American International University- Bangladesh

Faculty of Science and Technology (FST)

Electronic Devices

Fall 2024-2025

Section: Z, Group: 02

LAB REPORT - 08

Study of Single-Stage Bipolar Junction Transistor (BJT)-Based Common Emitter

Amplifier Circuit

Supervised By
SADIA YASMIN

Submitted by

Name ID Contribution
1.M.Sakib Sadman Arian 23-54986-3 Abstract, Procedure,
Experimental Data
2. Jarin Tasnim 23-54985-3 Analysis, Simulation
3. Md Tahsin Ur Rahman 23-54884-3 Simulation, Discussion
4. Asmaul Husna 23-54988-3 Theory, Data Table
5. Salman Arefin 22-47262-1 Data Table

Date of Submission : 26 December 2024

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Abstract:
This experiment studied a single-stage BJT-based Common Emitter (CE) amplifier. The main goals were to build the
circuit, find the DC operating point (Q-point), and measure the amplifier’s performance. Key tasks included
measuring the transistor’s gain (β), finding the maximum input signal without distortion, and calculating the voltage
gain at a frequency of 1 kHz. The impact of varying load resistances on the voltage gain was also analyzed to
understand its effect on the amplifier's behavior.
The experiment highlighted the importance of proper biasing for stable operation and clear signal amplification. We
tested the amplifier with various load resistor values, observing how these changes influenced the voltage gain. Both
simulated and experimental results confirmed the amplifier’s reliability for use in signal processing and electronic
systems.

Theory:
The aim of the AC analysis is to determine the Q point of a common emitter configuration which will ensure an
undistorted amplification of a signal. In this regard, a DC analysis will be performed to adjust Q at a suitable
location on the characteristic curve. After performing the DC analysis, the small signal parameters will be
calculated depending on the model being used. Gain dependency on the load resistors will also be observed.
The most common circuit configuration for an NPN transistor is that of the Common Emitter (CE) amplifier and
a family of curves known commonly as the output characteristics curves, which relate the collector current (IC),
to the output or collector voltage (VCE), for different values of base current (IB). All types of transistor amplifiers
operate using AC signal inputs which alternate between a positive value and a negative value. Presetting the
amplifier circuit to operate between these two maximum or peak values is achieved using a process known as
biasing. Biasing is very important in amplifier design as it establishes the correct operating point of the transistor
amplifier ready to receive signals, thereby reducing any distortion to the output signal.

It is a good idea to set the bias for a single stage amplifier to half the supply voltage, as this allows maximum
output voltage swing in both directions of an output waveform. For maximum symmetrical swing, it is clear from
the figure that the collector-to-emitter voltage, VCE should be equal to the half of the collector supply voltage, VCC
that is, VCE = VCC/2.

Circuit Configuration:
The common emitter configuration is used for voltage and current amplification and is the most common
configuration for transistor amplifiers. In this configuration, the emitter terminal is common between the input
(base) and collector (output) terminals of the transistor.

Biasing of Bipolar Junction Transistors:

Active Mode: The emitter junction is forward-biased, and the collector junction is reverse-biased. If the BJT is
operated in active mode, then the BJT can be used as an amplifier.

Output Characteristics:
The output characteristics curves for a common emitter configured BJT are plotted between the collector current,
IC, and the collector-to-emitter voltage drop by keeping the base current, IB constant as shown in Fig. 2. These
curves are almost horizontal. The output dynamic resistance again can be calculated from the ratio of the small
change of emitter-to-collector voltage drop to the small change of the collector current.

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 Figure 1: BJT Common Emitter Output Characteristics.

Apparatus:
SL# Apparatus Quantity
1 BJT (2N2222, C828, BD135) 1 each
Resistance (RB,POT = 0-500 k RL,POT = 0-100 k RC
2 = 470  RE = 560  R = 10 k RB2 = 10 k) 1 each

3 Capacitor (10 F and 100 F) 2+1

4 Project Board 1
5 Signal Generator and DC Power Supply 1+1
6 Oscilloscope and Probes 1+2
7 DC milliammeter (0-50 mA) 1
8 DC microammeter (0-500 A) 1
9 Multimeter 1
10 Connecting Leads 10

Precaution:
1. Transistors should never be removed or inserted into a circuit while voltage is applied.
2. It should be ensured that a replacement transistor is inserted into a circuit in the correct direction.
3. Transistors are prone to damage from electrical overloads, heat, humidity, and radiation. Such damage often occurs
when incorrect polarity voltage is applied to the collector circuit or excessive voltage is applied to the input circuit.
4. Electrostatic discharge from the human body is one of the most frequent causes of damage to transistors when
handled.
5. The applied voltage and current must not exceed the maximum rating of the given transistor.
6. Components or their properties should only be changed after turning off the power or stopping the simulation.

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Experimental Procedures:
1. The actual values of the base, emitter, and collector resistors should be measured.
2. The terminals of the transistor should be identified, and the value of Beta (β) should be measured.
3. The circuit should be connected, and the microammeter and milliammeter should be connected as shown in Fig. 3.
4. The multimeter (in voltmeter mode) should be connected to measure the base resistance voltage (VB) and input voltage (VBE).
5. Both the DC power supply and the voltage control knob should be set to 0 V, and the collector supply voltage, VCC, should
then be set to 15 V.
6. The 500 kΩ potentiometer should be adjusted until the collector-to-emitter voltage, VCE, is approximately equal to half of the
collector supply voltage, VCC (i.e., VCE = VCC/2).
7. The collector-to-emitter voltage (VCE), base-to-emitter voltage (VBE), base current (IB), emitter current (IE), and collector
current (IC) should be measured. The base current, IB, should be calculated from the collector current, IC, using the value of β.
The measured values should be recorded in Table 1.
8. A sinusoidal AC signal of 1 kHz with a 10 mV peak value should be applied at the input as shown in Fig. 3.
9. The input and output signals should be observed on the oscilloscope screen in DUAL mode.
10. The input signal should be increased until the output waveform starts showing distortion. This input signal, which represents
the maximum input signal the amplifier can handle without distortion, should be measured.
11. An AC signal less than the maximum signal-handling capacity of the amplifier should be applied. The input signal frequency
should be fixed at 1 kHz. The input and output voltage waveforms should be drawn, and the voltage gain, AV, should be
calculated.
12. A potentiometer (0-100 kΩ) should be connected as the load resistor. The potentiometer knob should be varied, and the output
voltages should be measured for each case. These values should be recorded in Table 2, and the voltage gain, AV, of the amplifier
should be calculated for each case.
13. The voltage gain, AV, of the amplifier circuit should be computed in decibels (AV,dB = 20 log AV).
14. Images of the hardware and simulation circuit diagrams, as well as various waveforms, should be recorded.
15. The DC power supply, function generator, and oscilloscope should be turned off.

RB
500kΩ 50 %
Key=A RC
C2

R 10µF
C1
Q VCC
10µF RL
3.6 %
100kΩ
Vs Key=A
RB2
10mVpk CE
1kHz RE 100µF
0°

Figure 3: Circuit diagram for the study of CE BJT amplifier circuit

Table 1 Measured data of the voltage divider bias circuit, operating point, and transistor parameter
VCC β VCE VBE IB IC IE
15 225 7.5 0.64 0.032 7.207 7.239

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 Table 2 Measured data of the voltage gain of the amplifier circuit against the load resistances.
Gain,
Load Resistor, Input voltage, Output Voltage, 𝑉𝑜𝑢𝑡 Gain in dB
RL (k) Vi (mV) Vo (mV) 𝐴𝑣 = 𝐴 𝑉,𝑑𝐵 = 20𝑙𝑜𝑔
𝐴𝑉
𝑉𝑖𝑛 10

1 430 0.0017 0.0000039 -108.17

430 0.00215 0.000005 -106.02

3.3
490 0.0022 0.0000044 -107.13
4.7
410 0.00225 0.0000054 -105.35
5.6
460 0.00235 0.0000051 -105.84
8.2
440 0.00235 0.0000053 -105.51
10
430 0.00235 0.0000054 -105.35
20
410 0.00235 0.0000057 -104.88
33
480 0.00245 0.0000051 -105.84
47
75
90
100

Simulation & Measurement:

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Page 6 of 8
 Table - 3 Simulated Data
VCC β VCE VBE IB IC IE
15 409 7.5 0.698 0.0177 7.242 7.263

Table - 4 Simulated Data

Gain,
Load Resistor, Input voltage, Output Voltage, 𝑉𝑜𝑢𝑡 Gain in dB
RL (k) Vi (mV) Vo (mV) 𝐴𝑣 = 𝐴𝑉,𝑑𝐵 = 20𝑙𝑜𝑔𝐴𝑉
𝑉𝑖𝑛 10

1 9.929 736.79 74.205 37.408

9.971 957.59 96.037 39.648
4.7
9.996 938.70 93.907 39.453
5.6
9.996 961.56 96.194 39.662
8.2
9.929 1022 102.930 40.250
10
9.929 1044 105.146 40.435
20

Discussion & Conclusion:

In this experiment we performed the following AC analysis to determine the Q point of a common emitter
configuration. We measured the VCE with the help of variable resistor. After we get VCE= VCC /2 which is 7.5V, we
connected the capacitors as shown in figure 3. We measure and observe the input and output on the load resistor RL
and determine the maximum input signal that the amplifier can amplify without any distortion . During the
experiment, We measured β from Table 1 and IB , IC ,and IE with multimeter. After that, we measured the input and
output voltage with the help of oscilloscope for different values of RL and calculated the gain 𝐴𝑣.
The results also showed that the amplifier's voltage gain (Av) increases with higher load resistance (RL). For
example, at RL=10 kΩ , the gain was 102.93, it increased to 105.15. This proves that the load resistance
significantly affects the amplifier's output.
The experiment explains the importance of proper biasing and the effect of load resistance on amplification, making
the CE amplifier an effective choice for voltage and current amplification in practical applications.

References:
[1] Robert L. Boylestad, Louis Nashelsky, Electronic Devices and Circuit Theory, 9th Edition, 2007-2008
[2] Adel S. Sedra, Kenneth C. Smith, Microelectronic Circuits, Saunders College Publishing, 3rd ed., ISBN: 0-03-
051648-X, 1991.
[3] American International University–Bangladesh (AIUB) Electronic Devices Lab Manual.
[4] David J. Comer, Donald T. Comer, Fundamentals of Electronic Circuit Design, John Wiley & Sons Canada, Ltd.,
ISBN: 0471410160, 2002.
[5] J. Keown, ORCAD PSpice and Circuit Analysis, Prentice Hall Press (2001)
[6] Resistor values: https://www.eleccircuit.com/how-to-basic-use-resistor/, accessed on 20 September 2023.

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