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SECTION 1
AUDIO AMPLIFIER
The purpose of the Audio Amplifier is to in-crease the
audio power to a level sufficient to drive an 8 ohm
speaker. To do this, DC (direct current) from the battery
is converted by the amplifier to an AC (alternating
current) in the speaker.
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 .5 or 50%, but in a Class B
amplifier (transistor on for 1/2 cycle) the maximum
theoretical efficiency is .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
Figure 2
shown in Figure 2.
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 R44, D5 and R47. 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 R44. 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. R45 provides
feedback to the base of Q10 which is biased at
approximately .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.
-6-
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