American International University- Bangladesh (AIUB)

Faculty of Engineering (FE)

ELECTRONIC DEVICES LAB

Course Name : Course Code : EEE 2104
Spring -2023-2024
Semester : Sec : J
MD. ALOMGIR KABIR
Lab Instructor : Group: 3

Experiment No : 06
Experiment Name : Study of BJT Biasing Circuit

Submitted by (NAME): Student ID:

Group Members ID Name

1. Istiaque Mahbub Isti 22-49167-3
2. Eshika Rani Pall 22-49200-3
3. Khadija Akter 22-48295-3
Ibrahim Khalil Ullah
4. 22-48301-3
Midul
5. Wasif Asad Alvi 22-46451-1

Performance Date : 19/03/24 Due Date : 3/04/2024

Marking Rubrics (to be filled by Lab Instructor)

Proficient Good Acceptable Secured
Category Unacceptable [1]
[6] [4] [2] Marks
All information, All Information Most information
Theoretical Much information
measures and variables provided that is correct, but some
Background, Methods missing and/or
are provided and sufficient, but more information may be
& procedures sections inaccurate.
explained. explanation is needed. missing or inaccurate.
Experimental results
All of the criteria are Most criteria are met,
don’t match exactly Experimental results
met; results are but there may be some
Results with the theoretical are missing or
described clearly and lack of clarity and/or
values and/or analysis incorrect;
accurately; incorrect information.
is unclear.
Demonstrates thorough Hypotheses are clearly
Conclusions don’t
and sophisticated stated, but some Some hypotheses
match hypotheses, not
understanding. concluding statements missing or misstated;
Discussion supported by data; no
Conclusions drawn are not supported by data conclusions not
integration of data from
appropriate for or data not well supported by data.
different sources.
analyses; integrated.
Title page, placement
of figures and figure
Minor errors in Major errors and/or
General formatting captions, and other Not proper style in text.
formatting. missing information.
formatting issues all
correct.
Writing is strong and
Writing is clear and
easy to understand;
easy to understand; Most of the required
ideas are fully
ideas are connected; criteria are met, but
elaborated and
Writing & effective transitions some lack of clarity, Very unclear, many
connected; effective
organization between sentences; typographic, spelling, errors.
transitions between
minor typographic, or grammatical errors
sentences; no
spelling, or are present.
typographic, spelling,
grammatical errors.
or grammatical errors.

Total Marks
Comments:
(Out of ):
Title: Study of BJT Biasing Circuit
Aim Of Objective:
1.The primary aim is to comprehend the principles and techniques involved in establishing the
appropriate biasing conditions for Bipolar Junction Transistors (BJTs).
2.The study focuses on achieving stable operating points to ensure reliable and predictable behavior of
BJT circuits while optimizing performance parameters such as linearity, efficiency, and distortion.
3. By mastering BJT biasing, engineers can effectively design, analyze, and troubleshoot transistor
circuits for various applications, enhancing their ability to create robust electronic systems.

Abstract: The operating point (Q) of BJT is very important for amplifiers, since a wrong ‘Q’ point selection
increases amplifier distortion. It is imperative to have a stable ‘Q’ point, meaning that the operating point
should not be sensitive to variation to temperature or BJT β, which can vary widely. In this experiment,
four different circuits will be analyzed for two different β to check the stability of biasing points. The
analysis of the BJT circuits is a systematic process. Initially, the operating point of a transistor circuit is
determined then the small signal BJT model parameters are calculated. Finally, the dc sources are
eliminated, the BJT is replaced with an equivalent circuit model and the resulting circuit is analyzed to
determine the voltage amplification (AV), current amplification (Ai), Input impedance (Zi), Output
Impedance (Zo), and the phase relation between the input voltage (Vi) and the output voltage (Vo). The
experiment is a very good practical realization of bipolar junction transistor (BJT) biasing circuit. A BJT
biasing circuit will be designed and simulated to find DC operation point using a circuit simulation tool.
Then a fixed-biasing and a self-biasing BJT circuits will be implemented on the trainer board to find DC
operation point for two different β of the transistor.
Theory
The dc analysis is done to determine the mode of operation of the BJT and to determine the voltages at all
nodes and currents in all branches. The operating point of a transistor circuit can be determined by
mathematical or graphical (using transistor characteristic curves) means. Here we will describe only the
mathematical solution.

V𝑡ℎ = 𝐼𝐵𝑅𝑡ℎ + 𝑅𝐵𝐸 +𝐼𝐸𝑅𝐸 = 𝐼𝐵(𝑅𝑡ℎ +(𝛽+1)𝑅𝐸)+𝑉𝐵𝐸

The dc analysis of the circuit is simple by applying two KVL's at the input and the output loop

V𝐶𝐶 = 𝐼𝐶𝑅𝐶 + 𝑉𝐶𝐸 +𝐼𝐸𝑅𝐸 = 𝐼𝐶(𝑅𝐶 +𝑅𝐵2/𝛼 ) 𝑉𝐶𝐸

𝑉𝐵 − 𝑉𝐵𝐸
𝐼𝐵 =
𝑅𝐵 + (1 + 𝛽)
𝐼𝐶𝑄 = 𝛽𝐼𝐵
𝐼𝐸𝑄 = (1 + 𝛽)
𝑉𝐶𝐸𝑄 = 𝑉𝐶𝐶 − 𝐼𝐶𝑅𝐶 − 𝐼𝐸𝑅𝐸

V𝐵𝐸 ≈ 0.7 𝑉and𝐼𝐶 ≈ 𝛽𝐼𝐵

If the BJT is in the active mode the following typical values can be observed:

Circuit Diagrams:

Apparatus:
1) Trainer Board
2) Transistor C828(NPN), BD135(NPN) R
3) Resistors 22KΩ, RC= 470Ω, RB1 = 10KΩ, RE = 560Ω, RB=500K(Potentiometer)
4) DC Power Supply (VCC = +15V DC)
5) Multimeter
6) Power Supply Cable

Circuit Figure:
Simulation:
Table:
β VCE VBE VRC IC IB
Fig. 1 (a) 488 7.46V 0.678V 4.53V 6.6368mA 13.63 uA
345 9.81V 0.637V 5.16V 3.238mA 9.385 uA
% of Change. 29.30% 23.96% 6.05% 12.09% 51.2% 31.14%
Fig. 1 (b) 488 7.51V 0.667V 3.434V 4.514mA 13.54 uA
345 9.28V 0.627V 2.58V 3.617mA 10.34 uA
% of Change. 29.30% 19.18% 5.997% 24.87% 19.88% 22.97%
Fig. 2 (b) 488 7.84V 0.670V 3.26V 6.318mA 12.94 uA
345 7.69V 0.632V 3.19V 4.46mA 12.88 uA
% of Change. 29.30% 1.91% 5.67% 2.15% 29.31% 0.46%

Discussion:
In this experiment, we studied a couple of BJT biasing circuits and calculated their operating points. First,
we measured the transistor’s β value and we implemented a circuit to obtain IB/IC values. By changing the
bias current (IB), we saw that the collector current (IC) changed almost β times. We noted these IC and
IB readings and calculated their β values by IC / IB. After completing all the necessary calculations, the values
of IC and IB were determined.
Conclusion:
The purpose of biasing circuits is to set the operating point of the transistor and ensure that it operates in
the active region, where it can amplify the input signal without distortion.
Fixed bias circuits use a voltage divider to set the base bias voltage, while self-bias circuits use the voltage
drop across an emitter resistor to set the base bias voltage. Self-bias circuits are generally more stable
against variations in the transistor gain factor β.
The Q-point is the operating point of the transistor in a DC circuit, and it is important to ensure that the Q-
point is stable and remains within the desired range under different operating conditions
References:
1. Robert L. Boyleston, Louis Natinsky, Electronic Devices and Circuit Theory, Ninth Edition, 2007-
2008
2. Adel S. Sidra, 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.