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Chapter 3:
Bipolar Junction Transistors

Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Transistor ConstructionTransistor Construction
There are two types of transistors:
• pnp
• npn
The terminals are labeled:
• E - Emitter
• B - Base
• C - Collector
pnppnp
npnnpn
22

Transistor OperationTransistor Operation
With the external sources, VEE and VCC, connected as shown:
• The emitter-base junction is forward biased
• The base-collector junction is reverse biased
33

Currents in a TransistorCurrents in a Transistor
The collector current is comprised of two
currents:
BICIEI +=
minority
COI
majority
CICI +=
Emitter current is the sum of the collector and
base currents:
44

Common-Base ConfigurationCommon-Base Configuration
The base is common to both input (emitter–base) and
output (collector–base) of the transistor.
55

Common-Base AmplifierCommon-Base Amplifier
Input CharacteristicsInput Characteristics
This curve shows the relationship
between of input current (IE) to input
voltage (VBE) for three output voltage
(VCB) levels.
66

This graph demonstrates
the output current (IC) to
an output voltage (VCB) for
various levels of input
current (IE).
Common-Base AmplifierCommon-Base Amplifier
Output CharacteristicsOutput Characteristics
77

Operating RegionsOperating 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.
88

E
I
C
I ≅
Silicon)(forV0.7BEV =
ApproximationsApproximations
Emitter and collector currents:
Base-emitter voltage:
99

Ideally: α = 1
In reality: α is between 0.9 and 0.998
Alpha (Alpha (αα))
Alpha (α) is the ratio of IC to IE :
EI
CI
α =dc
Alpha (α) in the AC modeAC mode:
EI
CI
α
Δ
Δ
ac =
1010

Transistor AmplificationTransistor Amplification
Voltage Gain:
V50kΩ5ma10
mA10
10mA
20Ω
200mV
===
=≅
≅
====
))((R
L
I
L
V
i
I
L
I
E
I
C
I
iR
iV
iIEI
Currents and Voltages:
1111
250
200mV
50V
===
iV
LV
vA

Common–Emitter ConfigurationCommon–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.
1212

Common-Emitter CharacteristicsCommon-Emitter Characteristics
Collector Characteristics Base Characteristics
1313

Common-Emitter Amplifier CurrentsCommon-Emitter Amplifier Currents
Ideal CurrentsIdeal Currents
IE = IC + IB IC = α IE
Actual CurrentsActual Currents
IC = α IE + ICBO
When IB = 0 µA the transistor is in cutoff, but there is some minority
current flowing called ICEO.
μA0=
−
= BI
CBO
CEO
α
I
I
1
where ICBO = minority collector current
1414
ICBO is usually so small that it can be ignored, except in high
power transistors and in high temperature environments.

Beta (Beta (ββ))
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
I
β =dc
constantac =
∆
∆
= CEV
B
C
I
I
β
1515

Determining β from a Graph
Beta (Beta (ββ))
108
A25
mA2.7
β 7.5VDC CE
=
µ
= =
100
μA10
mA1
μA)20μA(30
mA)2.2mA(3.2
β
7.5V
AC
CE
=
=
−
−
=
=
1616

Relationship between amplification factors β and α
1β
β
α
+
=
1α
α
β
−
=
Beta (Beta (ββ))
Relationship Between Currents
BC βII = BE 1)I(βI +=
1717

Common–Collector ConfigurationCommon–Collector Configuration
The input is on the
base and the output is
on the emitter.
1818

Common–Collector ConfigurationCommon–Collector Configuration
The characteristics are
similar to those of the
common-emitter
configuration, except the
vertical axis is IE.
1919

VCE is at maximum and IC is at
minimum (ICmax= ICEO) in the cutoff
region.
IC is at maximum and VCE is 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 ConfigurationOperating Limits for Each Configuration
2020

Power DissipationPower Dissipation
Common-collector:
CCBCmax IVP =
CCECmax IVP =
ECECmax IVP =
Common-base:
Common-emitter:
2121

Transistor Specification SheetTransistor Specification Sheet
2222

Transistor Specification SheetTransistor Specification Sheet
2323

Transistor TestingTransistor Testing
• Curve TracerCurve Tracer
Provides a graph of the characteristic curves.
• DMMDMM
Some DMMs measure βDC or hFE.
• OhmmeterOhmmeter
2424

Transistor Terminal IdentificationTransistor Terminal Identification
2525

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