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5-1
Electronics
Principles & Applications
Eighth Edition
Charles A. Schuler
Chapter 5
Transistors
McGraw-Hill
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5-2
INTRODUCTION
• Amplification
• Transistors
• Characteristic Curves
• Transistor Testing
• Other Transistor Types
• Transistors as Switches
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5-3
In
Gain =
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Amplifier
Out
Out
In
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5-4
NPN Transistor Structure
The collector is lightly doped.
N
C
The base is thin and
is lightly doped.
P
B
The emitter is heavily doped.
N
E
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5-5
NPN Transistor Bias
No current flows.
The C-B junction
is reverse biased.
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N
C
P
B
N
E
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5-6
NPN Transistor Bias
The B-E junction
is forward biased.
N
C
P
B
N
E
Current flows.
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5-7
NPN Transistor Bias
IC
Current flows
everywhere.
Most of the emitter carriers
diffuse through the thin base
When
both
junctions
region since they are attracted
biased....
by are
the collector.
Note that IB is smaller
than IE or IC.
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N
C
P
B
N
E
IB
IE
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5-8
Note: when the
switch opens, all
currents go to zero.
Although IB is smaller
it controls IE and IC.
Gain is something small
controlling something large
(IB is small).
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IC
N
C
P
B
N
E
IB
IE
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5-9
Transistor structure and bias quiz
The heaviest doping is found in the
___________ region.
emitter
The thinnest of all three regions is called
the ____________.
base
The collector-base junction is ___________
biased.
reverse
The base-emitter junction is ____________
biased.
forward
The majority of the emitter carriers flow to
the ___________.
collector
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5-10
IC = 99 mA
The current gain from
base to collector
is called b.
IB = 1 mA
b =
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99
ICmA
1IBmA
= 99
IE = 100 mA
C
P
B
N
E
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5-11
IC = 99 mA
Kirchhoff’s
current law:
IB = 1 mA
C
P
B
N
E
IE = I B + I C
= 1 mA + 99 mA
= 100 mA
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IE = 100 mA
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IC = 99 mA
In a PNP transistor,
holes flow from
emitter to collector.
IB = 1 mA
Notice the PNP
bias voltages.
5-12
C
B
E
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IE = 100 mA
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5-13
Transistor currents quiz
b is the ratio of collector current to ______
current.
base
The sum of the base and collector currents
is the __________ current.
emitter
In NPN transistors, the flow from emitter to
collector is composed of _______. electrons
In PNP transistors, the flow from emitter to
collector is composed of _______. holes
Both NPN and PNP transistors show
__________ gain.
current
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5-14
NPN schematic symbol
Collector
Base
C
B E
Emitter
Memory aid: NPN
means Not Pointing iN.
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5-15
PNP schematic symbol
Collector
Base
C
B E
Emitter
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IC
This circuit is used to
collect IC versus
VCE data for
several values of IB.
C
IB
5-16
VCE
B
E
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5-17
When graphed, the data provide an
NPN collector family of curves.
100 mA
14
12
10
IC in mA 8
6
4
2
80 mA
60 mA
40 mA
20 mA
0 2 4 6
8 10 12 14 16 18
0 mA
VCE in Volts
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5-18
100 mA
14
12
10
IC in mA 8
6
4
2
80 mA
60 mA
40 mA
20 mA
0 2 4 6
8 10 12 14 16 18
0 mA
VCE in Volts
b =
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14
mA
6ICmA
40
IB mA
100
mA
150
== 140
This type of gain
is called bdc or hFE.
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5-19
100 mA
14
12
10
IC in mA 8
6
4
2
80 mA
60 mA
40 mA
20 mA
0 2 4 6
bac =
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2.5DImA
C
20DImA
B
8 10 12 14 16 18
0 mA
VCE in Volts
= 125
Another type of gain
is called bac or hfe.
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5-20
100 mA
14
12
10
IC in mA 8
6
4
2
80 mA
60 mA
40 mA
20 mA
0 2 4 6
IB
C
8 10 12 14 16 18
VCE in Volts
0 mA
With these values of IB:
The C-E model is a resistor.
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E
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5-21
100 mA
14
12
10
IC in mA 8
6
4
2
0 2 4 6
IB
80 mA
60 mA
40 mA
20 mA
8 10 12 14 16 18
VCE in Volts
0 mA
When IB >> 100 mA
VCE @ 0
The model is a closed switch.
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5-22
100 mA
14
12
10
IC in mA 8
6
4
2
0 2 4 6
IB
80 mA
60 mA
40 mA
20 mA
8 10 12 14 16 18
VCE in Volts
0 mA
When IB = 0
IC = 0
The model is an open switch.
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5-23
Transistor operating conditions quiz
When IB is large and VCE @ 0, the transistor
acts as a ___________ switch. closed
When IB = 0 and IC = 0, the transistor
acts as an ___________ switch. open
When IB > 0 and VCE > 0, the transistor
acts as a ___________.
resistor
Two current gain measures are bdc and
__________.
bac
The symbol hfe is the same as _________.
bac
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5-24
NPN
V
mA
E
B
C
The E-B junction is forward biased by the ohmmeter.
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5-25
NPN
V
mA
E
B
C
The C-E resistance is very high.
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5-26
NPN
V
mA
E
B
C
100 kW
The meter reading is < 100 kW due to gain.
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5-27
Current In
Current
Amplifier
Current Out
The BJT is
current
controlled.
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5-28
Transistors
BJTs
FETs
JFETs
MOSFETs
PNP
NPN
B
P-Channel
N-Channel
P-Channel
N-Channel
D
D
D
D
D
G
B
E
N-Channel
C
C
E
Enhancement Mode
Depletion Mode
Depletion Mode
G
S
G
G
S
S
D
G
G
S
P-Channel
S
S
BJTs are current controlled and FETs are voltage controlled.
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5-29
Voltage In
Voltage
Amplifier
Current Out
The JFET is
a voltage
controlled
amplifier.
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5-30
Source
Gate
Drain
P
N-channel
P-type substrate
Structure of an
N-channel JFET
The channel has carriers so it
conducts from source to drain.
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Drain
Gate
Source
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5-31
Source
Gate
Drain
P
N-channel
P-type substrate
A negative gate voltage
can push the carriers from
the channel and turn
the JFET off.
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Drain
Gate
Source
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5-32
0V
-1 V
-2 V
ID in mA
-3 V
VGS
-4 V
0
-5 V
VDS in Volts
This is known as a depletion-mode device.
N-channel JFET drain family of characteristic curves
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It’s possible to make enhancement
type field effect transistors as well.
Metal
oxide
insulator
5-33
Drain
n
Gate
VGG
D
VDD
p
G
n
S
N-channel
MOSFET
Source
Gate bias enhances the channel and turns the device on.
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5-34
5V
4V
3V
ID in mA
2V
VGS
1V
0
0V
Drain
VDS in Volts
Enhancement mode MOSFET drain
family of characteristic curves Gate
Source
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5-35
The IGBT (insulated gate bipolar transistor)
Operation and structure
similar to a MOSFET
Voltage controlled
(like the MOSFET)
Has one more junction
than a MOSFET
Hole injection reduces
the collector resistance
RCE = 8.33 mW
Faster turn off than
BJTs but not as fast
as MOSFETS
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Three major power device technologies
5-36
hole injection
Extra
junction
Courtesy of Powerex, Inc.
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5-37
Typical IGBT driver circuit
Typically +15 V for turn on
Typically - 5 to -15 V for turn off
Control signal
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IGBT
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5-38
Powerex IGBT module structure
Powerex high voltage IGBT package
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5-39
Emitter voltage
The unijunction transistor fires
when its emitter voltage reaches VP.
VP
Then, the emitter voltage
drops due to its negative
resistance characteristic.
Base 2
Emitter current
Emitter
The UJT is not useful as an amplifier.
It is used in timing and control applications.
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Base 1
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5-40
Other transistor types quiz
BJTs are __________ -controlled
amplifiers.
current
FETs are __________ -controlled
amplifiers.
voltage
JFETs operate in the _________ mode.
depletion
MOSFETs operate in the __________
mode.
enhancement
IGBTs are __________ -controlled
amplifiers.
voltage
UJTs are not useful as __________.
amplifiers
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5-41
How do transistor switches work?
Can be viewed as solid state relays:
they are either ON or they are OFF.
BJT switches are characterized by:
high base current
(or no base current)
low resistance from collector to emitter
(or very high resistance)
low collector dissipation
PC = VCE x IC
PC = 0 x IC = 0 W
(or PC = VCE x 0 = 0 W)
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5-42
NPN SWITCH
LOAD
RCE @ 
W
0W
DRIVER
The
The
driver
driver
output
output
isiszero
positive,
volts,
IIBB>=I0
0and
andthe
theload
loadis
ison
off
The
driver
output
is
zero
volts,
B = 0 and ILOAD = 0
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5-43
PNP SWITCH
DRIVER
LOAD
The
Z):
Thedriver
driveroutput
outputisgoes
isoff
off(high
(highthe
Z):the
theresistor
resistor
low:
voltage
drop
pulls
the
base
voltage
up
that
VV
pulls
thethe
base
voltage
upso
soV
that
across
resistor
makes
BE
BE==00
BE negative
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PNP SWITCH WITH NPN DRIVER
5-44
LOAD
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NPN SWITCH WITH PNP DRIVER
(NEGATIVE POWER SUPPLY)
5-45
VV
0VV
BE
BE@ @+0.7
VVBE
0 VV
BE@@-0.7
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LOAD
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5-46
STEPPER MOTOR
A
B
C
D
A
Enhancement mode
power MOSFETs
used as switches
B
C
D
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5-47
This circuit is often
used when the driver
amplitude is not
great enough to
saturate the MOSFET.
LOAD
DRIVER
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5-48
The next slide shows a different type of transistor switch.
It does not have a digital output; it has an analog input and
output. It does have a digital input for controlling on and
off. Thus, these switches can be used to connect or
disconnect analog signals. Some are bidirectional left to
right or right to left.
Digital
on/off
input
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5-49
The output
is switched
on and off
by the
control.
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The signal source is a sine wave.
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5-50
Input signal
Output signal
Output signal
Control input
(logic 0 or 1)
The MAX4649
is an integrated
circuit (IC)
analog switch.
When the control input (pin 6) is at logic 0, the input signal applied to
the COM terminal (pin 1) is routed to the NC (normally closed)
terminal (pin 2) and when the control input is logic 1, the input signal
is connected to the NO (normally open) terminal (pin 8). The
MAX4649 integrated circuit acts as a static (no moving parts) SPDT
(single-pole-double-throw) relay. It is an example of a mixed-signal
integrated circuit. Mixed signal ICs have both digital and analog
features and are covered in more detail in subsequent chapters.
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5-51
REVIEW
• Amplification
• Transistors
• Characteristic Curves
• Transistor Testing
• Other Transistor Types
• Transistors as Switches
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