Elenco Electronics AM-550TK Instruction Manual download pdf (Page 8)

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%. 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 1

AUDIO AMPLIFIER

Figure 2

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Elenco Electronics AM-550TK Instruction Manual Page 8

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