Introduction to Bipolar Junction Transistor (BJT) which will cover some basic knowledge of this component.

1. Chapter 3 MEC523
BJT
(Bipolar Junction Transistor)
3a - Introduction
DR ROSHAKIMAH MOHD ISA
T2-A14-7C
0199503432

2. Chapter Objectives
 After completing this chapter, the student should be able to:
1. Describe the basic construction and operation of the Bipolar
JunctionTransistor.
2. Apply the proper biasing to insure operation in the active region.
3. Become aware of the saturation and cutoff conditions of a BJT
network and the expected voltage and current levels established by
each condition.
4. Recognize and explain the characteristics of an npn and pnp
transistor.
5. Identify the important parameters that define the response of a
transistor using DC analysis.
6. Be able to perform a load-line analysis of the most common BJT
configurations.
7. Understand the basic operation of transistor switching networks.
8. Learn to use the equivalent model to find the important ac
parameters for an amplifier.

3. Vacuum tube devices

4. Transistor construction
 A bipolar transistor is produced when a third layer
is added to a semiconductor.
 A transistor:
◦ Can amplify power, current, or voltage and switch
electronic signals on and off.
◦ Can be constructed of germanium or silicon – silicon is
more popular.
◦ Consists of three alternately doped regions.
◦ The regions are arranged two ways;
 - P-type material is sandwiched between two N-type materials,
NPN transistor.
 - N-type material is sandwiched between two P-type materials,
PNP transistor.
(A common p-type dopant for silicon is boron or gallium)
(A common n-type dopant for silicon is phosphorus or arsenic)

5. Bipolar Junction Transistor
There are two types of
transistors:
• pnp
• npn
The terminals are labeled:
• E - Emitter
• B - Base
• C - Collector
*The ratio of the total width to that of the center layer is150:1.
The doping of the sandwiched layer is less than that of the
outer layers (typically, 10:1 or less). This lower doping level
decreases the conductivity (increases the resistance) of this
material by limiting the number of “free” carriers.

6. Transistor operation
With the external sources, VEE and VCC, connected as
shown below:
• The emitter-base junction is forward biased
• The base-collector junction is reverse biased

7. Currents in transistor
The collector current is
comprised of two currents:
B
I
C
I
E
I 

COminority
I
Cmajority
I
C
I 

Emitter current is the sum of the
collector and base currents:
Leakage
current
If IB is very small, IE≈IC

8. Introduction
 A bipolar junction transistor (BJT) has three terminals connected to
input and output circuit loops.
 A transistor amplifier is a two-port network; one of the three
transistor terminals must be shared by both input and output ports as
the common terminal.
 This results in three possibilities, namely, common-emitter (CE)
amplifier, common-base (CB) amplifier and common-collector (CC)
amplifier circuits.
 Common Base Configuration – hasVoltage Gain but no Current Gain.
 Common Emitter Configuration – has both Current andVoltage Gain.
 Common Collector Configuration – has Current Gain but noVoltage
Gain.
 In electronics, a common-emitter amplifier circuit is generally used.

9. Configurations
Common base
Common collector
Common emitter

10. COMMON-BASE configuration
The base is common to both input (emitter–base) and output (collector–
base) of the transistor. (the base is usually at ground potential)
-current directions will refer to conventional (hole) flow rather than
electron electron flow.

11. Operating regions
This graph demonstrates the
output current (IC) to an output
voltage (VCB) for various levels of
input current (IE).
Output/collector Characteristics
for a common-base transistor
Operating
range of the
amplifier
The amplifier is
basically OFF.
There is voltage,
but little current
The
amplifier is
full ON.
There is
current,
but little
voltage.
E
I
C
I 

12. From the curves, it indicates that a first approximation to
the relationship between IE and IC in the active region is
given by;
Emitter and collector currents: E
I
C
I 
0.7
BE
V 
Once a transistor is in the “ON” state;
Base-emitter voltage:
Ideally:  = 1
In reality:  is between 0.9 and 0.998; thus
In the DC mode, the levels of IC and IE due to
the majority carriers are related by a quantity
called alpha;
Alpha () relates the DC currents IC and IE :
E
I
C
I
dc
α 
Alpha () in the AC mode:
E
ΔI
C
ΔI
ac
α 
IC = αIE + ICBO

13. Transistor amplification (C-B conf.)
Voltage Gain:
250
200mV
50V
i
V
L
V
v
A 


V
50
)
kΩ
5
)(
ma
10
(
R
L
I
L
V
mA
10
i
I
L
I
E
I
C
I
10mA
20Ω
200mV
i
R
i
V
i
I
E
I










Currents and Voltages:

14. COMMON-EMITTER configuration
The emitter is common to both input (base-emitter) and
output (collector-emitter).
-The most encountered transistor configuration
-The input is on the base and the output is on the collector.

15. Collector characteristics

16. Currents in C-E amplifier
Ideal Currents
IE = IC + IB IC =  IE
Actual Currents
IC =  IE + ICBO
When IB = 0 A the transistor is in cutoff, but there is
some minority current flowing called ICEO.
μA
0
I
CBO
CEO B
1
I
I 



where ICBO = minority collector current
This is usually so small that it can be
ignored, except in high power transistors
and in high temperature environments.

17. Amplification factor
In DC mode:
In AC mode:
 represents the amplification factor of a transistor. ( is
sometimes referred to as hFE (current gain or
amplification factor) - a term used in transistor modeling
calculations)
B
C
dc
I
I
β 
t
tan
cons
V
B
C
ac CE
I
I





Relationship between
amplification factors  and  1
β
β
α


1
α
α
β


Relationship Between
Currents
B
C βI
I  B
E 1)I
(β
I 

IC =  IE

18. Determining  from a graph
Note: AC = DC
108
A
25
mA
2.7
β 7.5
V
DC CE


 
100
μA
10
mA
1
μA)
20
μA
(30
mA)
2.2
mA
(3.2
β
7.5
V
AC
CE






Determine βac for a region
of the characteristics defined
by an operating point of IB
=25 µA and VCE = 7.5 V as
indicated in the graph.

19. COMMON-COLLECTOR Configuration
-This configuration is used primarily for impedance-matching
purpose; it has high input impedance & low output impedance-
opposite to CB and CE.
-The input is on the base and the output is on the emitter.

20. Limits of operation
VCE is at maximum
and IC is at minimum
(ICmax= ICEO) in the
cutoff region.
IC is at maximum and
VCE is at minimum
(VCE max = VCEsat =
VCEO) in the saturation
region.
The transistor
operates in the active
region between
saturation and cutoff.

21. Power dissipation
Common-collector:
C
CB
Cmax I
V
P 
C
CE
Cmax I
V
P 
E
CE
Cmax I
V
P 
Common-base:
Common-emitter:

22. Pdissipation=Ic⋅VCE
 Common Emitter Configuration:
◦ In the common emitter configuration, power dissipation can be
calculated using the formula mentioned above with the collector
current (Ic) and collector-emitter voltage (VCE) for the specific
operating point of the transistor.
 Common Collector Configuration (Emitter Follower):
◦ In the common collector configuration (also known as an emitter
follower), the power dissipation is typically lower than in the common
emitter configuration because the collector voltage (VCE) is close to
the supply voltage (VCC) when the transistor is saturated.The formula
still applies, but VCE is usually much smaller.
 Common Base Configuration:
◦ In the common base configuration, the power dissipation can also be
calculated using the same formula. However, common base
configurations are often used for low-power amplification, so the power
dissipation may be lower compared to common emitter circuits.

23. Transistor specification sheet