive load L1 of 1 mH and R2 of 6,200
, the gain of the ampllifier is about 10 at the middle of the AM broadcast band at
about 1 MHz.
The output of Q1 is the collector, which is connected to the second-stage amplifier
Q2 via C3. Amplifier Q2 is similar to amplifier Q1 but with a different collector load
