Iraq University College
Department of Communication Engineering
Analog Electronics II
Second Stage
Thre Class
Lecturer: Hamza Hadi
2023-2022
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Chapter one
Bipolar Junction Transistors
(BJT)
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CHAPTER OUTLINE
• Transistor construction
• Transistor operation
• Transistor amplifying action
• BJT Characteristics and Parameters
• Transistor connections and characteristics
• Common Base Connection
• Common Emitter configuration
• common collector connection
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Introduction
• When a third doped element is
added to a crystal diode in such a way
that two pn junctions are formed, the
resulting device is known as a
transistor.
• The transistor—an entirely new type
of electronic device—is capable of
achieving amplification of weak
signals.
• Transistors are mechanically strong,
have practically unlimited life and can
do some jobs better than vacuum
tubes.
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The First Transistor: Point-contact transistor
A point-contact transistor was the first type of solid state
electronic transistor ever constructed.
It was made by researchers John Bardeen & Walter
Houser Brattain at Bell Laboratories in December 1947.
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Point-Contact Transistor –
first transistor ever made
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Point-Contact Transistor –
first transistor ever made
W. Shockley
invented the p-n
junction transistor
The physically
relevant region is
moved to the bulk
of the material
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The Junction Transistor
• Initially known simply as the junction transistor.
• It did not become practical until the early 1950s.
• The term “bipolar "was tagged onto the name to
distinguish the fact that both carrier types play
important roles in the operation.
• Field Effect Transistors (FETs) are“unipolar”
transistors since their operation depends primarily
on a single carrier type.
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Bipolar Junction Transistor (BJT) structure
• The BJT is constructed with three
doped semiconductor regions separated
by two pn junctions.
• The three regions are called emitter,
base, and collector.
• One type consists of two n regions
separated by a p region (npn), and the
other type consists of two p regions
separated by an n region (pnp).
• The emitter is heavily doped
• The base region is lightly doped and very thin
• The collector regions is large and moderately
doped
Basic models of BJT
Diode
Diode
Diode
Diode
Note that emitter is shown by an arrow Which indicates
the direction of conventional current flow with forward
bias.
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Basic BJT operation (Bias)
In order for a BJT to operate properly as an amplifier, the two
pn junctions must be correctly biased with external dc
voltages.
Forward-reverse
bias of a BJT
The bias arrangement for both npn and pnp BJTs for operation as an amplifier.
Notice that in both cases the base-emitter (BE) junction is forward-biased
and the base-collector (BC) junction is reverse-biased with external dc
voltages.
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Inside the npn structure
• The heavily doped n-type emitter region
has a very high density of conduction
band (free) electrons,
• These free electrons easily diffuse
through the forward- based BE junction
into the lightly doped and very thin ptype
base region.
• A small percentage of the total number
of free electrons injected into the base
region recombine with holes and move
as valence electrons through the base
region and into the emitter region as
hole current.
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Inside the npn structure ,continue
• Most of the free electrons that have
entered the base do not recombine
with holes because the base is very
thin.
• As the free electrons move toward the
reverse-biased BC junction, they are
swept across into the collector region
by the attraction of the positive
collector supply voltage.
• The emitter current is slightly greater
than the collector current because of
the small base current that splits off
from the total current injected into the
base region from the emitter.
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Transistor Currents
The emitter current (IE) is the sum of the collector current (IC)
and the base current (IB), expressed as follows
IE = IB + IC
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Inside the pnp structure
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Basic models of BJT
PNP NPN
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DC Beta (βDC) and DC Alpha ( 𝜶DC)
The dc current gain of a transistor is the
ratio of the dc collector current (IC) to the
dc base current (IB) and is designated dc VCC reverse-biases the
base-collector junction
beta (𝛽DC).
Typical values of 𝛽DC range
From less than 20 to 200 or higher.
VBB forward-biases the
base-emitter junction
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DC Alpha (𝜶DC)
The ratio of the dc collector current (IC) to the dc emitter current (IE) is
the dc alpha 𝜶DC
Typically, values of 𝜶DC range from 0.95 to 0.99 or greater, but 𝜶DC is
always less than 1.
The reason is that IC is always slightly less than IE by the amount of IB.
The alpha is a less-used parameter than beta in transistor
circuits.
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Example
Determine the dc current gain 𝛽DC and the emitter current
IE for a transistor where IB = 50 𝜇A and IC = 3.65 mA.
Solution:
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HOMEWORK (1)
Q1: What is the 𝜶DC when IC =8.23 mA and IE =8.69 mA?
Q2:A certain transistor has an IC= 25 mA and an IB= 200
mA. Determine the βDC.
Q3:What is the βDC of a transistor if IC= 20.3 mA and IE=
20.5 mA?
Q4:What is the 𝜶DC if IC =5.35 mA and IB =50 mA?
Q5: A certain transistor exhibits an 𝜶DC of 0.96.Determine
IC when IE= 9.35 mA.
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