Transcript Base Bias - WordPress.com

BASIC ELECTRONICS
Bipolar Junction Transistor
By:
Engr. Syed Asad Ali
Acknowledgement
Large Part Of This Lecture is Taken From Manipal Institute India
Lecture Contents
•
•
•
•
•
Introduction to Bipolar Junction Transistor
BJT Operation
BJT Configurations
Transistor Application
BJT Biasing
Introduction
• Solid state transistor was invented by a team of scientists
at Bell laboratories during 1947-48
• 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
Introduction
Figure showing relative sizes of
transistor, IC and LED
Figure showing different transistor packages
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
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
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.
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
Transistor Operation
• Three Mode Of Operation
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)
Department of Electronics and Communication Engineering,
Manipal Institute of Technology, Manipal, INDIA
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.
Transistor Operation
•
1.
Three currents can be identified in BJT
Emitter current
•
2.
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)
•
3.
Collector current
–
Has two current components:
1.
2.
–
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
Transistor Operation
C
IC
C
IB
IB
B
B
IE
E
NPN
IC
IE
E
PNP
• Note the current directions(Conventional) in NPN and PNP
transistors
I E  IC  I B
• For both varieties:
---(1)
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
Input
2-port
device
Output
• Accordingly three configurations exist:
– Common Base (CB) configuration
– Common Emitter (CE) configuration
– Common Collector (CC) configuration
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
 dc 
IC
IE
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.
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 50-200
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
Summary
Amplifier Type
Common Base
Common Emitter
Common Collector
Voltage Gain
HIGH (e.g. 150)
Very High (e.g. 500)
Less Than 1
Current Gain
Less Than 1
High
High
Power Gain
Medium
High
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
Parameter
Transistor Application
1. Transistor As Amplifier
2. Transistor As a Switch
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
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
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
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.
Types of biasing:
1. Base Bias
2. Emitter Bias
3. Voltage Divider Bias
Transistor Biasing
• Base Bias
– Pros:
– Cons:
Simple circuit
Uses very few resistors
This Circuit is unstable
• Parameters can be changed with Temperature & from Transistor to
Transistors
Transistor Biasing
• Emitter Bias:
• Pros:
• Good stability compares to Base Bias
– Cons:
• This type can only be used when a split (dual) power supply
is available (Costly $)
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)