Elenco AM Radio Kit Manuel d'utilisateur télécharger pdf (Page 9)

-8-

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 1.35kΩ 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

R13

C12

C16

R14

C13

TP7

TP6

1 2 3 4 5 6 7 8 9 10 11 12 13 14 ... 31 32

Pas de commentaire

Elenco AM Radio Kit Manuel d'utilisateur Page 9