The project report details the design and analysis of a multistage audio amplifier using bipolar junction transistors (BJTs) to achieve higher gain and input/output flexibility. It discusses the operation of the common emitter amplifier, including biasing, phase inversion, and the influence of load resistance on gain. Simulation and implementation aspects emphasize the benefits of multistage design over single stage amplifiers, while challenges such as clipping and noise are also noted.

2. 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

3. 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.

4. Table of Contents
Introduction…………………………………………………………... 4
Operation……………………………………………………………. 5
Illustration………………………………………………………….... 6
Common Emitter………………………………………………………………………. 7
Simulation……………………………………………………............ 09
Implementation…………………………………………………….... 10
Conclusion…………………………………………………………… 11
References…………………………………………………………… 12
……………………………………………………………………………..

5. 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.

6. 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

7. Illustration

8. 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

9. 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.

10. Simulation

11. Implementation

12. 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

13. References
www.google.com
www.wikipedia.com
www.pearsonhighered.com
Electronic circuit devices by Floyd
Principles of electronics by V.K Mehta