ELECTRONIC CIRCUIT DESIGN 1
402058
Bipolar Junction
Transistors (BJT)
ACKNOWLEDGEMENT
This slide is adopted from lecture slides of
Microelectronic Circuits Text by Sedra and Smith,
Oxford Publishing.
Oxford University Publishing
Microelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033)
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402058 – Chap 3: BJT
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INTRODUCTION
§ IN THIS CHAPTER YOU WILL LEARN
§ The physical structure of the bipolar transistor and how it
works.
§ How the voltage between two terminals of the transistor
controls the current that flows through the third terminal,
and the equations that describe these current-voltage
relationships.
§ How to analyze and design circuits that contain bipolar
transistors.
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INTRODUCTION
§ IN THIS CHAPTER YOU WILL LEARN
§ How the transistor can be used to make an amplifier.
§ How to obtain linear amplification from the fundamentally
nonlinear BJT.
§ The three basic ways for connecting a BJT to be able to
construct amplifiers with different properties.
§ Practical circuits for bipolar-transistor amplifiers that can
be constructed by using discrete components.
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INTRO TO BJT
§ BJT was invented in 1948 at Bell Telephone
Laboratories.
§ Ushered in a new era of solid-state circuits.
§ It was replaced by MOSFET as predominant
transistor used in modern electronics.
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1. DEVICE STRUCTURE AND
PHYSICAL OPERATION
§ Consists of three semiconductor regions:
§ Emitter region
§ Base region
§ Collector region
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1.1. SIMPLIFIED STRUCTURE AND
MODES OF OPERATION
§ Transistor consists of two pn-junctions:
§ emitter-base junction (EBJ)
§ collector-base junction (CBJ)
§ Operating mode depends on biasing.
§ active mode – used for amplification
§ cutoff and saturation modes – used for switching.
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1.2. OPERATION OF THE NPNTRANSISTOR IN THE ACTIVE MODE
§ Active mode is
“most important.”
§ Two external
voltage sources are
required for biasing
to achieve it.
IE = IB + IC
IB << IC
IE ~ IC
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402058 – Chap 3: BJT
IC = beta. IB
=> beta: current gain
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CURRENT FLOW
§
§
§
§
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Current flow
The Collector current
The Base current
The Emitter current
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THE COLLECTOR CURRENT
§ Magnitude of iC is
independent of vCB.
§ Saturation current (IS)
§ Typically between 10-12
and 10-18A
§ Also referred to as
scale current.
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(eq6.3) iC = IS evBE / VT
−−−−−−−−−−−−−−−−−−
AE qDn np 0
saturation current: IS =
W
−−−−−−−−−−−−−−−−−−
AE qDn ni2
(eq6.4) IS =
W N
1 4 4 4 2 4 4 4 3A
ni = intrinsic carrier density
NA= doping concentration of base
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THE BASE CURRENT
§ Base current (iB) –
composed of two
components:
§ ib1 – due to holes injected
from base region into
emitter.
§ ib2 – due to holes that
have to be supplied by
external circuit to replace
those recombined.
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402058 – Chap 3: BJT
β = transistor parameter
6 44 7 4 48
iC
(eq6.5) iB =
β
−−−−−−−−−−−−
IS vBE / VT
(eq6.6) iB = e
β
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THE EMITTER CURRENT
§ All current which
enters transistor
must leave.
§ iE = iC + iB
6this 4expression 4 4 4is generated 4 4 4through 7 4combination 4 4 4 of 4(6.5) 4and 4(6.7)
8
β +1
β +1
(eq6.8/6.9) iE =
iC =
IS evBE / VT )
(
β
β 14 2 43
iC
−−−−−−−−−−−−−−−−−−−−−−−
(eq6.10) iC = α iE
−−−−−−−−−−−−−−−−−−−−−−−
this parameter is reffered to
as common-­‐base current gain 6 4 44 7 4 4 48
(eq6.11) α =
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β
β +1
, (eq6.13) β =
α
1 −α
−−−−−−−−−−−−−−−−−−−−−−−
IS vBE / VT
2) iE = e
12
402058 –(eq6.1
Chap 3: BJT
α
LARGE SIGNAL EQUIVALENTCIRCUIT MODELS
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EXAMPLE 6.1.
§ Refer to textbook for Example 6.1. pg 313 – 314
Sedra/Smith, Microelectronic Circuits, 7e.
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1.3. THE PNP TRANSISTOR
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2. CURRENT-VOLTAGE
CHARACTERISTICS
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2.1. CIRCUIT SYMBOLS AND
CONVENTIONS
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CURRENT VOLTAGE RELATIONSHIP
IN ACTIVE MODE
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CURRENT VOLTAGE RELATIONSHIP
IN ACTIVE MODE
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EXAMPLE 6.2
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2.2. DEPENDENCE OF iC ON THE
COLLECTOR VOLTAGE
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3. BJT CIRCUITS AT DC
Simplified models for the operation
npn pnp Ac&ve mode Sat. mode 13/3/2016
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EXAMPLE 6.4.
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4. TRANSISTOR BREAKDOWN
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4. TRANSISTOR BREAKDOWN
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TEMPERATURE EFFECT
Typical dependence of
β on IC and on
temperature in an
integrated-circuit npn
silicon transistor
intended for operation
around 1 mA. 13/3/2016
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HOMEWORK
Sedra/Smith, Microelectronic Circuits, 7e.
Chapter 6 problems:
6.7, 6.8, 6.13
6.28, 6.29, 6.32, 6.34, 6.51, 6.53, 6.54, 6.56, 6.61
Optional problems: 6.66, 6.69
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5. APPLYING THE BJT IN AMPLIFIER
DESIGN
§ An amplifier may be designed by transistor and
series resistance.
§ Appropriate biasing is important to ensure linear
gain, and appropriate input voltage swing.
§ Small-signal model is employed to model the amp’s
operation.
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5.1 DC BIAS POINT
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5.2 COLLECTOR CURRENT &
TRANSCONDUCTANCE
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TRANSCONDUCTANCE
§ The transconductance
IC
gm =
VT
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Electrical Engineering, “BJT: What exactly is a loadline?”, Web. 15 May 2014, 10 Jan 2015. <hHp://
electronics.stackexchange.com/ques&ons/110404/bjt-­‐what-­‐exactly-­‐is-­‐a-­‐load-­‐line> 13/3/2016
402058 – Chap 3: BJT
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Electrical Engineering, “BJT: What exactly is a loadline?”, Web. 15 May 2014, 10 Jan 2015. <hHp://
electronics.stackexchange.com/ques&ons/
110404/bjt-­‐what-­‐exactly-­‐is-­‐a-­‐load-­‐line> 13/3/2016
402058 – Chap 3: BJT
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6. MODELS FOR SMALL SIGNAL
OPERATION OF BJT
§ The Hybrid-π model
Voltage-­‐controlled current source 13/3/2016
Current-­‐controlled current source 402058 – Chap 3: BJT
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6. MODELS FOR SMALL SIGNAL
OPERATION OF BJT
§ The T model
Voltage-­‐controlled current source 13/3/2016
Current-­‐controlled current source 402058 – Chap 3: BJT
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7. CHARACTERIZING AMPLIFIERS
vin
Rin ≡
iin
vx
Ro ≡
ix
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Rin
vin =
vsig
Rin + Rsig
RL
vo =
Avo vi
RL + Ro
vi = 0
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7. CHARACTERIZING AMPLIFIERS
vo
Open circuit voltage gain: Avo ≡
RL=∞
vi
vo
RL
= Avo
Voltage gain of the amplifier: Av ≡
vi
RL + Ro
vo
Rin
RL
Overall voltage gain: Gv ≡
=
Avo
vsig Rin + Rsig
RL + Ro
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8. THREE-BASIC CONFIGURATIONS
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8.1. COMMON-EMITTER AMPLIFIER
signal course (vsig)
source resistance (Rsig)
input resistance (Rin)
gain (Avo)
output resistance (Ro)
transconductance (Gv)
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8.2. THE COMMON-BASE (CB)
AMPLIFIER
(a) CB amplifier with bias details omitted;
(b) Amplifier equivalent circuit with the BJT represented by its T Model.
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8.3. THE COMMON-COLLECTOR
(CC) AMPLIFIER
(a) CB amplifier with
bias details omitted;
(b) Amplifier
equivalent circuit with
the BJT represented
by its T Model.
(c) Simplified circuit
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EXAMPLE 7.5
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SUMMARY
IN THIS CHAPTER, YOU HAVE LEARNED:
§ the physical structure of the bipolar transistor and how it
works.
§ how to analyze and design circuits that contain bipolar
transistors.
§ how the transistor can be used to make an amplifier.
§ the three basic ways for connecting a BJT.
§ Practical circuits for bipolar-transistor amplifiers
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HOMEWORK
Sedra/Smith, Microelectronic Circuits, 7e.
Chapter 7 problems:
7.15, 7.16, 7.21
7.48, 7.50, 7.52, 7.53, 7.58
Prepare Chapter 6 & 7: sections related to FET
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