Session 4 BJT Introduction
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saadaghaleb1973Session 4 BJT Introduction
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saadaghaleb1973Electronics Circuits
4- BJT Introduction
Dr. Eng. Yousef E. M. Hamouda
Bipolar Junction Transistors
•• Introduction
Introduction
•• DC
DC biasing
biasing
•• Applications
Applications
•• Summary
Summary
Dr. Yousef Hamouda
Introduction
• Essential component of many electronic
circuits
– used in amps, oscillators, digital computers
– provide power gain, voltage gain, current
gain
– IC's composed of transistor arrays
fabricated onto single chip of Si
Dr. Yousef Hamouda
History of BJT
• The bipolar junction transistor (BJT) was
the first solid-state amplifier element and
started the solid-state electronics
revolution.
• Bardeen, Brattain and Shockley, while at
Bell Laboratories, invented it in 1948 as
part of a post-war effort to replace
vacuum tubes with solid-state devices.
• The work at Bell Laboratories was highly
successful and culminated in Bardeen,
Brattain and Shockley receiving the
Nobel Prize in 1956.
Dr. Yousef Hamouda
Limitation of Vacuum Tubes
• The vacuum tube looks
and behaves very much
like a light bulb;
• it generates a lot of heat
and has a tendency to
burn out.
• Also, compared to the
transistor it is slow, big
and bulky.
Dr. Yousef Hamouda
The first Transistor
• The first transistor ever
assembled, invented in Bell
Labs in 1947.
• They used germanium as the
semiconductor of choice
because it was possible to
obtain high purity material.
Dr. Yousef Hamouda
Fundamentals of BJT
collector collector
c c c c
n p
base b b base b b
p n
n p
e e e e
emitter emitter
npn TRANSISTOR pnp TRANSISTOR
Dr. Yousef Hamouda
Equilibrium conditions (npn)
• Base is thin and lightly C
doped electrons
(majority carriers in n-type)
• No external potential is n
applied holes
B (majority carriers in p-type)
• Diffusion of electrons from p
np and holes from pn results Depletion
in formation of depletion regions
regions at boundaries n
• A potential barrier forms for
majority carriers and flow E
stops (as for junction diode) Equilibrium Conditions
Dr. Yousef Hamouda
Bipolar Junction Transistors
•• Introduction
Introduction
•• DC
DC biasing
biasing
•• AC
AC analysis
analysis
•• Applications
Applications
•• Summary
Summary
Dr. Yousef Hamouda
DC Operating conditions
• C: collector;
• B: base;
• E: emitter
Dr. Yousef Hamouda
DC Operating conditions
Distance
IC
C
V CB
n
I Potential
B
p
B
Electron flow
n
VBE
E
IE
CE mode Configuration
Dr. Yousef Hamouda
DC Operating conditions
• Under normal operating IC
conditions (common
C
n
emitter mode)
I
B
p
B
– BE junction is connected Electron flow
with forward potential n
– BC junction is connected IE
E
with reverse potential
CE mode
n
Dr. Yousef Hamouda
DC Operating conditions
• As a result,
– BE junction width is reduced over
equilibrium values
– BC junction is increased
Dr. Yousef Hamouda
DC Operating conditions
• Electrons are injected from EB (np) under forward bias,
• At the same time, holes injected from BE
– net flow of current from BE
• Emitter n-region is more heavily doped than p-type base
• injected electrons diffuse to BC junction
• electrons are swept across to collector because of high potential
gradient
– collector current IC
Dr. Yousef Hamouda
NPN BJT Current flow
Dr. Yousef Hamouda
DC Operating conditions
• Transistor is a current- C
controlled device IC
– current will not flow IB
unless power supply B V CE
connected
VBE
– Total current is IE
IE = IB + IC E
npn Configuration
Dr. Yousef Hamouda
Operating modes of the BJT
• The operating mode of a BJT depends on how its
junctions
– active mode in applications where it is used as an amplifier.
– In the cut-off and saturation modes, the BJT behaves like an
open and closed switch, respectively. Used in digital circuits
(logic gates, memory)
Dr. Yousef Hamouda
Basic BJT Configurations
• Common base configuration (CB)
• Common emitter configuration (CE)
• Common collector configuration (CC)
Dr. Yousef Hamouda
Alpha
• In the dc mode, the levels of IC and IE
due to the majority carriers are related to
a quantity called alpha:
αdc=IC/IE
αac=∆IC/∆IE
Dr. Yousef Hamouda
Common Emitter Configuration
• This is the most
frequently accounted
transistor
configuration
Dr. Yousef Hamouda
Characteristics of CE amplifier
• Although the
configuration
is changed, but
– IE = IB + IC
– α=IC/IE
Dr. Yousef Hamouda
Example
• Determine IC at
IB=30μA and
VCE=10V
Dr. Yousef Hamouda
Beta for CE amplifier
β = IC/IB
• IC and IB are determined at a particular
operating point on the characteristics. The
typical value ranges from 50 to 400.
Dr. Yousef Hamouda
Beta for CE amplifier
βac = ΔIC/ΔIB when VCE=const.
• It is called as common-emitter, forward
current amplification factor. (Also denoted
by hfe)
• While hFE is denoted as βdc
Dr. Yousef Hamouda
Examples
IC2 IB2
IC1 IB1
Dr. Yousef Hamouda
Examples
• βac = ΔIC/ΔIB when VCE=const.
= (3.2mA-2.2mA)/(30μA-20μA)
=100
• βdc = IC/IB
= 2.7mA/25 μA
=108
Dr. Yousef Hamouda
Relations between alpha and beta
• IE = IB + IC
• Ic /α = IC + IC/β
• α = β / (β+1)
β = α / (1- α)
• For CE amplifiers:
– Ic = α IB
– IE = (α +1)IB
Dr. Yousef Hamouda
Summary
We have looked at:
• History of Transistors
• Operation of Transistors
• Different configuration of
Transistors
Dr. Yousef Hamouda
Bipolar Junction Transistors
•• Introduction
Introduction
•• DC
DC biasing
biasing
•• Applications
Applications
•• Summary
Summary