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BASIC ELECTRONICS
MODULE 3

Bipolar Junction Transistor
Prepared by

H V Balachandra Achar
Senior Lecturer,
Dept
...
,
M I T, Manipal

Department of Electronics and Communication Engineering,
Manipal Institute of Technology, Manipal, INDIA

Syllabus
•
•
•
•
•
•
•
•

Introduction to Bipolar Junction Transistor
BJT Operation
BJT Configurations
Tutorials
BJT Biasing
Tutorials
BJT Amplifier
Tutorials

Department of Electronics and Communication Engineering,
Manipal Institute of Technology, Manipal, INDIA

Reference Books
1
...
“Integrated Electronics” by Millman & Halkias,

Department of Electronics and Communication Engineering,
Manipal Institute of Technology, Manipal, INDIA

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

Department of Electronics and Communication Engineering,
Manipal Institute of Technology, Manipal, INDIA

Introduction

Figure showing relative sizes of
transistor, IC and LED

Figure showing different transistor packages
Department of Electronics and Communication Engineering,
Manipal Institute of Technology, Manipal, INDIA

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

Department of Electronics and Communication Engineering,
Manipal Institute of Technology, Manipal, INDIA

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
...
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

Department of Electronics and Communication Engineering,
Manipal Institute of Technology, Manipal, INDIA

Introduction
• Transistor symbols:

Note: Arrow direction from P to N (like diode)
Department of Electronics and Communication Engineering,
Manipal Institute of Technology, Manipal, INDIA

Introduction
• BJT has two junctions – Emitter-Base (EB) Junction and
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

Department of Electronics and Communication Engineering,
Manipal Institute of Technology, Manipal, INDIA

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

Department of Electronics and Communication Engineering,
Manipal Institute of Technology, Manipal, INDIA

Transistor Operation

Un-biased transistor showing barriers at the junctions
Department of Electronics and Communication Engineering,
Manipal Institute of Technology, Manipal, INDIA

Transistor Operation

C-B junction is reverse biased – increased barrier height
Department of Electronics and Communication Engineering,
Manipal Institute of Technology, Manipal, INDIA

Transistor Operation

E-B junction is forward biased – aids charge flow
Department of Electronics and Communication Engineering,
Manipal Institute of Technology, Manipal, INDIA

Transistor Operation

Electron-hole combination – leads to small base current
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
• Some free electrons combine with holes in the base to form
small base current
• Inside the base region (p-type), free electrons are minority
carriers
...


Three currents can be identified in BJT
Emitter current
•
•

2
...


This is due to combination of free electrons and holes in the base
region
Small in magnitude (usually in micro amperes)

Collector current
•
•
•

Has two current components:
One is due to injected free electrons flowing from base to collector
Another is due to thermally generated minority carriers
Department of Electronics and Communication Engineering,
Manipal Institute of Technology, Manipal, INDIA

Transistor Operation
C

IC

C

IB

IB

B

IC

B
IE
E

IE
E

NPN

PNP

• Note the current directions in NPN and PNP transistors
• For both varieties:
---(1)

I E  IC  I B

Department of Electronics and Communication Engineering,
Manipal Institute of Technology, Manipal, INDIA

Transistor Operation
• As noted earlier, collector current has two components:
– One due to injected charge carriers from emitter
– Another due to thermally generated minority carriers
• Both results in current in the same direction
...

• So ICBO cannot be neglected at higher temperatures
• The parameter αdc is called common-base dc current gain
• Value of αdc is around 0
...
99, ICEO >> ICBO
• However, ICEO is still very small compared to IC
IC
• Hence approximation of (4) gives: I C   dc I B or  dc 

IB

• Parameter βdc is called common emitter dc current gain
• Values of αdc and βdc vary from transistor to transistor
...


2
...


A BJT has alpha (dc) 0
...
If emitter current is 5mA, calculate the
collector and base currents
...
99 mA, 10 μA)
An npn transistor has collector current 4mA and base current
10 μA
...
9975, 400)
neglecting the reverse sat current ICBO
In a transistor, 99% of the carriers injected into the base
cross over to the collector region
...
034 mA, 34 μA)

Department of Electronics and Communication Engineering,
Manipal Institute of Technology, Manipal, INDIA

Transistor Configurations
• BJT has three terminals
• For two-port applications, 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
• (The last one is not discussed in this course)
Department of Electronics and Communication Engineering,
Manipal Institute of Technology, Manipal, INDIA

Transistor Configurations
• Common Base configuration

(Resistors are not shown here
for simplicity)

• Base is common between input and output
– Input voltage: VEB
Input current: IE
– Output voltage: VCB Output current: IC
Department of Electronics and Communication Engineering,
Manipal Institute of Technology, Manipal, INDIA

Transistor Configurations
• CB Input characteristics
– A plot of IE versus VEB
for various values of VCB
– It is similar to forward
biased diode
characteristics
– As VCB is increased, IE
increases only slightly
– Note that second letter in
the suffix is B (for base)
Department of Electronics and Communication Engineering,
Manipal Institute of Technology, Manipal, INDIA

Transistor Configurations
• Input resistance ri

VEB
ri 
with VCB const
I E
• Voltage amplification factor AV

VCB
AV 
with I E const
VEB
• Both can be determined from the CB input characteristics
Department of Electronics and Communication Engineering,
Manipal Institute of Technology, Manipal, INDIA

Transistor Configurations

VEB1  VEB 2
ri 
I E1  I E 2

VCB1  VCB 2
AV 
VEB1  VEB 2

Department of Electronics and Communication Engineering,
Manipal Institute of Technology, Manipal, INDIA

Transistor Configurations
• CB Output characteristics
– A plot of IC versus VCB for various values of IE
– Three regions are identified: Active, Cutoff, Saturation
– Active region:
• E-B junction forward biased
• C-B junction reverse biased
• IC is positive, VCB is positive
• IC increases with IE
• For given IE, IC is almost constant; increases only
slightly with increase in VCB
...
Part of this
depletion region lies in the base layer
...
Hence number of electron-hole
combination at the base decreases
...

– Note that IE = IC + IB

Department of Electronics and Communication Engineering,
Manipal Institute of Technology, Manipal, INDIA

Transistor Configurations
• When IE = 0, IC = ICBO
– ICBO is collector to base current with emitter open
– Below this line we have cut-off region
– Here both junctions are reverse biased
• Region to the left of y-axis (VCB negative) is saturation region
– Here both junctions are forward biased
– IC decreases exponentially, and eventually changes
direction

Department of Electronics and Communication Engineering,
Manipal Institute of Technology, Manipal, INDIA

Transistor Configurations
• Output resistance ro
rO 

VCB
with I E const
I C

• Current amplification factor AI or αac
 ac 

I C
with VCB const
I E

• Both can be measured from output characteristics
Department of Electronics and Communication Engineering,
Manipal Institute of Technology, Manipal, INDIA

Transistor Configurations
• Common Emitter configuration

(Resistors are omitted for simplicity)

• Emitter is common between input and output
– Input voltage: VBE
– Output voltage: VCE

Input current: IB
Output current: IC

Department of Electronics and Communication Engineering,
Manipal Institute of Technology, Manipal, INDIA

Transistor Configurations
CE input characteristics
• Plot of IB versus VBE for
various values of VCE
• Similar to diode
characteristics
• As VCE is increased, IB
decreases only slightly
• This is due to base-width
modulation
• Note that second suffix is E
(for emitter)
Department of Electronics and Communication Engineering,
Manipal Institute of Technology, Manipal, INDIA

Transistor Configurations
• CE output characteristics
– A plot of IC versus VCE for various values of IB
– Three regions identified: Active, Cut-off, Saturation
– Active region:
• Linear region in the output characteristics
• E-B junction forward biased
• C-B junction reverse biased
• IC increases with IB
• For given IB, IC increases slightly with increase in VCE;
this is due to base-width modulation (Early effect)
Department of Electronics and Communication Engineering,
Manipal Institute of Technology, Manipal, INDIA

Transistor Configurations
CE output characteristics

Department of Electronics and Communication Engineering,
Manipal Institute of Technology, Manipal, INDIA

Transistor Configurations
• Note that VCE = VCB + VBE
• So if VCE is increased, effectively VCB also increases
• For saturation to take place, C-B junction should be forward
biased
...
3 V (or less) for Si
• Note that when VCE= 0
...
6 V, VCB= –0
...
3 V)
• So region to the left of the vertical line VCE=VCE(sat)=0
...
99

• The values of αdc & αac, and βdc & βac are almost the same
...
If the transistor is connected in commonemitter operation, find the collector current for base current
(a) 0 (b) 40 µA
...


When IB = 0,
IC = ICEO = (β+1)ICBO = 505 µA
When IB = 40 µA,
IC = βIB + ICEO
= (100 × 40 × 10–6) + (505 × 10–6)
= 4
...
A Ge Transistor has collector current of 51 mA when the base
current is 0
...
If β = 125, then what is its collector cutoff
current ICEO?
(Ans: 1 mA)
3
...
32 mA to 0
...
968, 30
...
Find αac and βac values
...
A transistor with α = 0
...

Find IC and IE
...
15 mA, 5

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