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1
Chapter 4 Physics of Bipolar Transistors
4.1 General Considerations
4.2 Structure of Bipolar Transistor
4.3 Operation of Bipolar Transistor inActive Mode
4.4 Bipolar Transistor Models
4.5 Operation of Bipolar Transistor inSaturation Mode
4.6 The PNP Transistor
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CH4 Physics of Bipolar Transistors 2
Bipolar Junction Transistor (BJT)
Bipolar Junction Transistor invented in 1945 (Bell Lab.)
Based on pn junction theory in Ch.2, study the physics ofBJT, derive I-V characterist ics and develop large and smallsignal equivalent models.
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CH4 Physics of Bipolar Transistors 3
General Considerations: Voltage-Dependent
Current Source
A voltage-dependent current source can act as an amplifier.
If KRL is greater than 1, then the signal is amplified.
L
in
outV KR
V
VA ==
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CH4 Physics of Bipolar Transistors 4
Voltage-Dependent Current Source with Input
Resistance
Regardless of the input resistance, the magnitude ofamplification remains unchanged.
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CH4 Physics of Bipolar Transistors 5
Exponential Voltage-Dependent Current Source
A three-terminal exponential voltage-dependent currentsource is shown above.
Ideally, bipolar transistor can be modeled as such.
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CH4 Physics of Bipolar Transistors 6
Structure and Symbol of Bipolar Transistor
Bipolar (junction) transistor can be thought of as asandwich of three doped Si regions. The outer two regionsare doped with the same polarity, while the middle region isdoped with opposite polarity.
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CH4 Physics of Bipolar Transistors 7
Injection of Carriers
Reverse biased PN junction creates a large electr ic f ieldthat sweeps any injected minority carriers to their majori tyregion.
This abili ty proves essential in the proper operation of abipolar transistor.
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CH4 Physics of Bipolar Transistors 8
Forward Active Region
Forward active region: VBE > 0, VBC < 0.
Figure b) presents a wrong way of modeling f igure a).
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CH4 Physics of Bipolar Transistors 9
Accurate Bipolar Representation
Collector also carries current due to carrier injection from
base.
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CH4 Physics of Bipolar Transistors 10
Carrier Transport in Base
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CH4 Physics of Bipolar Transistors 11
Collector Current
Applying the law of diffusion, we can determine the chargeflow across the base region into the collector.
The equation above shows that the transistor is indeed avoltage-controlled element, thus a good candidate as anamplifier.
BE
inES
T
BESC
T
BE
BE
inEC
WN
nqDAI
V
VII
VV
WNnqDAI
2
2
exp
1exp
=
=
=
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CH4 Physics of Bipolar Transistors 12
Parallel Combination of Transistors
When two transistors are put in parallel and experience thesame potential across all three terminals, they can bethought of as a single transistor with twice the emitter area.
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CH4 Physics of Bipolar Transistors 13
Simple Transistor Configuration
Although a transistor is a voltage to current converter,output voltage can be obtained by inserting a load resistorat the output and allowing the controlled current to passthru it.
Vout= 3 - ICRL
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CH4 Physics of Bipolar Transistors 14
Constant Current Source
Ideally, the collector current does not depend on thecollector to emitter voltage. This property allows thetransistor to behave as a constant current source when itsbase-emitter voltage is fixed.
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CH4 Physics of Bipolar Transistors 15
Base Current
Base current consists of two components: 1) Reverse
injection of holes into the emitter and 2) recombination of
holes with electrons coming from the emitter.
BC II =
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CH4 Physics of Bipolar Transistors 16
Emitter Current
Applying Kirchoffs current law to the transistor, we can
easily f ind the emitter current.
B
C
CE
BCE
I
I
II
III
=
+=
+=
11
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CH4 Physics of Bipolar Transistors 17
Summary of Currents
=
+
+=
=
=
1
exp1
exp
1
exp
T
BE
SE
T
BE
SB
T
BE
SC
V
VII
V
VII
V
VII
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CH4 Physics of Bipolar Transistors 18
Bipolar Transistor Large Signal Model
A diode is placed between base and emitter and a voltage
controlled current source is placed between the col lector
and emitter.
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CH4 Physics of Bipolar Transistors 19
Example: Maximum RL
IC
= 1.153 mA and VBE
=0.8 V
As RL increases, Vx drops and eventually forward biases thecollector-base junction. This wil l force the transistor out offorward active region.
Therefore, there exists a maximum tolerable collectorresistance.
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CH4 Physics of Bipolar Transistors 20
I/V Characteristics of Bipolar Transistor
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CH4 Physics of Bipolar Transistors 21
Example: IV Characteristics
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CH4 Physics of Bipolar Transistors 22
Transconductance
A voltage-dependent current source (in the act ive-mode) Transconductance, gm shows a measure of how well the
transistor converts voltage to current.
It wi ll later be shown that gm is one of the most importantparameters in circui t design.
T
Cm
T
BES
T
m
T
BES
BE
m
V
Ig
V
VI
Vg
VVI
dVdg
=
=
=
exp1
exp
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CH4 Physics of Bipolar Transistors 23
Visualization of Transconductance
gm can be visualized as the slope of IC versus VBE.
A large IC has a large slope and therefore a large gm.
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CH4 Physics of Bipolar Transistors 24
Transconductance and Area
When the area of a transistor is increased by n, IS increases
by n. For a constant VBE, IC and hence gm increases by a
factor of n.
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CH4 Physics of Bipolar Transistors 25
Transconductance and Ic
The figure above shows that for a given VBE swing, thecurrent excursion around IC2 is larger than it would bearound IC1. This is because gm is larger IC2.
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CH4 Physics of Bipolar Transistors 26
Small-Signal Model: Derivation
Small signal model is derived by perturbing voltagedifference every two terminals while fixing the third terminal
and analyzing the change in current of all three terminals.
We then represent these changes with control led sources
or resistors.
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CH4 Physics of Bipolar Transistors 27
Small-Signal Model: VBE Change
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CH4 Physics of Bipolar Transistors 28
Small-Signal Model: VCE Change
Ideally, VCE has no effect on the collector current. Thus, it
will not contribute to the small signal model.
It can be shown that VCB has no effect on the small signal
model, either.
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CH4 Physics of Bipolar Transistors 29
Small Signal Example I
Here, small signal parameters are calculated from DC
operating point and are used to calculate the change in
collector current due to a change in VBE.
==
==
375
75.3
1
m
T
Cm
g
r
V
Ig
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CH4 Physics of Bipolar Transistors 30
Small Signal Example II
In this example, a resistor is placed between the powersupply and collector, therefore, providing an output voltage.
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CH4 Physics of Bipolar Transistors 31
AC Ground
Since the power supply voltage does not vary with
time, it is regarded as a ground in small-signal
analysis.
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CH4 Physics of Bipolar Transistors 32
Early Effect
The claim that collector current does not depend on VCE isnot accurate.
As VCE increases, the depletion region between base andcollector increases. Therefore, the effective base widthdecreases, which leads to an increase in the collectorcurrent.
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CH4 Physics of Bipolar Transistors 33
Early Effect Illustration
With Early effect, collector current becomes larger than
usual and a function of VCE.
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CH4 Physics of Bipolar Transistors 34
Early Effect Representation
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CH4 Physics of Bipolar Transistors 35
Early Effect and Large-Signal Model
Early effect can be accounted for in large-signal model by
simply changing the collector current with a correction
factor.
In this mode, base current does not change.
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CH4 Physics of Bipolar Transistors 36
Early Effect and Small-Signal Model
C
A
T
BE
S
A
C
CE
o
I
V
V
VI
V
I
Vr =
=
exp
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CH4 Physics of Bipolar Transistors 37
Summary of Ideas
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CH4 Physics of Bipolar Transistors 38
Bipolar Transistor in Saturation
When collector voltage drops below base voltage and
forward biases the collector-base junction, base current
increases and decreases the current gain factor, .
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CH4 Physics of Bipolar Transistors 39
Large-Signal Model for Saturation Region
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CH4 Physics of Bipolar Transistors 40
Overall I/V Characteristics
The speed of the BJT also drops in saturation.
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CH4 Physics of Bipolar Transistors 41
Example: Acceptable VCC Region
In order to keep BJT at least in soft saturation region, thecollector voltage must not fall below the base voltage by
more than 400mV.
A linear relationship can be derived for VCC and RC and an
acceptable region can be chosen.
)400( mVVRI BECCCC +
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CH4 Physics of Bipolar Transistors 42
Deep Saturation
In deep saturation region, the transistor loses its vol tage-
controlled current capability and VCE becomes constant.
VCE,sat = 0.2 V
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CH4 Physics of Bipolar Transistors 43
PNP Transistor
With the polarities of emitter, collector, and base reversed,a PNP transistor is formed.
All the pr inciples that applied to NPN's also apply to PNPs,with the exception that emitter is at a higher potential thanbase and base at a higher potential than collector.
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CH4 Physics of Bipolar Transistors 44
A Comparison between NPN and PNP Transistors
The figure above summarizes the direction of current flowand operation regions for both the NPN and PNP BJTs.
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CH4 Physics of Bipolar Transistors 45
PNP Equations
+
=
+=
=
=
A
EC
T
EB
SC
T
EBSE
T
EBSB
T
EBSC
V
V
V
VII
V
VII
V
VII
V
VII
1exp
exp1
exp
exp
Early Effect
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CH4 Physics of Bipolar Transistors 46
Large Signal Model for PNP
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CH4 Physics of Bipolar Transistors 47
PNP Biasing
Note that the emitter is at a higher potential than both the
base and collector.
PNP ~ 50 (
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CH4 Physics of Bipolar Transistors 48
Small Signal Analysis
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CH4 Physics of Bipolar Transistors 49
Small-Signal Model for PNP Transistor
The small signal model for PNP transistor is exactly
IDENTICAL to that of NPN. This is not a mistake because
the current direction is taken care of by the polarity of VBE.
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CH4 Physics of Bipolar Transistors 50
Small-Signal Model for Diode-Connected Transistor
Fig. 4.44 NPN and PNP Diode-Connected Transitors
Small-signal impedance of ~ 1/gm
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CH4 Physics of Bipolar Transistors 51
Small Signal Model Example I
S S
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CH4 Physics of Bipolar Transistors 52
Small Signal Model Example II
Small-signal model is identical to the previous ones.
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CH4 Physics of Bipolar Transistors 53
Small Signal Model Example III
Since during small-signal analysis, a constant voltage
supply is considered to be AC ground, the final small-signal
model is identical to the previous two.
S ll Si l M d l E l IV
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Small Signal Model Example IV
