ECE 4991 Electrical and Electronic ECE 4991 Electrical and Electronic CircuitsCircuits

Chapter 10Chapter 10

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Where We AreWhere We Are• Chapter 2 - The basic concepts and practice at

analyzing simple electric circuits with sources and resistors

• Chapter 3 – More harder networks to analyze and the notion of equivalent circuits

• Chapter 4 – Capacitors and inductors added to the mix

• Chapter 5 – Analyzing transient situations in complex passive networks

• Chapter 8 – New subject – the wonders of operational amplifiers as system elements

• Chapter 9 – Introduction to semiconductors – the basics and diodes – more network analysis

• Chapter 10 – Bipolar junction transistors and how they work – now you can build your own op amp

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What’s Important in What’s Important in Chapter 10Chapter 10

1. Definitions

2. How bipolar transistors work

3. Building a transistor

4. Transistor basics

5. Collector characteristics

6. Load lines and operating points

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1. Definitions1. Definitions

• Bipolar transistor

• Emitter

• Base

• Collector

• NPN, PNP

• Gain ()

• Cutoff region

• Active linear region

• Saturation region

• Breakdown region

• Load lines

• Operating points

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2. How Bipolar Transistors 2. How Bipolar Transistors WorkWork

• Transistors are made by creating a PNP (or NPN) “sandwich”

• Carriers are “emitted” by the emitter and “collected” by the collector

• The base controls how much gets through

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Transistor OperationTransistor Operation• A small amount of base current can

control a large amount of collector current

• Gain = IC / IB =

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Transistor Operation in Transistor Operation in SiliconSilicon

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3. Building a Transistor3. Building a Transistor

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Transistor FabricationTransistor Fabrication

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4. Transistor Basics4. Transistor Basics

VBB

RB

+

VCC

RC

+

Typical NPN transistor schematic

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There are four operating There are four operating regions for a transistorregions for a transistor

VBB

RB

+

VCC

RC

+

1. Cutoff Region – Both junctions reverse-

biased – Very little base or collector current

2. Active Linear Region – BE junction forward

biased, CB junction reverse-biased –

amplifying mode

3. Saturation Region – Both junctions forward

biased – low VCE – switch mode

4. Breakdown Region – Determines the

operating limits of the device

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5. Collector Characteristics5. Collector Characteristics

VBB

RB

+

VCC

RC

+

“Collector characteristic” is a plot of

collector current versus collector-

emitter voltage for different base

currents

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Collector CharacteristicsCollector Characteristics

“Collector characteristic” is a plot of

collector current versus collector-

emitter voltage for different base

currents

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Real Transistor Collector Real Transistor Collector CharacteristicsCharacteristics

IB

= 10 µA, 20 µA, 30 µA, ... , 80 µA

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6. Load Lines and Operating 6. Load Lines and Operating PointsPoints

IB

= 10 µA, 20 µA, 30 µA, ... , 80 µA

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Consider this schematic…Consider this schematic…

VBB

RB

+

VCC

RC

+

• VCC = 15V, RC = 1000, VBB = 3V

• Design a circuit that operates around

the center of the active region collector

characteristics

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First, generate a load line, First, generate a load line, then set the operating pointthen set the operating point

VBB

RB

+

VCC

RC

+

• VCC = 15V, RC = 1000 ,

VBB = 3V

• What is IC @ VCE = 0?

• What is VCE @ IC = 0?

• Generate load line

• Pick desired IB

• Set RB to get it IB

= 10 µA, 20 µA, 30 µA, ... , 80 µA

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Practice with BipolarsPractice with Bipolars

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Practice with BipolarsPractice with Bipolars

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Practice with BipolarsPractice with Bipolars

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Practice with BipolarsPractice with Bipolars

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Practice with BipolarsPractice with Bipolars

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Practice with BipolarsPractice with Bipolars