Elenco AM/FM Radio Kit - Manual (Page 16)

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SECTION 1B
TRANSISTOR AUDIO AMPLIFIER
If you have successfully completed the IC audio
amplifier, you are now ready to build the five-transistor
audio amplifier. The transistor audio amplifier is built on
a separate PC board and will plug into the IC socket. It
will be necessary to remove the IC from its socket.
The ratio of the power delivered to the speaker and the
power taken from the battery is the efficiency of the
amplifier. In a Class A amplifier (transistor on over
entire cycle) the maximum theoretical efficiency is 0.5
or 50%, but in a Class B amplifier (transistor on for 1/2
cycle) the maximum theoretical efficiency is 0.785 or
78.5%. Since transistor characteristics are not ideal, in
a pure Class B amplifier, the transistors will introduce
crossover distortion. This is due to the non-linear
transfer curve near zero current or cutoff. This type
distortion is shown in Figure 12.
In order to eliminate crossover distortion and maximize
efficiency, the transistors (Q11 and Q12) of the audio
amplifier circuit are biased on for slightly more than 1/2
of the cycle, Class AB. In other words, the transistors
are working as Class A amplifiers for very small levels
of power to the speaker, but they slide toward Class B
operation at larger power levels.
Transistor Q10 is a Class A amplifier that drives Q11
and Q12 through the bias string R46, D5 and R49. Q13
and Q14 are current amplifiers that amplify the current
of transistors Q11 and Q12. The AC and DC gain are
set by the DC current in transistor Q10 and the collector
resistor R46. The AC gain of the Audio Amplifier is
approximately equal to 100, while the DC gain equals
approximately 50. The transistors Q13 and Q14 self
bias so that the voltage at their emitters is
approximately 1/2 the supply voltage. R47 provides
feedback to the base of Q10 which is biased at
approximately 0.7 volts. Capacitor C40 couples the
audio signal from the volume control to the input of the
audio amplifier. Capacitor C43 blocks the DC to the
speaker, while allowing the AC to pass.
Figure 12