study of gain based on bypass capacitor and its removal.

1. Small signal amplifier for Public Address (PA) system
Experiment 2
Batch B1 Bock 1
Omkar Rane SETB118
Rohit Mane SETB102
Abhishek Sainkar SETB104
Tanmay Kale SETB112
JULY 24, 2017
MIT ACADEMY OF ENGINEERING, DEPARTMENT OF ENTC ENGINEERING
ojrane@mitaoe.ac.in

2. EXPERIMENT No: 2
Small signal amplifier for Public Address (PA) system
Introduction:-
A preamplifier is an electronic amplifier that converts a weak electrical signal ( in mV or
micro volts) into an output signal. In an audio system, preamplifiers are typically used to
amplify signals from analog sensors to line level. They provides voltage gain (e.g., from
10 mV to 1 V) but no significant current gain. The second amplifier is typically a power
amplifier .The power amplifier provides the higher current necessary to drive loudspeakers.
For these systems some common sensors are microphones, instrument pickups, and
phonographs. Preamplifiers are often integrated into the audio inputs on mixing consoles,
DJ mixers, and sound cards. They can also be stand-alone devices.
Preamplifiers increase a microphone signal to line level (i.e. the level of signal strength
required by such devices) by providing stable gain while preventing induced noise
thatwould otherwise distort the signal
Objective:-

3.  To identify role of amplifier circuit in PA system.
 To design and test the amplifier circuit
Pre lab Questions:-
 Why CE configuration is preferably used for an amplifier?
 Give important characteristics of a good amplifier in terms of Gain,
Bandwidth, Rin and Rout with proper justification. (e.g. Give
justification of why input resistance should be high)
Design of Preamplifier circuit:
a. DC biasing circuit
Design a voltage divider biasing circuit for a single stage amplifier such
that VCE~__Vcc/2___ and Ic = ___0______
Lab Exercise: -
1. Draw voltage divider circuit

4. 2. Procedure: -
1. Find all dc biasing level and component values
2. Find Icmax and VCE max
3. Draw DC load line and show Q point of your circuit
4. Simulate your circuit on Multisim and compare theoretical and
Practical values
5. Find ICQ and VCEQ of the circuit given to you on breadboard and
explain in which state your transistor is operating
b. AC Analysis:
Connect Coupling and Bypass capacitor. Find the gain of your
circuit. (Simulate the circuit on Multisim and find gain of circuit
with and without bypass capacitor)
Find the gain of the circuit if r’e = 25 ohms
Av = RC/(re’+RE)= 0.1269
Lab Exercise: -
1. Draw the circuit diagram here
Amplifier with capacitor

5. Amplifier without capacitor
Post Lab. Question:-
2. Procedure: -
1. Test your circuit on Multisim
2. Find the gain of your circuit with and without bypass capacitor
3. Show input and output waveform on Oscilloscope
4. Support your reading by attaching the screen shot of your circuits

6. 5. Build the circuit given by your instructor o breadboard and
measure gain of your circuit with and without bypass capacitor
3. List the Equipments / Component required for the experiment
Equipment / Components Specifications/Values
Function Generator CADDO,4061,3MHz Pulse
Generator with 40 MHz Frequency
Counter
CRO HAL-TEC plus, HT4030,30MHz,
Oscilloscope
Resistor R1=8k, R2=2.7k,RC=3.3k, RE=1k
Breadboard
Variable Power Supply Radial Industry SVP030002D
Transistor BC547{npn}
4. Test component using CRO & DMM & Write test Report.
Component Tools for testing Status
Transistor DMM HFE value=322
Resistors DMM Working
Diode DMM (Diode continuity
test)
Working
5. Observation Table
Parameters
Measured Values
(Theoretical)
Measured Values
(Software)
DC Analysis
ICQ
2.32mA 2.325mA
IEQ
2.32mA -2.33mA
VCEQ
2.02V 1.994V
VB
0.7V 0.666V
Parameters Measured Values
(Theoretical)
Measured Values
(Bread board)

7. ICQ 2.32mA 1.92mA
IEQ
2.32mA 1.96mA
VCEQ
2.02V 1.899V
VB
0.7V 0.6V
AC Analysis
Parameters Measured Values
(Theoretical)
Measured Values
(Software)
Gain with Bypass capacitor 800mV/40mV=20 818mV/40mV=20.45
Gain without Bypass
capacitor
80mV/40mV=2 129mV/40mV=3.225
Parameters Measured Values
(Theoretical)
Measured Values
(Breadboard)
Gain with Bypass Capacitor
on BB
560mV/40mV=14 600mV/40mV=15
Gain without Bypass
Capacitor on BB
90mV/40mV=2.25 100mV/40mV=2.5
6. Draw Corresponding waveforms of input and output voltage

9. Conclusion: When an emitter resistance is added in
a CE amplifier, its voltage gain is reduced but the input
impedance increases. When a bypass capacitor is connected in
parallel with an emitter resistance, voltage gain of CE amplifier
increases.
If the bypass capacitor is removed, excessive degeneration
produced in the amplifier circuit and voltage gain will be
reduced.
Post Lab. Questions: -
1. Why is it convenient to measure gain in dB?
Ans 1) Response curves normally use a logarithmic scale of frequency,
plotted along the horizontal x-axis. This allows for a wider range of
frequency to be accommodated than if a linear scale were used.
The vertical y-axis is marked in linear divisions but using the logarithmic
units of decibels allowing for a much greater range within the same
distance. The logarithmic unit used is the decibel, which is one tenth of
a Bel, a unit originally designed for measuring losses of telephone
cables, but as the Bel is generally too large for most electronic uses, the
decibel (dB) is the unit of choice. Apart from providing a more
convenient scale the decibel has another advantage in displaying audio
information, the human ear also responds to the loudness of sounds in
a manner similar to a logarithmic scale, so using a decibel scale gives a
more meaningful representation of audio levels.
2. Explain the role of following components in a single stage amplifier
a. Coupling and bypass capacitors
When an emitter resistance is added in a CE amplifier, its voltage gain
is reduced but the input impedance increases. When a bypass
capacitor is connected in parallel with an emitter resistance, voltage

10. gain of CE amplifier increases.
If the bypass capacitor is removed, excessive degeneration produced
in the amplifier circuit and voltage gain will be reduced.
b. Resistors R1, R2, Rc and RE:
R1 and R2 are voltage dividers, Rc is current limiting resistors, Re
increases the range of input bias where the circuit operates linearly, makes
the circuit gain more stable if the BJT properties vary, and it increases the
input resistance of the circuit.
If you removed RE and replaced it with a wire you'd just have a standard
common-emitter amplifier.
Note :- Kindly attach screenshot of your performed experiments
Circuit setup:

11. Decrease in gain on removal of by pass capacitor

12. Removal Of By capacitor photograph:
Increased in gain on adding by pass capacitor: