Audio Amplifier - Elenco Electronics SUPERHET AM-550TK Assembly And Instruction Manual

AUDIO AMPLIFIER

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 .5 or 50%, but in a Class B
amplifier (transistor on for 1/2 cycle) the maximum
theoretical efficiency is .785 or 78.5%.
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.
SECTION 1
Transistor Q4 is a Class A amplifier that drives the
base of transistor Q5 directly.
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.
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
Since 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.
speaker, while allowing the AC to pass.
Figure 2
-7-
Capacitor C13 blocks the DC to the
Q5 is a current
Bias stability is