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Copyright 2004 by Oxford University Press, Inc.
ELZ 303 - Elektronik II
Dr. Mehmet Siraç Özerdem
Elektrik Elektronik Mühendisliği Bölümü
Dicle Üniversitesi
Microelectronic Circuits – Fourth Edition
Adel S. Sedra, Kenneth C. Smith, 1998 Oxford University Press
Bipolar Junction
Transistors (BJTs)
Copyright 2004 by Oxford University Press, Inc.
Basic Single-Stage BJT Amplifier Configurations
Three basic configuration of BJT amplifiers
CE The Common - Emitter
CB The Common - Base
CC The Common - Collector
Dr. MS Ozerdem
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Copyright 2004 by Oxford University Press, Inc.
(a) A common-emitter amplifier
(b) Equivalent circuit obtained by replacing the transistor with its hybrid-p model.
The Common-Emitter Amplifier
Copyright 2004 by Oxford University Press, Inc.
A common-emitter amplifier with
an emitter resistance Re
The Common-Emitter Amplifier
with a Resistance in the Emitter
Equivalent circuit obtained by replacing the
transistor with its T model.
Dr. MS Ozerdem
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Copyright 2004 by Oxford University Press, Inc.
(a) A common-base amplifier
(b) Equivalent circuit obtained by replacing the transistor with its T model.
The Common-Base Amplifier
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(a) An emitter-follower circuit
(b) Small-signal equivalent circuit of the emitter
follower with the transistor replaced by its T model
augmented with ro.
(c) The circuit in (b) redrawn to emphasize that ro
is in parallel with RL. This simplifies the analysis
considerably.
The Common-Collector Amplifier
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Cutoff Region
Active Region
Saturation Region
The Transistor As a Switch-Cutoff and Saturation
These two extreme modes of operation are very useful if
the transistor is to be used as a switch.
Dr. MS Ozerdem
Copyright 2004 by Oxford University Press, Inc.
Cutoff Region
The Transistor As a Switch-Cutoff and Saturation
vi is smaller than about 0.5V
Since the CBJ is reverse-
biased (Vcc positive)
CBJ Reversed
EBJ Reversed
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Copyright 2004 by Oxford University Press, Inc.
Active Region
The Transistor As a Switch-Cutoff and Saturation
vi above 0.5V
Since the CBJ is reverse-
biased (Vcc positive)
CBJ Reversed
EBJ Forward
For appreciable current to flow,
vBE ≈ 0.7 and vi > 0.7
Dr. MS Ozerdem
Copyright 2004 by Oxford University Press, Inc.
Active Region
The Transistor As a Switch-Cutoff and Saturation
How do we know that the device is in the active mode ?
1. Assume that it is in active mode.
2. Calculate ic and vc=Vcc-Rcic
3. Check whether vCB ≥ 0 or not.
4. Merely check whether vC ≥0.7V or not
5. If vC ≥ 0.7V, then our assumption is correct.
If vC < 0.7V, then the device has left the active region and
entered the saturation region.
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Saturation Region
The Transistor As a Switch-Cutoff and Saturation
vi ↑ => iB ↑ => iC ↑ => vC ↓
CBJ Forward
EBJ Forward
If vC < vB The device will enter
the saturation region
Dr. MS Ozerdem
Copyright 2004 by Oxford University Press, Inc.
Saturation Region
The Transistor As a Switch-Cutoff and Saturation
Max. current that the collector “can take” without the transistor
leaving the active mode can be evaluated by setting vCB=0
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Saturation Region
The Transistor As a Switch-Cutoff and Saturation
Since in saturation the vB > vC by about 0.4 to 0.6V,
it follows that vC > vE by about 0.3 to 0.1V.
VCEsat ≈ 0.2V
ICsat is constant in saturation region
npn
Edge Of Saturation
Normally IB is higher than IB(EOS) by factor of 2 to 10
(called the overdrive factor)
Dr. MS Ozerdem
Copyright 2004 by Oxford University Press, Inc.Microelectronic Circuits - Fifth Edition Sedra/Smith
Example
The minimum value of β
should be used in testing for
saturation.
β is specified to be at least 50
VCEsat = 0.2V
8
Copyright 2004 by Oxford University Press, Inc.Microelectronic Circuits - Fifth Edition Sedra/Smith
ExampleFind the value of RB that results
in saturation with an overdrive
factor of at least 10.
50 ≤ β ≤ 150
VCEsat = 0.2V
Dr. MS Ozerdem
Copyright 2004 by Oxford University Press, Inc.Microelectronic Circuits - Fifth Edition Sedra/Smith
Example βmin = 30
VECsat = 0.2V
Testing of the device
in saturation region.
Dr. MS Ozerdem
9
Copyright 2004 by Oxford University Press, Inc.
A General Large – Signal Model for BJT
The Ebers-Moll (EM) Model
The transport form of the Ebers-Moll model for an npn BJT.
ISE, ISC saturation or scale currents.
Dr. MS Ozerdem
Copyright 2004 by Oxford University Press, Inc.
A General Large – Signal Model for BJT
The Ebers-Moll (EM) Model
Since the CBJ is usually of large area than the EBJ,
ISC is usually larger than ISE (by a factor of 2 to 50).
IS is proportional to the area
of the emitter-base junction
10
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A General Large – Signal Model for BJT
The Ebers-Moll (EM) Model
The transistor terminal currents
{
Dr. MS Ozerdem
Copyright 2004 by Oxford University Press, Inc.
A General Large – Signal Model for BJT
The Ebers-Moll (EM) Model
EM Model – The Forward Active Mode
EM Model – The Saturation Mode
EM Model – The Inverse Mode
Dr. MS Ozerdem
11
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A General Large – Signal Model for BJT
The Ebers-Moll (EM) Model
EM Model – The Forward Active Mode
vBE > 0 Reverse
vBC < 0 Forward
The 2.term can be neglected.
Dr. MS Ozerdem
Copyright 2004 by Oxford University Press, Inc.
A General Large – Signal Model for BJT
The Ebers-Moll (EM) Model
EM Model – The Saturation Mode
vBE > 0 Forward
vBC > 0 Forward
Assume that a current IB is pushed into
the base and that its value is sufficient to
drive the transistor into saturation. Thus
Dr. MS Ozerdem
12
Copyright 2004 by Oxford University Press, Inc.
A General Large – Signal Model for BJT
The Ebers-Moll (EM) Model
EM Model – The Saturation Mode
Copyright 2004 by Oxford University Press, Inc.
A General Large – Signal Model for BJT
The Ebers-Moll (EM) Model
EM Model – The Inverse Mode
Collector and emitter interchanged. I1, I2 and IB are positive.
Since the roles of E and C are interchanged, the transistor
will operate in the active mode (reverse active mode).
When EBJ is reverse biased.
The transistor will saturate when the EBJ becomes
forward biased. In this case
Dr. MS Ozerdem
13
Copyright 2004 by Oxford University Press, Inc.
A General Large – Signal Model for BJT
The Ebers-Moll (EM) Model
EM Model – The Inverse Mode
We can use the EM equations to find an expression for VECsat
}
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The Basic BJT Logic Inverter
Basic BJT digital logic inverter.
The Voltage Transfer
Characteristic
Dr. MS Ozerdem
14
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Complete Static Characteristics, internal Capacitance, and
Second – Order Effects
Common – Base Characteristics
Including the rµ, which models
the effect of vc on ib
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Complete Static Characteristics, internal Capacitance, and
Second – Order Effects
Common – Emitter Characteristics
As a result of the slope in the active region is different from 1/ro; in fact,
the slope here is greater.
Dr. MS Ozerdem
15
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Complete Static Characteristics, internal Capacitance, and
Second – Order Effects
The Transistor β
β is the forward h parameter in CE configuration
When the transistor is used as
an amplifier, it is first biased at
a point such as Q. Applied
signals then cause incremental
changes in iB, iC and vCE around
the Q point. We may therefore
define an ac β.
Copyright 2004 by Oxford University Press, Inc.
Complete Static Characteristics, internal Capacitance, and
Second – Order Effects
Transistor Breakdown
The maximum voltages that can be applied to a BJT are limited by EBJ and CBJ
breakdown effects that follow the avalanche multiplication mechanism.
CBJ breaks down at a voltage
donoted by BVCBO.
For iE = 0, BVCBO ≈ 50V
For iE > 0, BVCBO < 50V
For CB Configuration
Dr. MS Ozerdem
16
Copyright 2004 by Oxford University Press, Inc.
Complete Static Characteristics, internal Capacitance, and
Second – Order Effects
Transistor Breakdown
Breakdown occuring at a
voltage BVCEO
BVCEO ≈ BVCBO / 2
For CE Configuration
Copyright 2004 by Oxford University Press, Inc.
Complete Static Characteristics, internal Capacitance, and
Second – Order Effects
Transistor Breakdown
Example
BVBCO = 70V
Vo = ?
Dr. MS Ozerdem
17
Copyright 2004 by Oxford University Press, Inc.
Complete Static Characteristics, internal Capacitance, and
Second – Order Effects
Internal Capacitances of the BJT
The Base Charging or Diffusion Capacitance Cde
When the transistor is operating in the active or saturation modes, minority –
carrier charge in the base region.
For npn Qn charge was written
For large signals, iC is exponentially
related to vBE, Qn will similarly depend
on vBE
Thus this charge-storage mechanism represents a nonlinear capacitive effect.
Small signal diffusion capacitance
Copyright 2004 by Oxford University Press, Inc.
Complete Static Characteristics, internal Capacitance, and
Second – Order Effects
Internal Capacitances of the BJT
The BEJ Capacitance - Cje
The BEJ or depletion – layer capacitance Cje
Cjeo : The value Cje at zero voltage
m : Grading coefficient (0.5)
Voe : EBJ built-in voltage (0.9V)
Alternative term
Dr. MS Ozerdem
18
Copyright 2004 by Oxford University Press, Inc.
Complete Static Characteristics, internal Capacitance, and
Second – Order Effects
Internal Capacitances of the BJT
The CBJ Capacitance - Cµ
In active mode operation, CBJ is reverse-biased, and its depletion
capacitance
Cµo : The value Cµ at zero voltage
m : Grading coefficient (0.2-0.5)
Voc : CBJ built-in voltage (0.75V)
Copyright 2004 by Oxford University Press, Inc.
Complete Static Characteristics, internal Capacitance, and
Second – Order Effects
The High-Frequency Hybrid – π Model
EB capacitance Cπ = Cde + Cje
CB capacitance Cµ
Dr. MS Ozerdem
19
Copyright 2004 by Oxford University Press, Inc.
Complete Static Characteristics, internal Capacitance, and
Second – Order Effects
The Cutoff Frequency
In order to determine Cπ and Cµ we shall drive an expression for hfe as a
function of frequency in terms of the hybrid components.
}
Copyright 2004 by Oxford University Press, Inc.
Complete Static Characteristics, internal Capacitance, and
Second – Order Effects
The Cutoff Frequency
ǀhfeǀ drops to unity, which is called unity-
gain bandwidth
ǀhfeǀ drops to unity, which is called unity-
gain bandwidth
Dr. MS Ozerdem
20
Copyright 2004 by Oxford University Press, Inc.
Complete Static Characteristics, internal Capacitance, and
Second – Order Effects
Example
ǀhfeǀ drops to unity, which is called unity-
gain bandwidth
IC =1mA
CBJ reverse bias of 2V
The device has τF=20ps
Cjeo=20fF
Cµ=20fF
Voe=0.9V
Voc=0.5V
mCBJ=0.33
Find
Cde
Cje
Cπ
Cµ
fT
Dr. MS Ozerdem
