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Electronics

Principles & Applications Seventh Edition

Charles A. Schuler Chapter 5

Transistors (student version)

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

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

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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.

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Amplifier

In

Gain =

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Out

Out In Š 2008 The McGraw-Hill Companies Inc. All rights reserved.


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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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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NPN Transistor Bias

The B-E junction is forward biased.

N

C

P

B

N

E

Current flows. McGraw-Hill

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NPN Transistor Bias 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. McGraw-Hill

IC N

C

P

B

N

E

IB IE Š 2008 The McGraw-Hill Companies Inc. All rights reserved.


Note: when the switch opens, all currents go to zero. Although IB is smaller it controls IE and IC.

IC N

C

P

B

N

E

IB

Gain is something small controlling something large (IB is small).

IE

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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 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 McGraw-Hill

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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 McGraw-Hill

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Concept Preview • The base to collector gain is called β (beta). • To find β, 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. McGraw-Hill

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IC = 99 mA The current gain from base to collector is called β. IB = 1 mA

β =

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99 IC mA 1IBmA

= 99

IE = 100 mA

C P

B

N

E

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IC = 99 mA Kirchhoff’s current law: IB = 1 mA

C P

B

N

E

IE = IB + IC = 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.

C B E

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IE = 100 mA

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Transistor currents quiz β 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 McGraw-Hill

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Concept Review • The base to collector gain is called β (beta). • To find β, 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 McGraw-Hill

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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 (βdc) and ac beta (βac) 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. McGraw-Hill

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NPN schematic symbol Collector

Base

C B E

Emitter Memory aid: NPN means Not Pointing iN. McGraw-Hill

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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

VCE

B E

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When graphed, the data provide an NPN collector family of curves. 100 µA

14 12 10 IC in mA 8 6 4 2 0 2 4 6

80 µA 60 µA 40 µA 20 µA 8 10 12 14 16 18

0 µA

VCE in Volts McGraw-Hill

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100 µA

14 12 10 IC in mA 8 6 4 2

80 µA 60 µA 40 µA 20 µA

0 2 4 6

β = McGraw-Hill

14 mA 6ICmA 40 IB µA 100 µA

8 10 12 14 16 18

0 µA

VCE in Volts 150 == 140

This type of gain is called β dc or hFE. © 2008 The McGraw-Hill Companies Inc. All rights reserved.


100 µA

14 12 10 IC in mA 8 6 4 2

80 µA 60 µA 40 µA 20 µA

0 2 4 6

β ac = McGraw-Hill

2.5∆ImA C 20∆IµA B

8 10 12 14 16 18

0 µA

VCE in Volts = 125

Another type of gain is called β ac or hfe. © 2008 The McGraw-Hill Companies Inc. All rights reserved.


100 µA

14 12 10 IC in mA 8 6 4 2

80 µA 60 µA 40 µA 20 µA

0 2 4 6 IB

C

8 10 12 14 16 18 VCE in Volts

0 µA

With these values of IB: The C-E model is a resistor.

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E

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100 µA

14 12 10 IC in mA 8 6 4 2 0 2 4 6 IB

80 µA 60 µA 40 µA 20 µA 8 10 12 14 16 18 VCE in Volts

0 µA

When IB >> 100 µA VCE ≅ 0 The model is a closed switch.

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100 µA

14 12 10 IC in mA 8 6 4 2 0 2 4 6 IB

80 µA 60 µA 40 µA 20 µA 8 10 12 14 16 18 VCE in Volts

0 µA

When IB = 0 IC = 0 The model is an open switch.

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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 resistor acts as a ___________. Two current gain measures are β dc and β ac __________. The symbol hfe is the same as _________. β ac McGraw-Hill

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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 (βdc) and ac beta (βac) 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 McGraw-Hill

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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.

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0

NPN

V mA

E B

C

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

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0

NPN

V mA

E B

C

The C-E resistance is very high. McGraw-Hill

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0

NPN

V mA

E B C 100 kΩ

The meter reading is < 100 kΩ due to gain. McGraw-Hill

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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 McGraw-Hill

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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.

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Current In

Current Amplifier

Current Out

The BJT is a current amplifier. McGraw-Hill

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Voltage In

Voltage Amplifier

Current Out

The JFET is a voltage controlled amplifier. McGraw-Hill

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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. McGraw-Hill

Drain Gate Source

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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. McGraw-Hill

Drain Gate Source

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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 McGraw-Hill

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Itâ&#x20AC;&#x2122;s possible to make enhancement type field effect transistors as well. Metal oxide insulator

Drain n

Gate

VGG

D

VDD

p

G

n

S N-channel MOSFET

Source

Gate bias enhances the channel and turns the device on. McGraw-Hill

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5V 4V 3V

ID in mA

2V

VGS

1V 0

VDS in Volts

0V Drain

Enhancement mode MOSFET drain family of characteristic curves Gate Source McGraw-Hill

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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 mΩ

Faster turn off than BJTs but not as fast as MOSFETS

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Three major device technologies

hole injection Extra junction

Courtesy of Powerex, Inc. McGraw-Hill

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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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Powerex IGBT module structure

Powerex high voltage IGBT package McGraw-Hill

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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. McGraw-Hill

Base 1

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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

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UJTs are not useful as __________. amplifiers

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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 McGraw-Hill

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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.

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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) McGraw-Hill

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NPN SWITCH

LOAD

RCE ≅ ∞ 0 ΩΩ CE DRIVER

The The driver driver output output isiszero positive, volts, IIBB>=I00 and andthe theload loadis ison off The driver output is zero volts, B = 0 and ILOAD = 0

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PNP SWITCH DRIVER

LOAD

The Z): Thedriver driveroutput outputisgoes isoff off(high (highthe Z):the theresistor resistor The driver output low: voltage drop pulls the base voltage up that VV ==00 pulls thethe base voltage upso soV that across resistor makes BEBE BE negative

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PNP SWITCH WITH NPN DRIVER

LOAD

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NPN SWITCH WITH PNP DRIVER (NEGATIVE POWER SUPPLY) VV 0VV BE BE≅ ≅+0.7

VVBE 0 VV BE≅≅ -0.7

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LOAD

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STEPPER MOTOR

A

B

C

D

A

Enhancement mode power MOSFETs used as switches

B C D

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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 McGraw-Hill

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

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