Page 154 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

from T2's secondary winding. However, because the emitter of Q2 has a 24-V

resistor to ground, the actual input resistance is about the transistor's current gain
(beta or Hfe, typically 100) times 24 V, which is about 2,400 V. Thus the emitter
24-V resistor raises the input resistance while essentially providing about the same
voltage gain as if Q2 were biased at (typically) 1 mA of collector current with the
emitter bypassed to ground with a capacitor.
The amplified IF signal then is connected to the second IF transformer T3, and the

secondary winding of T3 is connected to D3 for demodulation of the 4S5-kHz AM
signal. Because the demodulated AM signal is going to be recirculated into the
converter oscillator transistor Ql, it is essential to remove any 455-kHz signal prior
to connection to the base of Ql via the secondary winding of Ll, the antenna coil.
Therefore, a low-pass filter formed by C8, L2, and C9 removes substantially the

455-kHz IF signal component from the detector diode D3 while coupling an audio
signal to the base of Ql via the secondary winding of Ll.
At audio frequencies, the converter oscillator circuit has an input resistance of
about 100,000

(beta times R2 = 1,000

, where beta = 100). The primary impedance of T4 is in fact about 10 kV because

the tapped secondary of T4 is being loaded by the input resistance of Q2, which is
2,400 V. Recall that the tapped secondary winding of T4 should be 2,500 V. Thus
there is close to an optimal power transfer in impedance matching for T4 into Q2,
and the aUdio-voltage gain from the base of Ql to the base of Q2 is about 5. The
audio gain from the base of Ql to its collector is 10 k

/1 k

= 10. But because the center tap of the secondary of T4 is used, we only get half
the audio-signal voltage, which is connected to the base of Q2.
With the amplified (T4) audio signal coupled to the base of Q2, the collector of Q2
supplies sufficient current to output transformer TS for driving a loudspeaker at its
secondary winding. The DC collector current of Q2 is set at about 15 mA with a

SOO-V impedance load to allow for 7.5 volts peak or 15 volts peak to peak of
alternating-current (AC) signal swing. Note that the term impedance is used instead
of resistance. Transformer Ts allows the audio-signal voltage at the collector of Q2
to swing above the 9-volt power supply. Thus the 15 mA of collector current allows
the collector to swing about 7.5 volts above the 9 volts. For power conservation,

the reader may connect a power switch in series with the battery.