Elenco AM Radio Kit - Manual (Page 9)

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SECTION 1
AUDIO AMPLIFIER
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. For the Audio
Amplifier, we use the integrated circuit (IC) LM-386. In Figure 2, you can see equivalent schematic and connection diagrams.
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 of distortion is shown in Figure 3.
In order to eliminate crossover distortion and maximize efficiency, the transistors 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.
To make the LM-386 a more versatile amplifier, two pins (1 and 8) are
provided for gain control. With pins 1 and 8 open, the 1.35k
Ω resistor sets
the gain at 20 (see Figure 4a). The gain will go up to 200 (see Figure 4b)
if a resistor is placed in series with the capacitor. The gain can be set to
any value from 20 to 200. The amplifier with a gain of 50 is shown in
Figure 4c.
The amplifier in our kit with a gain of 150 is shown in Figure 5. Capacitor C11
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.
Figure 3
Figure 4a
Figure 4b
Figure 4c
Figure 2
Figure 5
from detector
C15
C11
C14
9V
R13
C12
C16
R14
C13
TP7
TP6
U1
6
1
8
5
7
4
2
3