Design of an Audio Amplifier
Abstract
The main objective of this project is to design and audio amplifier. Low voltage audio
power amplifier LM386 is used for this purpose. An audio amplifier is designed to
amplify frequencies between 15 Hz and 20 kHz. Any amplifier that is designed for this
entire band of frequencies or any band of frequencies contained in the audio range is
considered to be an audio amplifier.
Introduction
An amplifier is an electronic device that amplifies the voltage, current or power of a
signal. Amplifiers are classified in two main ways: The first classification is by function
and other is by frequency response [1]. The functional op-amps are voltage amplifiers
and power amplifiers. In these op-amps output voltage and power gets amplified
respectively. Frequency response of an amplifier refers to the band of frequencies that the
amplifier can be designed to amplify. The components of an amplifier respond differently
at different frequencies hence selected components of the amplifier can amplify certain
range or band of frequencies [2]. An audio amplifier is one, which can amplify a band of
frequencies. LM386 audio power amplifier chip is used in this project.
LM386: The LM386 is an audio power amplifier that is designed for use in low voltage
applications. The gain for this amplifier is usually 20 but can go up to 200 by introducing
components like resistor and capacitors at appropriate pins [3]. Since the gain can be
adjusted anywhere from 20 to 200 using external circuitry, this audio power amplifier
becomes an ideal choice for most of the applications. It has applications in AM-FM radio
amplifiers, ultrasonic drivers, power converters etc., The pin diagram of an LM386 power
amplifier is shown in fig (1).
Fig (1): Pin Diagram of LM386
Method
An audio power amplifier of gain around 50 is designed using different resistors and
capacitors. The circuit diagram of an audio amplifier is shown in Fig (2).
Fig (2): Audio Amplifier Circuit Diagram
Circuit Description: In the audio power amplifier circuit, power input is given to noninverting terminal through a 10K pot resistor. The inverting terminal is grounded along
with the pin 4. DC voltage between +5 to +15 V is applied to pin 6 through a resistor,
capacitor combination. This combination forms a High pass filter configuration so that
the only frequencies in required band can be allowed to pass. By connecting a resistor of
1K in series with 10f capacitor from pin 1 to pin 8 the gain of the circuit can be further
increased. Without this combination the gain will be limited to 20. A Bypass capacitor is
connected to pin 7 of the amplifier. The output of the circuit is obtained from pin 5. The
output from pin 5 is connected to speaker through a combination of resistor and
capacitors in order to obtain an output that is free from noise. A 10  resistor, 220f,
0.05f capacitors are used for this purpose.
Circuit Connections:
1. Make all the connections as per the circuit diagram.
2. Supply input to the non-inverting terminal from any audio source.
3. Ground the inverting terminal.
4. Supply a DC source voltage between +5 V to +15 V to the pin 6 and ground pin 4.
5. Receive the output from a speaker connected to pin 5 of the amplifier.
6. The result obtained is free from noise and is an amplified signal.
Bill of Materials:
Part
Cost ($)
Description
R1
0.15
10 Resistor
C1
0.15
220f Electrolytic Capacitor
R2
0.15
1k Resistor
C2
0.10
10f Electrolytic Capacitor
R3
0.15
10k Pot Resistor
Bypass
0.10
100pf Ceramic Capacitor
R4
0.15
10 Resistor
C4
0.10
0.05f Ceramic Capacitor
C3
0.15
220f Electrolytic Capacitor
U1
0.55
LM386 Audio Power Amplifier Chip
O/P
Speaker
From the bill of materials it is evident that the total cost involved in building an audio
amplifier circuit is around $1.75.