Chapter 3 & 4:
Bipolar Junction Transistors and Applications© Modified by Yuttapong Jiraraksopakun
ENE, KMUTT 2009
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Transistor Construction
There are two types of transistors:
• pnp
• npn
The terminals are labeled:
• E - Emitter
• B - Base
• C - Collector
pnp
npn
2
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Transistor OperationWith the external sources, V
EEand V
CC, connected as shown:
• The emitter-base junction is forward biased
• The base-collector junction is reverse biased
3
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Currents in a Transistor
The collector current is comprised of two
currents:
BI
CI
EI +=
minorityCOI
majorityCI
CI +=
Emitter current is the sum of the collector and
base currents:
4
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Common-Base Configuration
The base is common to both input (emitter–base) and
output (collector–base) of the transistor.
5
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Common-Base Amplifier
Input Characteristics
This curve shows the relationship
between of input current (IE) to input
voltage (VBE) for three output voltage
(VCB) levels.
6
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
This graph demonstrates
the output current (IC) to
an output voltage (VCB) for
various levels of input
current (IE).
Common-Base Amplifier
Output Characteristics
7
CBOI
COI =
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Operating Regions
• Active – Operating range of the
amplifier.
• Cutoff – The amplifier is basically
off. There is voltage, but little
current.
• Saturation – The amplifier is full on.
There is current, but little voltage.
8
Regions Base-Emitter Collector-Base
Active Forward-biased Reverse-biased
Cutoff Reverse-biased Reverse-biased
Saturation Forward-biased Forward-biased
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
EI
CI ≅
Silicon)(for V 0.7BEV =
Approximations
Emitter and collector currents:
Base-emitter voltage:
9
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Ideally: α = 1
In reality: α is between 0.9 and 0.998
Alpha (α)
Alpha (α) is the ratio of ICto I
E:
EI
CI
α =
dc
Alpha (α) in the AC mode:
EI
CI
α
∆
∆
ac =
10
constant=
CBV
CBOI
EαI
CI +=
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Transistor Amplification
Voltage Gain:
V 50kΩ 5ma 10
mA 10
10mA
20Ω
200mV
===
=≅
≅
====
))((RL
IL
V
iI
LI
EI
CI
iR
iV
iI
EI
Currents and Voltages:
11
250
200mV
50V===
iV
LV
vA
Omit DC biasing to demonstrate AC response
Assume Riand R
ofrom input & output
characteristic curves
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Common–Emitter Configuration
The emitter is common to both input
(base-emitter) and output (collector-
emitter).
The input is on the base and the
output is on the collector.
12
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Common-Emitter Characteristics
Collector Characteristics Base Characteristics
13
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Common-Emitter Amplifier Currents
Ideal Currents
IE= I
C+ I
BIC= α I
E
Actual Currents
IC= α I
E+ I
CBO
When IB= 0 µA the transistor is in cutoff, but there is some minority
current flowing called ICEO
.
µA 0=−
=BI
CBO
CEO
α
II
1
where ICBO
= minority collector current
14
ICBO is usually so small that it can be ignored, except in high
power transistors and in high temperature environments.
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Beta (β)
In DC mode:
In AC mode:
β represents the amplification factor of a transistor. (β is
sometimes referred to as hfe, a term used in transistor modeling
calculations)
B
C
I
Iβ =
dc
constantac =
∆
∆=
CEV
B
C
I
Iβ
15
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Determining β from a Graph
Beta (β)
108
A 25
mA 2.7β 7.5VDC CE
=
µ=
=
100
µA 10
mA 1
µA) 20 µA (30
mA) 2.2mA (3.2β
7.5V
AC
CE
=
=
−
−
=
=
16
Both β values are usually reasonably close and are often used interchangeably
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Relationship between amplification factors β and α
1β
βα
+
=
1α
αβ
−
=
Beta (β)
Relationship Between Currents
BC βII =BE 1)I(βI +=
17
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Common–Collector Configuration
The input is on the
base and the output is
on the emitter.
18
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Common–Collector Configuration
The characteristics are
similar to those of the
common-emitter
configuration, except the
vertical axis is IE.
19
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
VCEis at maximum and I
Cis at
minimum (ICmax
= ICEO
) in the cutoff
region.
ICis at maximum and V
CEis at
minimum (VCE max
= VCEsat
= VCEO
) in
the saturation region.
The transistor operates in the active
region between saturation and cutoff.
Operating Limits for Each Configuration
20
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Power Dissipation
Common-collector:
CCBCmax IVP =
CCECmax IVP =
ECECmax IVP =
Common-base:
Common-emitter:
21
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Transistor Specification Sheet
22
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Transistor Specification Sheet
23
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Transistor Terminal Identification
24
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Biasing
Biasing: The DC voltages applied to a transistor in
order to turn it on so that it can amplify the AC signal.
( )
BC
CBE
BE
II
II1I
V7.0V
β
β
=
≅+=
=
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Operating Point
The DC input
establishes an
operating or
quiescent point
called the Q-point.
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
The Three States of Operation
• Active or Linear Region Operation
Base–Emitter junction is forward biased
Base–Collector junction is reverse biased
• Cutoff Region Operation
Base–Emitter junction is reverse biased
• Saturation Region Operation
Base–Emitter junction is forward biased
Base–Collector junction is forward biased
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
DC Biasing Circuits
• Fixed-bias circuit
• Emitter-stabilized bias circuit
• Collector-emitter loop
• Voltage divider bias circuit
• DC bias with voltage feedback
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Fixed Bias
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
The Base-Emitter Loop
From Kirchhoff’s voltage
law:
Solving for base current:
+VCC– I
BR
B– V
BE= 0
B
BECCB
R
VVI
−
=
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Collector-Emitter Loop
Collector current:
From Kirchhoff’s voltage law:
BIICβ=
CCCCCE RIVV −=
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Saturation
When the transistor is operating in saturation, current
through the transistor is at its maximum possible value.
CR
CCV
CsatI =
V 0CEV ≅
This approximation is equivalent to move the region below
VCEsat
of the output curves to align on the output current
axis.
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Load Line Analysis
ICsat
IC= V
CC/ R
C
VCE= 0 V
VCEcutoff
VCE= V
CC
IC= 0 mA
• where the value of RBsets the value of
IB
• that sets the values of VCE
and IC
The Q-point is the operating point:
The end points of the load line are:
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Circuit Values Affect the Q-Point
more …
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Circuit Values Affect the Q-Point
more …
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Circuit Values Affect the Q-Point
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Emitter-Stabilized Bias Circuit
Adding a resistor
(RE) to the emitter
circuit stabilizes
the bias circuit.
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Base-Emitter Loop
From Kirchhoff’s voltage law:
This image cannot currently be displayed.
0R1)I(β-V-RI-VEBBEBBCC=+
0 RI-V-RI-VEEBEBBCC=+
EB
BECCB
1)R(R
V-VI
+β+=
Since IE= (β + 1)I
B:
Solving for IB:
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Collector-Emitter Loop
From Kirchhoff’s voltage law:
0 CC
VC
RC
I CE
V E
RE
I =−++
Since IE≅ I
C:
)R (RI– V V ECCCCCE +=
Also:
This image cannot currently be displayed.
EBEBRCCB
CCCCECEC
EEE
V V RI– V V
RI - V V V V
RI V
+==
=+=
=
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Improved Biased Stability
Stability refers to a circuit condition in which the currents and voltages
will remain fairly constant over a wide range of temperatures and
transistor Beta (β) values.
Adding RE to the emitter improves the stability of a transistor.
EB
BECCB
1)R(R
V-VI
+β+=
B
BECCB
R
VVI
−
=
BIICβ=
Fixed-bias circuit Emitter-stabilized bias circuit
IBin fixed-bias circuit cannot change, so change in β results in large
change in output current and voltage.
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Saturation Level
VCEcutoff
: ICsat:
The endpoints can be determined from the load line.
mA 0 I
V V
C
CCCE
=
=
ERCR
CCV
CI
CE V 0V
+
=
=
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Voltage Divider Bias
This is a very stable
bias circuit.
The currents and
voltages are nearly
independent of any
variations in β.
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Approximate Analysis
Where IB<< I
1and I
1≅ I
2 :
Where βRE> 10R
2:
From Kirchhoff’s voltage law:
21
CC2B
RR
VRV
+
=
E
EE
R
VI =
BEBE VVV −=
EECCCCCERI RI V V −−=
)R (RIV V
II
ECCCCCE
CE
+−=
≅
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Voltage Divider Bias Analysis
Transistor Saturation Level
EC
CCCmaxCsat
RR
VII
+
==
Load Line Analysis
Cutoff: Saturation:
mA0I
VV
C
CCCE
=
=
V0VCE
ER
CR
CCV
CI
=
+
=
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Voltage Divider Bias
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Voltage Divider Bias (Exact)
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
DC Bias with Voltage Feedback
Another way to
improve the stability
of a bias circuit is to
add a feedback path
from collector to
base.
In this bias circuit
the Q-point is only
slightly dependent on
the transistor beta, β.
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Base-Emitter Loop
)R(RR
VVI
ECB
BECCB
+β+
−=
From Kirchhoff’s voltage law:
0RI–V–RI–RI– V EEBEBBCCCC =′
Where IB<< I
C:
CI
BI
CI
CI' ≅+=
Knowing IC= βI
Band I
E≅ I
C, the loop
equation becomes:
0RIVRIRI– V EBBEBBCBCC =β−−−β
Solving for IB:
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Collector-Emitter Loop
Applying Kirchoff’s voltage law:
IE+ V
CE+ I’
CR
C– V
CC= 0
Since I′C≅ I
Cand I
C= βI
B:
IC(R
C+ R
E)+ V
CE– V
CC=0
Solving for VCE:
VCE= V
CC– I
C(R
C+ R
E)
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Base-Emitter Bias Analysis
Transistor Saturation Level
EC
CCCmaxCsat
RR
VII
+
==
Load Line Analysis
Cutoff: Saturation:
mA 0I
VV
C
CCCE
=
=
V 0VCE
ER
CR
CCV
CI
=
+
=
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Transistor Switching Networks
Transistors with only the DC source applied can be used
as electronic switches.
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Switching Circuit Calculations
C
CCCsat
R
VI =
dc
CsatB
II
β>
Csat
CEsatsat
I
VR =
CEO
CCcutoff
I
VR =
Saturation current:
To ensure saturation:
Emitter-collector resistance
at saturation and cutoff:
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Switching Time
Transistor switching times:
dron ttt +=
fsoff ttt +=
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Troubleshooting Hints
• Approximate voltages
– VBE
≅ .7 V for silicon transistors
– VCE
≅ 25% to 75% of VCC
• Test for opens and shorts with an ohmmeter.
• Test the solder joints.
• Test the transistor with a transistor tester or a curve tracer.
• Note that the load or the next stage affects the transistor operation.
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
PNP Transistors
The analysis for pnp transistor biasing circuits is the same
as that for npn transistor circuits. The only difference is that
the currents are flowing in the opposite direction.
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Homework 3 (Chapter 3)
• Common-Base Configuration
– 3.4 (13)
• Transistor Amplifying Action
– 3.5 (18)
• Common-Emitter Configuration
– 3.6 (23)
56
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Homework 3 (Chapter 4)
• Fixed-Bias Configuration
– 4.3 (1)
• Emitter-Bias Configuration
– 4.4 (8)
• Voltage Divider Configuration
– 4.5 (13)
• Collector-Feedback Configuration
– 4.6 (23)
• Miscellaneous Bias Configuration
– 4.59 (30)
57