Page 153 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

+9

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510K
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20 pf VR1
60 pt
4 .7 mH 50K
lN270
ca
C9
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C10
+
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l01 421F100 Pin Out +9 22K
To T2 6 3 R4
22K
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1N914
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lN914
Bottom View
FIGURE 10-4 Schematic diagram of the two-transistor supe.rheterodyne

radio.
RF signals are received via antenna coil Ll, which is tuned with variable capacitor
VCl RF. These RF signals are connected to the base of converter oscillator
transistor Q 1, which has a DC collector current at the typical hundreds of

microamperes. The oscillator circuit consists of the collector output of Ql coupling
back to the input (emitter) of Ql via the secondary winding of Tl and its tapped
primary winding, respectively.
Because the emitter of Ql has a large oscillator signal, Ql is driven into sufficient
distortion that the small RF signal coupled with the base of Ql is effectively
multiplied with the oscillator signal. Stated in another way, the large-amplitude

oscillator signal results in a time-varying transconductance, where the time-varying
function is the oscillator's signal.
From the collector output terminal of Ql then is a 455-kHz IF signal that is
connected to the primary winding of the first IF transformer T2. The signal from the
secondary winding of the IF transformer then is connected to the base of Q2 for

amplification of the 455-kHz signal.
Because Q2's collector is operating at a direct current of about 15 mA, generally
one would think that the input resistance at the base of Q2 would be in the
hundreds of ohms (e.g., 250 V or less), which can excessively load down the signal