1. Large Part Of This Lecture is Taken From Manipal Institute India
BASIC ELECTRONICS
Bipolar Junction Transistor
By:
Engr. Syed Asad Ali
Acknowledgement
Large Part Of This Lecture is Taken From Manipal Institute India

2. Lecture Contents Introduction to Bipolar Junction Transistor
BJT Operation
BJT Configurations
Transistor Application
BJT Biasing

3. Introduction
Solid state transistor was invented by a team of scientists at Bell laboratories during
It brought an end to vacuum tube era
Advantages of solid state transistor over vacuum devices:
Smaller size, light weight
No heating elements required
Lower power consumption and operating voltages
Low price

4. Introduction Figure showing relative sizes of transistor, IC and LED
Figure showing different transistor packages

5. Introduction
Bipolar Junction Transistor (BJT) is a sandwich consisting of three layers of two different types of semiconductor
Two kinds of BJT sandwiches are: NPN and PNP

6. Introduction
The three layers of BJT are called Emitter, Base and Collector
Base is very thin compared to the other two layers
Base is lightly doped. Emitter is heavily doped. Collector is moderately doped
NPN – Emitter and Collector are made of N-type semiconductors; Base is P-type
PNP – Emitter and Collector are P-type, Base is N-type
Both types (NPN and PNP) are extensively used, either separately or in the same circuit

7. Introduction Transistor symbols: Arrow is Always Drawn On Emitter.
Arrow Always Points towards N Type.
Emitter and Collector are always made up of same type of Material.

8. Introduction BJT has two junctions
Emitter-Base (EB) Junction
Collector-Base (CB) Junction
Analogous to two diodes connected back-to-back:
EB diode and CB diode
The device is called “bipolar junction transistor” because current is due to motion of two types of charge carriers – free electrons & holes

10. Transistor Operation
Three Mode Of Operation

11. Transistor Operation
Operation of NPN transistor is discussed here; operation of PNP is similar with roles of free electrons and holes interchanged
For normal operation (amplifier application)
EB junction should be forward biased
CB junction should be reverse biased
Depletion width at EB junction is narrow (forward biased)
Depletion width at CB junction is wide (reverse biased)

12. Department of Electronics and Communication Engineering,
Manipal Institute of Technology, Manipal, INDIA

13. Transistor Operation
When EB junction is forward biased, free electrons from emitter region drift towards base region.
Base is Thin and Lightly Doped, hence have very minimum Holes.
Only small percentage of Emitter free electrons combine with holes in the base to form small base current.
While majority of the free electrons are swept away into the collector region due to reverse biased CB junction.

14. Transistor Operation Three currents can be identified in BJT
Emitter current
This is due to flow of free electrons from emitter to base
Base current
This is due to recombination of free electrons and holes in the base region
Small in magnitude (usually in micro amperes)
Collector current
Has two current components:
One is due to injected free electrons flowing from base to collector
Another is due to thermally generated minority carriers (Leakage)
Ideally equals to IE

15. Transistor Operation C E B IC IE IB
NPN C E B IC IE IB
PNP
Note the current directions(Conventional) in NPN and PNP transistors
For both varieties: ---(1)

16. Transistor Configurations
BJT has three terminals.
To Properly use it, it Must have 2 or 4 Leads
Due to this reasons one of the BJT terminals needs to be made common between input and output
Accordingly three configurations exist:
Common Base (CB) configuration
Common Emitter (CE) configuration
Common Collector (CC) configuration
Input
Output
2-port device

17. Transistor Configurations
Common Base configuration
I/p Is Applied At Emitter & Base
O/p Is Taken From Collector & base
Base is common between input and output
Current Amplification Factor (Gain)
Ratio Of O/p Current to I/P Current
In Common Base
Input current: IE
Output current: IC
Alpha is Always Less then Unity
Usually between 0.9 to 0.99
We can improve the value of current amplification factor (but not more than one) by minimizing the recombination in the base and this is achieved by doping the base very lightly and making it very thin.

18. Transistor Configurations
Common Emitter configuration
I/p Is Applied At Base & Emitter
O/p Is Taken From Collector & Emitter
Emitter is common between input and output
Current Amplification Factor (Gain)
Input current: IB
Output current: IC
β= IC / IB
β is always Greater then 1
Usually between

19. Transistor Configurations
Common Collector configuration
I/p Is Applied At Base & Collector
O/p Is Taken From Emitter & Collector
Collector is common between input and output
Current Amplification Factor
Input current: IB
Output current: IC
γ =IE/IB
As IE=IC so Above Formula Will be equal to Beta, Hence C.C provide same Current Gain As C.B

20. Summary Amplifier Type Common Base Common Emitter Common Collector
Parameter
Voltage Gain
HIGH (e.g. 150)
Very High (e.g. 500)
Less Than 1
Current Gain
High
Power Gain
Medium
Input Resistance
Very Low(e.g. 100Ω)
Medium(e.g. 750Ω)
Very High(e.g. 750KΩ)
Output Resistance
Very High(e.g. 450Ω)
High(e.g. 45Ω)
Low(e.g. 50Ω)
Application
High Frequency App
Audio Frequency App
Impedance Matching

21. Transistor Application
Transistor As Amplifier
Transistor As a Switch

22. Transistor As Amplifier
Amplification is the process of linearly increasing the strength of electrical signal
Weak Signal Is Applied at the Input and Amplified(Strong Signal Is Collected at the Output)
For amplifier application, transistor should operate in Active Region

23. Transistor As Switch
When Transistor is Operated as A switch it is normally Operated in Cutoff & Saturation Mode
Conditions For Cutoff
B.E Junction Is Reverse Bias
No Input Current
B.C Junction Is Reverse Bias
No Output Current or No Collector Current
Neglecting Leakage Current
Transistor Behave as Open Switch
Condition For Saturation
B.E & B.C Junction Is Forward Bias
Maximum Or Large Input (base)Current
Maximum Output Current (Collector Current )
Transistor Behave as Close Switch

24. Advantages Fast Switching Millions of Time in a Sec
Low Operating Voltage/Current
mAmp, mV
Less or No Noise in Switching
Switch On/Off By Light, Heat or Wireless Signal
Applications Of Transistor Switch
Fridge Door Alarm, Dimming Light, Computers

25. Transistor Biasing What is meant by biasing the transistor?
Applying external dc voltages to ensure that transistor operates in the desired region/Mode
Which is the desired region?
For amplifier application, transistor should operate in Active Region
For switch application, it should operate in cut-off and saturation.

26. Types of biasing:
Base Bias
Emitter Bias
Voltage Divider Bias

27. Transistor Biasing Base Bias Pros: Simple circuit
Uses very few resistors
Cons: This Circuit is unstable
Parameters can be changed with Temperature & from Transistor to Transistors

28. Transistor Biasing Emitter Bias: Pros: Cons:
Good stability compares to Base Bias
Cons:
This type can only be used when a split (dual) power supply is available (Costly $)

29. Transistor Biasing Voltage Divider:
Pros: Unlike above circuits, only one dc supply is necessary.
Stable Circuit
Cons:
Circuit Analysis Is Complex
Requires Many Components (Resistors)