Electronics Principles & Application

McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved.

ElectronicsElectronics Principles & ApplicationsPrinciples & Applications

Seventh EditionSeventh Edition

Chapter 5Transistors

(student version)

Charles A. Schuler



• Amplification• Transistors• Characteristic Curves• Transistor Testing• Other Transistor Types• Transistors as Switches

INTRODUCTION


Dear Student:

This presentation is arranged in segments. Each segmentis preceded by a Concept Preview slide and is followed by aConcept Review slide. When you reach a Concept Reviewslide, you can return to the beginning of that segment byclicking on the Repeat Segment button. This will allow youto view that segment again, if you want to.


Concept Preview• Amplifiers provide gain (the output is larger than

the input).• Transistors have gain.• Transistors have a collector, a base, and an

emitter.• The C-B junction is reverse biased.• The B-E junction is forward biased.• Most of the emitter carriers reach the collector.• The base current is relatively small but controls

the larger currents.


Amplifier Out

InGain =

In

Out


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


The C-B junction is reverse biased.

N

P

N

NPN Transistor Bias

C

B

E

No current flows.


The B-E junction is forward biased.

N

P

N

NPN Transistor Bias

C

B

E

Current flows.


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


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).


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


Concept Review• Amplifiers provide gain (the output is larger than

the input).• Transistors have gain.• Transistors have a collector, a base, and an

emitter.• The C-B junction is reverse biased.• The B-E junction is forward biased.• Most of the emitter carriers reach the collector.• The base current is relatively small but controls

the larger currents.

Repeat Segment


Concept Preview• The base to collector gain is called (beta).• To find divide the collector current by the base

current.• The emitter current is the largest since it is the

sum of the base and collector currents.• PNP transistors have opposite polarity from NPN

transistors.• In an NPN transistor, the major flow is made up of

electrons.• In a PNP transistor, the major flow is made up of

holes.


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


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:


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.


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


Concept Review• The base to collector gain is called (beta).• To find divide the collector current by the base

current.• The emitter current is the largest since it is the

sum of the base and collector currents.• PNP transistors have opposite polarity from NPN

transistors.• In an NPN transistor, the major flow is made up of

electrons.• In a PNP transistor, the major flow is made up of

holes.

Repeat Segment


Concept Preview• The NPN schematic symbol shows the emitter

arrow as Not Pointing iN.• The collector curves are a graph of collector

voltage versus collector current.• Both dc beta (dc) and ac beta (ac) can be

determined from the collector curves.• The collector circuit of a transistor can be modeled

as a resistor, as a closed switch or as an open switch.

• The amount of base current determines which of the three models applies.


Emitter

NPN schematic symbol

Base

Collector

Memory aid: NPNmeans Not Pointing iN.

EBC


Collector

Base

Emitter

PNP schematic symbol

EBC


IB

IC

VCE

B

C

E

This circuit is used tocollect IC versus

VCE data forseveral values of IB.


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 A80 A

60 A

40 A


0 2 4 6 8 10 12 14 16 18

2468

101214

VCE in Volts

IC in mA

20 A

0 A

100 A80 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.


0 2 4 6 8 10 12 14 16 18

2468

101214

VCE in Volts

IC in mA

20 A

0 A

100 A80 A

60 A

40 A

ac = Another type of gainis called ac or hfe.

IC

IB

= 12520 A

2.5 mA


0 2 4 6 8 10 12 14 16 18

2468

101214

VCE in Volts

IC in mA

20 A

0 A

100 A80 A

60 A

40 A

IBWith these values of IB:

The C-E model is a resistor.

C

E


0 2 4 6 8 10 12 14 16 18

2468

101214

VCE in Volts

IC in mA

20 A

0 A

100 A80 A

60 A

40 A

IB When IB >> 100 AVCE 0

The model is a closed switch.


0 2 4 6 8 10 12 14 16 18

2468

101214

VCE in Volts

IC in mA

20 A

0 A

100 A80 A

60 A

40 A

IB When IB = 0IC = 0

The model is an open switch.


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


Concept Review• The NPN schematic symbol shows the emitter

arrow as Not Pointing iN.• The collector curves are a graph of collector

voltage versus collector current.• Both dc beta (dc) and ac beta (ac) can be

determined from the collector curves.• The collector circuit of a transistor can be modeled

as a resistor, as a closed switch or as an open switch.

• The amount of base current determines which of the three models applies.

Repeat Segment


Concept Preview• It is possible to test transistors out-of-circuit using

an ohmmeter.• The E-B and C-B junctions act as diodes during

ohmmeter testing.• The C-E test shows a high resistance because two

junctions are involved; one of which is reverse biased by the ohmmeter.

• Gain can be verified by using a resistor in conjunction with the ohmmeter test.


0

The E-B junction is forward biased by the ohmmeter.

V

mA

NPN

E

BC


0

The C-E resistance is very high.

V

mA

NPN

E

BC


0

The meter reading is < 100 kdue to gain.

V

mA

NPN

EB

C

100 k


Concept Review• It is possible to test transistors out-of-circuit using

an ohmmeter.• The E-B and C-B junctions act as diodes during

ohmmeter testing.• The C-E test shows a high resistance because two

junctions are involved; one of which is reverse biased by the ohmmeter.

• Gain can be verified by using a resistor in conjunction with the ohmmeter test.

Repeat Segment


Concept Preview• Bipolar junction transistors (BJTs) are controlled by base

current.• Junction field effect transistors (JFETs) are controlled by

gate voltage.• JFETs operate in the depletion mode (as normally on

devices).• Metal oxide semiconductor field effect transistors

(MOSFETs) usually operate in the enhancement mode (as normally off devices).

• Insulated gate bipolar transistors (IGBTs) are modified MOSFETs and have very low on-resistance.

• Unijunction transistors (UJTs) are not used as amplifiers.


Current OutCurrent In CurrentAmplifier

The BJT isa currentamplifier.


Current OutVoltage In VoltageAmplifier

The JFET isa voltage

controlledamplifier.


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.


DrainSource

Drain

Source

Gate

Gate

P

N-channelP-type substrate

A negative gate voltagecan push the carriers from

the channel and turn the JFET off.


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.


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


0VDS in Volts

ID in mA

1 V

0 V

5 V4 V

3 V

2 VVGS

Enhancement mode MOSFET drain family of characteristic curves

Drain

Source

Gate


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


Courtesy of Powerex, Inc.

Three major device technologies

hole injection

Extrajunction


Typical IGBT driver circuit

IGBT

Typically +15 V for turn on

Typically - 5 to -15 V for turn off

Control signal


Powerex IGBT module structure

Powerex high voltage IGBT package


Base 2

Base 1

Emitter

The unijunction transistor fires when its emitter voltage reaches VP.

VP

Emitter current

Em

itter

vol

tage

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.


Other transistor types quizBJTs 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


Concept Review• Bipolar junction transistors (BJTs) are controlled by base

current.• Junction field effect transistors (JFETs) are controlled by

gate voltage.• JFETs operate in the depletion mode (as normally on

devices).• Metal oxide semiconductor field effect transistors

(MOSFETs) usually operate in the enhancement mode (as normally off devices).

• Insulated gate bipolar transistors (IGBTs) are modified MOSFETs and have very low on-resistance.

• Unijunction transistors (UJTs) are not used as amplifiers.

Repeat Segment


Concept Preview• BJTs can be used as switches.• No base current = switch is off.• High base current = switch is on.• The dissipation is always zero in an ideal switch:

off = no current flow and on = no voltage drop.• MOSFETs can also be used as switches: no gate

voltage = switch is off and high gate voltage = switch is on.


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 (switch is on)

(or no base current … off)

low resistance from collector to emitter (on)(or very high resistance … off)

low collector dissipation (on or off)PC = VCE x IC

PC = 0 x IC = 0 W (on)(or PC = VCE x 0 = 0 W … off)


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


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


PNP SWITCH WITH NPN DRIVER

LOADLOADLOAD


NPN SWITCH WITH PNP DRIVER(NEGATIVE POWER SUPPLY)

LOADLOADLOADVBE -0.7 V

VBE +0.7 V

VBE 0 V

VBE 0 V


A

B

C

D

A B C D

STEPPER MOTOR

Enhancement modepower MOSFETsused as switches


Concept Review• BJTs can be used as switches.• No base current = switch is off.• High base current = switch is on.• The dissipation is always zero in an ideal switch:

off = no current flow and on = no voltage drop.• MOSFETs can also be used as switches: no gate

voltage = switch is off and high gate voltage = switch is on.

Repeat Segment


REVIEW• Amplification• Transistors• Characteristic Curves• Transistor Testing• Other Transistor Types• Transistors as Switches

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Electronics Principles & Application