The document provides an overview of bipolar junction transistors (BJT), detailing their structure, operation, and configurations. It explains the types of BJTs, specifically PNP and NPN transistors, and outlines their operating regions: active, saturation, and cut-off. Additionally, the document describes the common configurations for connecting BJTs in circuits, namely common emitter, common collector, and common base configurations.

1. BIPOLAR JUNCTION
TRANSISTORS
(BJT)
Neeta Ukirade(Gatkal)
Pratibha College of Commerce and Computer Studies

2. CONTENTS
• INTRODUCTION TO BJT
• TYPES OF BIPOLAR JUNCTION TRANSISTOR
• BIPOLAR JUNCTION TRANSISTOR STRUCTURE
• BIPOLAR JUNCTION TRANSISTOR OPERATION
• BIPOLAR JUNCTION TRANSISTOR CONFIGURATIONS
Common Emitter Configuration input and output characteristics

3. INTRODUCTION
What is a transistor?
A transistor is a type of semiconductor device that can be used to conduct and
insulate electric current or voltage. A transistor basically acts as a switch and an
amplifier.
Parts of a Transistor
A typical transistor is composed of three layers of semiconductor materials or,
more specifically, terminals which help to make a connection to an external circuit
and carry the current.
There are three terminals for a transistor. They are listed below:
Base: This is used to activate the transistor.
Collector: It is the positive lead of the transistor.
Emitter: It is the negative lead of the transistor.

4. TYPES OF TRANSISTOR

5. BIPOLAR JUNCTION TRANSISTOR (BJT)
• A Bipolar junction transistor, shortly termed as BJT is called so as it has
two PN junctions for its function.
• The three terminals of BJT are the base, emitter and collector. A very small
current flowing between the base and emitter can control a larger flow of
current between the collector and emitter terminal.
• The concept of BJT refers to a three-terminal semiconductor device
consisting of two p-n-junctions that are formed by sandwiching either p-
type or n-type semiconductors between two p-type semiconductors.
Transistors can be used to transfer input signals through the circuits having
lower to higher resistance.

6. TYPES OF BJTS
 PNP Bipolar Junction Transistor
 NPN Bipolar Junction Transistor

7. BIPOLAR JUNCTION TRANSISTOR OPERATION
BJT operates in three regions:
a) Active region b) Saturation region c) Cut-off region
Active region: The transistor is on in this region. When the collector-base
junction is reverse-biased while the emitter-base junction is in forward-bias,
then BJT works in the active area. As a result, BJT is used as an amplifier. The
collector current is proportional to and controlled by the base current and
relatively insensitive to VCE. In this region the transistor can be an amplifier.
Saturation region: The saturation region arises when the collector-base
junctions and the emitter-base junction are both forward-biased. The transistor
is on. The collector current varies very little with a change in the base current
in the saturation region.
Cut-off region: Because the emitter and collector junctions are both in
the reversed bias, so the BJT is off in this condition and vary small amount
of current flows from collector to the emitter. There is no current between
emitter and the collector.

8. OPERATION OF NPN TRANSISTOR
 With the forward-biased emitter-base junction and reverse-biased
collector-base junction, it can be seen that the forward bias causes the
flow of electrons from the n-type emitter into the p-type base.
 This constitutes the emitter current (). As these electrons flow through
the p-type base, they tend to combine with the holes.
 Since the base is lightly doped and very thin, hence, only a small
number electrons (less than 5%) combine with the holes to constitute
the base current ().
 The remaining (more than 95%) electrons cross over the base region
and reach to the collector region to constitute the collector current ().

9. BIPOLAR JUNCTION TRANSISTOR CONFIGURATIONS
There are basically three possible ways to connect BJT in
an electric circuit are:
Common Collector Configuration
Common Base Configuration
Common Emitter Configuration.
Common Base Configuration
• The base of transistor is grounded in
common base characteristics , with the
emitter acting as the input and the collector
as the output.
• The emitter-base junction is biased forward,
whereas the junction between the collector
and the base is reverse biased.
• Low voltage gain but high current gain is
offered by the CB arrangement.
Current Amplification factor (α)

10. Common Collector Configuration
 The configuration in which the collector is
common between emitter and base is known
as CC configuration.
 In CC configuration, the input circuit is
connected between emitter and base and the
output is taken from the collector and
emitter.
 The collector is common to both the input
and output circuit and hence the name
common collector connection or common
collector configuration.
Current Amplifier Factor (Y) Relation Between Υ and α

11. Common Emitter Configuration
 The configuration in which the emitter is
connected between the collector and base is
known as a common emitter configuration.
 The input circuit is connected between
emitter and base, and the output circuit is
taken from the collector and emitter.
 Thus, the emitter is common to both the
input and the output circuit, and hence the
name is the common emitter configuration.
Current Amplification Factor (β) The relation between Β and α can be
derived as

12. COMMON EMITTER CONFIGURATION INPUT AND
OUTPUT CHARACTERISTICS

13. THANK YOU