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CHAPTER 1
DEFINITION
1.1 Definition
“Audio amplifier is defined as the device which is used to convert the audio signal into a electrical signal and then amplify it to a specific level.”
Or
“Audio amplifier is a device which selects the frequency of audio range i.e. 20Hz to 200 kHz and amplifies that frequency signal.”
Audio amplifier works with some basic components of electronics these are microphone, operational amplifier, transistors & speaker. The microphone collects the audio signal and converts it into the electrical signal. This electrical signal is used by summing operational amplifier. This is used to eliminate the noise by comparing it with a reference voltage level. Push pull amplifier is used to amplify the final signal. Two transistors one NPN & other PNP type transistor is used in this configuration. This amplify signal converted back into audio signal with the help of speaker. Push pull amplifier is used because the speaker has low impedance & push pull amplifier has low output impedance. Which easily matched with speaker & more power can be transmitted.
CHAPTER 2
INTRODUCTION
2.1 Introduction
Audio amplifier is based on amplification property of transistor. When any signal gives to transistor at input terminal then transistor amplify that signal at the output terminal because of its high output impedance as compare to its input impedance. Operational amplifier works as summing amplifier which controls the level of input signal. Condenser mike receive the audio signal and convert these signal into the electrical signals. The capacitor used at the input of operational amplifier to block the dc voltage & pass the audio signal only.
The condenser mike connected to a resistor of 10kΩ which provide it a dc voltage to it. Operational amplifier (IC741) has two inputs one is inverting at pin 2 & another is non inverting at pin 3. It has one output at pin 6. Pin 7 of operational amplifier is connected to +Vcc (+5volts) & pin 4 is connected to –Vcc (zero volt).
Then we are using the push pull amplifier which is used to amplify the output signal of operational amplifier which is nothing but the input audio signal. We are using push pull amplifier because the output impedance of push pull amplifier is match able with the low voltage speaker. We get the amplified signal at the output of push pull amplifier then this amplified electrical signal is converted back into the audio signal. This is the basic introduction of simple audio amplifier.
The variable resistance is used to control the gain of operational amplifier. Gain of the circuit is depends upon the feedback of the circuit.
CHAPTER 3
BLOCK DIAGRAM
3.1 Block Diagram
Block diagram has four blocks as given bellow:-
Audio signal (input)
Amplified audio signal (output)
3.2 Description of Block Diagram
The block diagram of audio amplifier is divided into 4 blocks
a) Input Stage (Condenser mike)
b) Operational amplifier
c) Push pull amplifier
d) Output Stage (Speaker)
First of all there is a condenser mike which is used to collect the audio signal (input signal) and to convert it into electrical signal. This electrical signal is goes to inverting input of the operational amplifier through a capacitor which is used to block the dc signal.
Second stage of audio amplifier is operational amplifier which is acting as summing amplifier and control the voltage of the signal. This is connected to Vcc as well as ground terminal of supply. This stage controls the level of voltage of input signal as well as output signal because of feedback of the output signal.
Third stage of audio amplifier is push pull amplifier which is used to amplify the audio signal (converted into electrical signal by a condenser mike). This amplified signal is goes to the output stage.
Last stage of amplifier is low impedance speaker whose impedance is matched with the push pull amplifier and maximum power of the amplified signal can be transferred. The amplified auditable signal comes out from the output of audio amplifier.
CHAPTER 5
WORKING
5.1 Working of Audio Amplifier
The working of audio amplifier is very simple. Mike is connected to IC 741at pin 2 (inverting input) through a capacitor C1 (0.04µF) which is used to block the dc voltage and pass the audio signal to the operational amplifier. Actually this audio signal is converted into electrical signal by the condenser mike. The resistances R2 and R3 (both of 10kΩ) are working as a voltage divider circuit for operational amplifier. This zero reference voltage is connected to IC 741 at pin 3 (non inverting input). The resistance R4 and R5 are used for biasing purpose. Positive 5 volts supply is connected to pin 7 & pin no 4 is connected to ground terminal of IC 741. Operational amplifier amplifies signal and give output at pin 6.
Operational amplifier has a feedback from pin 6 to pin 2 which inverting input through a variable resistance R1 (100kΩ) which control the gain of operational amplifier. Resistance R6 is used if variable resistance is zero then IC can be damaged due to amplified signal so a small resistance is used.
Output signal of op amp is given to a combination of two transistors T1 548 (NPN) & T2 (PNP) which are working as push pull amplifier. The small amplified signal by op amp is working as input signal for push pull amplifier. Push pull amplifier amplify signal to a large value with low output impedance.
Amplified signal goes to a speaker of low impedance through a capacitor C2 (4.7µF) which blocks dc voltage and also prevents from stage load to the speaker. This speaker convert’s amplified electrical signal to audio signal. And we get auditable audio signal output of speaker. The resistances used are of value 10kΩ except variable resistance.
A supply circuit is used to get positive 5 volts from a 9 volts battery. Voltage regulator 7805 is user which gives output of 5 volts then a π filter is used to filter the ripples. Ripple free dc is given to main circuit at Vcc.