Elenco SEE AMFM108CK - Manual (Page 7)

-6-
SECTION 1
AUDIO AMPLIFIER
Figure 3
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 capacitor is placed between pins 1 and 8. The gain can
be set to any value from 20 to 200 if a resistor is placed
in series with the capacitor. The amplifier with a gain of
150 is shown in Figure 4c.
The amplifier in our kit with a gain of 150 is shown in
Figure 5. Capacitor C40 couples the audio signal from the
volume control to the input of the audio amplifier.
Capacitor C43 blocks the DC to the speaker, while
allowing the AC to pass.
Figure 2
Figure 4a
Figure 4c
Figure 4b
Figure 5
Typical Applications
Amplifier with Gain = 20
Minimum Parts
V IN
V S
2
6
1
8
5
7
4
LM386
+
+
–
.05 μ F
10 Ω
10k Ω
Amplifier with Gain = 150
Amplifier with Gain = 200
3
V IN
V S
2
6
1
8
5
7
4
LM386
+
–
10k Ω
3
+
.05 μ F
10 Ω
BYPASS
+
10 μ F
V IN
V S
2
6
1
8
5
7
4
LM386
+
–
10k Ω
3
.05 μ F
10 Ω
BYPASS
47 Ω
10 μ F
+
+
Equivalent Schematic and Connection Diagrams
V OUT
V S
6
5
7
4
15k Ω
BYPASS
GND
15k Ω
2
– INPUT
150 Ω 1.35k Ω
8
GAIN
1
GAIN
15k Ω
50k Ω
50k Ω
+ INPUT
Dual-In-Line and Small Outline Packages
Top View
GAIN
– INPUT
+ INPUT
GND
GAIN
BYPASS
V S
V OUT
4
1
2
3
5
8
7
6
3