Page 129 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

diodes D3 and D4, which also provide a DC level-shifted voltage of the

demodulated signal. The demodulated and DC level-shifted signal is further
low-pass filtered to remove audio information via R5 and CS. Thus the voltage at
CS represents a DC shifted-voltage version of the average RF carrier level of the
signal from the radio station. And thus this voltage at CS provides an AVC voltage
that is connected to the bases of Q2 and Q3 via the secondary winding of Ll and
primary winding of T2. Since both bases of Q2 and Q3 require at least 0.6 volt of
biasing voltage, the voltage from D3, D4, and R7 provides about 1.0 volt of DC to

start with that allows biasing of Q2 and Q3.
Automatic volume control then is accomplished by varying the signal output from
mixer 2 and first IF amplifier Q3 in an inverse manner. The higher the received
signa:l, the lower is the AVC voltage to turn down the gain. Conversely, the lower

the received signal, the higher is the AVC voltage to turn up the gain.
The demodulated signal from D4 is further audio amplified by Q5 and Q6, and its
volume is controlled by VRl. Amplifier QS is an inverting-gain amplifier that is
connected to a driver amplifier Q6. The output of amplifier Q6 is connected to an
interstage transformer T5 to boost the audio signal current into transistors Q7 and
Q8 in a push-pull manner. The phase of the audio signal into the base of Q7 is 180
degree in reference to the phase of the audio signal at Q8. Biasing of Q7 and Q8 is

accomplished by the voltage generated by diodes D5 and D6 and R13. Signal
currents in a push-pull manner from Q7 and Q8 are connected to output
transfonmer T6 so that its secondary winding can drive a loud speaker or
low-impedance earphone.

This radio will work fine off 2.4 volts to 3 volts but also will run off 1.2 volts. Also
because of the current drain, a power switch in series with the battery is
suggested.
Alternative Oscillator and Antenna Coil Circuit

In Chapter 3 it was noted that one of the sections of a twin variable capacitor can
be used for the oscillator circuit. A series capaCitor of 110 to 120 percent of the
variable capacitor's maximum capacitance in an oscillator circuit allows for accurate

tracking with the tuned RF stage. For example, with a twin variable capacitor of 270
pF, a 330-pF series capacitor is needed for the oscillator circuit. And for a twin
335-pF variable capacitor, a 390-pF series capacitor is required. Figure 8-7 shows
an alternative oscillator and antenna coil circuit for the eight-transistor radio.