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© 2013 The McGraw-Hill Companies, Inc. All rights reserved.McGraw-Hill
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ElectronicsElectronics
Principles & ApplicationsPrinciples & ApplicationsEighth EditionEighth Edition
Chapter 5Transistors
Charles A. Schuler
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.McGraw-Hill
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• Amplification• Transistors• Characteristic Curves• Transistor Testing• Other Transistor Types• Transistors as Switches
INTRODUCTION
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Amplifier Out
InGain =
In
Out
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N
P
N
NPN Transistor Structure
The collector is lightly doped. C
The base is thin and is lightly doped.
B
The emitter is heavily doped. E
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The C-B junction is reverse biased.
N
P
N
NPN Transistor Bias
C
B
E
No current flows.
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The B-E junction is forward biased.
N
P
N
NPN Transistor Bias
C
B
E
Current flows.
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When both junctionsare biased....
N
P
N
NPN Transistor Bias
C
B
E
Current flowseverywhere.
Most of the emitter carriersdiffuse through the thin base
region since they are attractedby the collector.
Note that IB is smallerthan IE or IC.
IC
IB
IE
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N
P
N C
B
E
Although IB is smallerit controls IE and IC.
IC
IB
IE
Note: when the switch opens, all
currents go to zero.
Gain is something smallcontrolling something large
(IB is small).
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Transistor structure and bias quiz
The heaviest doping is found in the___________ region. emitter
The thinnest of all three regions is calledthe ____________. base
The collector-base junction is ___________biased. reverse
The base-emitter junction is ____________biased. forward
The majority of the emitter carriers flow tothe ___________. collector
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N
P
C
B
E
IC = 99 mA
IB = 1 mA
IE = 100 mA
= IC
IB
The current gain frombase to collector
is called
99 mA
1 mA= 99
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N
P
C
B
E
IC = 99 mA
IB = 1 mA
IE = 100 mA
IE = IB + IC
99 mA= 1 mA +
= 100 mA
Kirchhoff’scurrent law:
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5-12
C
B
E
IC = 99 mA
IB = 1 mA
IE = 100 mA
In a PNP transistor,holes flow from
emitter to collector.
Notice the PNPbias voltages.
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5-13
Transistor currents quiz
is the ratio of collector current to ______current. base
The sum of the base and collector currentsis the __________ current. emitter
In NPN transistors, the flow from emitter tocollector is composed of _______. electrons
In PNP transistors, the flow from emitter tocollector is composed of _______. holes
Both NPN and PNP transistors show__________ gain. current
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Emitter
NPN schematic symbol
Base
Collector
Memory aid: NPNmeans Not Pointing iN.
EBC
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Collector
Base
Emitter
PNP schematic symbol
EBC
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IB
IC
VCE
B
C
E
This circuit is used tocollect IC versus
VCE data forseveral values of IB.
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0 2 4 6 8 10 12 14 16 18
2468
101214
VCE in Volts
IC in mA
When graphed, the data provide anNPN collector family of curves.
20 A
0 A
100 A
80 A
60 A
40 A
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0 2 4 6 8 10 12 14 16 18
2468
101214
VCE in Volts
IC in mA
20 A
0 A
100 A
80 A
60 A
40 A
= IC
IB
= 15040 A
6 mA
100 A
14 mA= 140 This type of gain
is called dc or hFE.
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0 2 4 6 8 10 12 14 16 18
2468
101214
VCE in Volts
IC in mA
20 A
0 A
100 A
80 A
60 A
40 A
ac = Another type of gainis called ac or hfe.
IC
IB
= 12520 A
2.5 mA
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0 2 4 6 8 10 12 14 16 18
2468
101214
VCE in Volts
IC in mA
20 A
0 A
100 A
80 A
60 A
40 A
IBWith these values of IB:
The C-E model is a resistor.
C
E
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0 2 4 6 8 10 12 14 16 18
2468
101214
VCE in Volts
IC in mA
20 A
0 A
100 A
80 A
60 A
40 A
IB When IB >> 100 A
VCE 0
The model is a closed switch.
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0 2 4 6 8 10 12 14 16 18
2468
101214
VCE in Volts
IC in mA
20 A
0 A
100 A
80 A
60 A
40 A
IB 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 transistoracts as a ___________ switch. closed
When IB = 0 and IC = 0, the transistoracts as an ___________ switch. open
When IB > 0 and VCE > 0, the transistoracts as a ___________. resistor
Two current gain measures are dc and__________. ac
The symbol hfe is the same as _________.ac
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0
The E-B junction is forward biased by the ohmmeter.
V
mA
NPN
E
BC
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0
The C-E resistance is very high.
V
mA
NPN
E
BC
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0
The meter reading is < 100 kdue to gain.
V
mA
NPN
E
BC
100 k
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Current OutCurrent In CurrentAmplifier
The BJT iscurrent
controlled.
Transistors
BJTs FETs
JFETs MOSFETs
NPN PNP
Depletion Mode Depletion Mode Enhancement Mode
N-Channel N-Channel N-ChannelP-Channel P-Channel P-Channel
D D D D D D
GGGGGG
S S S S S S
CC
E E
BB
BJTs are current controlled and FETs are voltage controlled.© 2013 The McGraw-Hill Companies, Inc. All rights reserved.McGraw-Hill
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Current OutVoltage In VoltageAmplifier
The JFET isa voltage
controlledamplifier.
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DrainSource
Drain
Source
Gate
Gate
Structure of anN-channel JFET
P-type substrate
P
N-channel
The channel has carriers so it conducts from source to drain.
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DrainSource
Drain
Source
Gate
Gate
P
N-channel
P-type substrate
A negative gate voltagecan push the carriers from
the channel and turn the JFET off.
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0VDS in Volts
ID in mA
-4 V
-5 V
0 V
-1 V
-2 V
-3 VVGS
N-channel JFET drain family of characteristic curves
This is known as a depletion-mode device.
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n
Source
Gate
Drain
VDD
p
n
It’s possible to make enhancementtype field effect transistors as well.
G
S
D
VGG
Gate bias enhances the channel and turns the device on.
Metaloxide
insulator
N-channelMOSFET
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0VDS in Volts
ID in mA
1 V
0 V
5 V
4 V
3 V
2 VVGS
Enhancement mode MOSFET drain family of characteristic curves
Drain
Source
Gate
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The IGBT (insulated gate bipolar transistor)
Operation and structuresimilar to a MOSFET
Voltage controlled(like the MOSFET)
Has one more junctionthan a MOSFET
Hole injection reducesthe collector resistance
Faster turn off thanBJTs but not as fast
as MOSFETS
RCE = 8.33 m
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Courtesy of Powerex, Inc.
Three major power device technologies
hole injection
Extrajunction
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Typical IGBT driver circuit
IGBT
Typically +15 V for turn on
Typically - 5 to -15 V for turn off
Control signal
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Powerex IGBT module structure
Powerex high voltage IGBT package
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Base 2
Base 1
Emitter
The unijunction transistor fires when its emitter voltage reaches VP.
VP
Emitter current
Em
itte
r vo
ltag
e
Then, the emitter voltagedrops due to its negativeresistance characteristic.
The UJT is not useful as an amplifier.It is used in timing and control applications.
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5-40Other transistor types quiz
BJTs are __________ -controlledamplifiers. current
FETs are __________ -controlledamplifiers. voltage
JFETs operate in the _________ mode.depletion
MOSFETs operate in the __________mode. enhancement
UJTs are not useful as __________.amplifiers
IGBTs are __________ -controlledamplifiers. voltage
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How do transistor switches work?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 dissipationPC = VCE x IC
PC = 0 x IC = 0 W(or PC = VCE x 0 = 0 W)
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LOADLOADLOAD
DRIVER
The driver output is zero volts, IB = 0 and ILOAD = 0The driver output is positive, IB > 0 and the load is onThe driver output is zero volts, IB = 0 and the load is off
RCE RCE 0RCE
NPN SWITCH
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LOAD
DRIVER
The driver output is off (high Z): the resistorpulls the base voltage up so that VBE = 0
PNP SWITCH
The driver output goes low: the voltage dropacross the resistor makes VBE negative
The driver output is off (high Z): the resistorpulls the base voltage up so that VBE = 0
LOADLOAD
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PNP SWITCH WITH NPN DRIVER
LOADLOADLOAD
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NPN SWITCH WITH PNP DRIVER(NEGATIVE POWER SUPPLY)
LOADLOADLOADVBE -0.7 V
VBE +0.7 V
VBE 0 V
VBE 0 V
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A
B
C
D
A B C D
STEPPER MOTOR
Enhancement modepower MOSFETsused as switches
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LOADLOADLOAD
DRIVER
This circuit is oftenused when the driver
amplitude is notgreat enough to
saturate the MOSFET.
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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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The signal source is a sine wave.
The output is switched on and off
by the control.
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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.
Control input (logic 0 or 1)
Input signal
Output signal
Output signal
The MAX4649 is an integrated
circuit (IC) analog switch.