Project Report Audio Amplifier

Department of Electrical Engineering

Sukkur IBA

By:

Zeeshan Ahmed

033-14-0062

Raheel Nadeem

033-14-0074

Shahryar

Prepared for

Sir Engr. Afaque Manzoor Soomro

July 20,2016

Abstract

A bipolar junction transistor is a semiconductor device used to amplify or

switch electronic signals. The device is a solid piece of semiconductor material

with three connection terminal. The purpose of this project is to design, build,

and study the operation of a multistage amplifier. We will accomplish this by

using a Common Emitter amplifier. There are multiple stages because a single

stage may not be sufficient for most applications. Two big advantages of

multistage over single stage are that it has a higher gain, and has flexibility for

higher input and output impedances. That’s why it’s chosen as a major project.

Table of Contents

Introduction…………………………………………………………... 4

Operation……………………………………………………………. 5

Illustration………………………………………………………….... 6

Common Emitter………………………………………………………………………. 7

Simulation……………………………………………………............ 09

Implementation…………………………………………………….... 10

Conclusion…………………………………………………………… 11

References…………………………………………………………… 12

……………………………………………………………………………..

Introduction

A bipolar junction transistor (BJT) is a three terminal electronic device that

is constructed of semiconductor material and is used for switching or

amplifying applications. There are two types of BJTs, which are PNP and

NPN. Both are made of the same material but their physical characteristics

are different. The different regions of the BJT are connected to its separate

terminal E, B, and C. Many applications cannot be handled with single-

transistor amplifiers in order to meet the specification of the project we

need to add more transistor amplifiers, in a multistage amplifier, a number

of single amplifiers are connected in cascade arrangement i.e. output of first

stage is connected to the input of the second stage through a suitable

coupling device and so on. The purpose of coupling device (e.g. a capacitor

is to transform ac. output of one stage to the input of the next stage

and to isolate the dc conditions of one stage from the next stage. Each

stage consist of one transistor and associated circuitry and is coupled

to the next stage through a coupling device name of the amplifier is usually

given after the type of coupling used. By following above structure we will

get accurate amplification with lower noise and higher stability. In this

project all the cascaded stages are common emitter amplifier.

Operation

The operation begins with analysis of common emitter amplifier with

voltage-divider bias and coupling capacitors C1 and C3 on the input and output

and a bypass capacitor, C2, from emitter to ground. The analysis should be of

both dc and ac .it will help us to set the resistors and capacitors values and

particular operating point to achieve accurate amplification. In that the input

signal, Vin, is capacitively coupled to the base terminal, the output signal, Vout,

is capacitively coupled from the collector to the load. The amplified output is

180° out of phase with the input. Because the ac signal is applied to the base

terminal as the input and taken from the collector terminal as the output the

emitter is common to both the input and output signals. There is no

signal at the emitter because the bypass capacitor effectively shorts the emitter

to ground at the signal frequency. All amplifiers have a combination of both ac

and dc operation, which must be considered, but keep in mind that our focus is

on common-emitter amplifier. Moreover, the output signal is out of phase with

the input signal. As the input signal voltage changes, it causes the ac

base current to change, resulting in a change in the collector current from its

Q-point value. If the base current increases, the collector

current increases above its Q-point value, causing an increase in the voltage

drop across RC. This increase in the voltage across RC

means that the voltage at the collector decreases from its Q-point. So, any

change in input signal voltage results in an opposite change in

collector signal voltage, which is a phase inversion

Illustration

This is the main source of audio amplifier which amplifies the small signal given in

form of sound. there are many types but this amplifier is chosen because this circuit

has large voltage gain and large current gain, the concept behind this is to amplify

small variations of current input into the base terminal. IB is increased by an input

voltage variation and this current is amplified by a factor beta in IC (the current

flowing into the Collector terminal), the connection to the collector terminal has

resistance in series so when current increases into C there is voltage drop at C and

vice versa. Biasing of the Common emitter is the design work for this stage. It is

required that the BJT always stay in forward active mode, meaning that the

voltage in the collector is always higher than that of the base which in turn

must be higher than that at the emitter terminal. For undistorted amplification,

this relationship must remain true up to the highest and lowest peaks of the

output. Since there is a single voltage source of 12V, maximum amplification

can occur with 12V peak to peak, providing 6V peaks from a bias point of 6V.

Additionally, when the base voltage increases slightly (from input), the

collector voltage will be at its minimum, meaning that peaks will create the

absolute worst case for maintaining forward active mode. There is a single

voltage source (in this case +12V DC) with which to create the bias, and the

balance is adjusted with values of the resistors RB1, RB2, RC, and RE. The

source and load resistances connected to the Common emitter do not affect its

operation because they are coupled to it with capacitors, which act as open

circuits to DC values and short circuits to signals with high enough frequency.

This phenomenon comes about from the equation for impedance of a capacitor,

Z = 1 / jwC. As can be seen, impedance is reduced by a high frequency w and by

a high capacitance C. In order to create a minimal resistance for the inputs and

outputs of the circuit for the entire frequency range, it is necessary to use as

high a capacitance value as possible. Note that the load resistance does greatly

affect the gain of the CE by acting as a resistance to ground in parallel with RC

Common Emitter Amplifier

in the small signal. This reduces the current running though RC (by half if Rload

= RC), which in turn reduces the gain. This is one reason why the Common

emitter must be designed for a gain much higher than the required gain.

Roughly, the highest gain possible out of the CE is 24 V/V. The bias point is

controlled differently and in complex ways by each of the resistors in the

circuit. The values RB1 and RB2 are used to control the bias voltage and current

of the base. A larger value of RC will increase possible gain but alter the bias

point of the collector. It was observed that varying the value of RE could control

the output gain, but that it was also easy to create distortion this way. It is

important to note that the standard BJT is capable of immense amplification,

but that this is actually harmful

given the voltage swing limits

imposed by the power supply. In

order to reduce the gain and take

account for the variability of Beta

from BJT to BJT, the resistor Re

must not be shorted to ground for

the small signal. The presence of

Re in the small signal domain causes the gain formula to contain a Beta value

in the numerator and the denominator, and so these large numbers balance

each other. There are techniques to control the Re seen for DC bias and the AC

signal independently, and these involve the use of capacitors to bypass parts of

Re and introduce different Re’s to the small signal.

Simulation

Implementation

A multistage amplifier is more useful than a single stage amplifier in that it has

a higher gain and has more flexibility for higher input and output impedances.

A single npn BJT was used to drive this common emitter amplifier. The

collector current entering the npn BJT will be the current source driving the

common emitter amplifier. Clipping occurs when the amplifier tries to output

a higher power than what the amplifier can achieve. It was also tough finding a

good frequency to achieve a non-noisy input.

Conclusion

References

www.google.com

www.wikipedia.com

www.pearsonhighered.com

Electronic circuit devices by Floyd

Principles of electronics by V.K Mehta