-7-
Theory of Operation
- The purpose of the Audio
Amplifier is to increase 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 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 2.
In order to eliminate crossover distortion and maximize
efficiency, the transistors (Q6 and Q7) 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 Q4 is a Class A amplifier that drives the base
of transistor Q5 directly. Q5 is a current amplifier that
multiplies the collector current of Q4 by the beta (current
gain, B) of Q5. The current from Q5 drives the output
transistors Q6 and Q7 through the bias string R17, D2
and R18. Bias stability is achieved by using 100% DC
feedback from the output stage to the emitter of Q4
through resistor R16. This gives the Audio Amplifier a DC
gain of one. The AC gain is set by resistors R16, R15
and capacitor C12. In this circuit, the value of R16 is
1000 ohms and R15 is 100 ohms. Their ratio is 10 to 1,
therefore the AC gain of the amplifier is 10 times.
Resistors R13 and R14 set the DC voltage at the base
of Q4 to approximately 5.2V. The emitter of Q4 is set at
4.5V, which is the same voltage at this output to the
speaker. Note that this voltage is 1/2 the battery voltage.
Capacitor C11 AC couples the audio signal from the
volume control to the input of the Audio Amplifier.
Capacitor C13 blocks the DC to the speaker, while
allowing the AC to pass.
SECTION 1A
TRANSISTOR AUDIO AMPLIFIER
Figure 2
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