Lecture_3

Lecture 3 Electronic Devices 1

ETU 07103LECTURE 3

Lecture 3 Electronic Devices 2

Lecture 3Coverage

• Bipolar Junction Transistor (BJT) Structure and principals of operationBJT configurationsBJT characteristic curvesBJT DC biasing Analysis under various biasing conditions

References • Electronic devices and circuit theory, 7th Ed. by Robert

L. Boylestad & Louis Nashelsky, pg. 112 to 166, • Electronic devices and circuits, by Theodore F.

Bogart, Jr., page 85 to 120

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Bipolar Junction Transistor (BJT)Introduction

• BJT is a three layers semiconductor device with either NPN or PNP structure

• BJT has three terminals described as follow Base – the control terminal Emitter – the source of majority carriers Collector – the collector of majority carriers

• Applications include the use as amplifier and switch• There are two types of BJT namely as

PNP transistorNPN transistor

• BJT is a current controlled device

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BJT continue…Structure

Figure 3.1 Structure and schematic symbols of NPN and PNP transistors

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BJT continue…Mode of operation

• The basic operation of BJT will be described using the PNP transistor of Fig. 3.2

• For linear operation The base-emitter junction is always forward biased, while The base-collector junction is always reversed biased

• A small base-emitter current controls a much large collector-emitter current

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BJT continue…

Figure 3.2 Majority and minority carriers flow of a PNP transistor

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BJT continue...

From Fig. 3.2 • Base-Emitter junction (Forward biased)

(1) Holes diffusing from E into the B (2) Electrons diffusing from the B into the E

• At the Base region (3) Recombination of holes injected into the base (4) Most holes reach the C

• Base-Collector junction (Reverse biased) (5) Electron minority carrier current from C to B (6) Hole minority carrier current from B to C

• In most practical purposes, current (5) and (6) can be neglected

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BJT continue…• For both NPN and PNP transistors

IE = IB + IC ……………………………………….......................(3.1)

• The IC is comprised of two components, the majority and minority carriers as indicated in Fig. 3.2

• The minority current component is called the leakage current (ICO)

• The IC, therefore is determined in total by Eqn. 3.2

IC = ICmajority + ICOminority ……………………………………………(3.2)

• Alpha is an important transistor parameter defined asα = ICmajority/IE = IC/IE ……………………………………............(3.3)

• α measures the portion of the IE that managed to cross the base and become IC (0.90 < α < 1)

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BJT Configurations Introduction

• A transistor can be connected in three configurations Common Base Configuration (CB) Common Emitter Configuration (CE) Common Collector Configuration (CC)

Common base configuration• The base terminal is common to both input and output

as shown in Fig. 3.3

• IC and IE are related by Eqn. 3.3 (i.e. α = IC/IE )

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BJT config. Continue…

Figure 3.3 Common base configuration

Common emitter configuration• The emitter terminal is common to both input and

output terminals as shown in Fig. 3.4

• The IC is related to the IB by a factor known as beta (β) as shown in Eqn. 3.4

β = IC/IB……………………………………………..(3.4)

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BJT config. Continue…• β is known as common-emitter, forward-current,

amplification factor

Figure 3.4 Common emitter configuration

Common collector configuration• Collector terminal is common to both input and output

as shown in Fig. 3.5

• Eqn. 3.5 shows on how IB and IE relates in CC

IE/IB = β / α ……………………………………………………..(3.5)

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BJT config. Continue…

Figure 3.5 Common collector configuration

BJT Characteristic curve• The behavior of BJT can be described using two sets

of characteristic curves namely Input characteristics: To show the relation between input I

and V Output characteristics: To show the relation between output

I and V

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BJT char. curve continue…Consider the CB configuration• Input characteristics

They are shown in Fig. 3.6a They relate an input IE to an input VBE for various

levels of output VCB

• Output characteristics The characteristics are as shown in Fig. 3.6b They relate an output IC to an output VCB for

various levels of input IE

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BJT char. curve continue…

(a) (b)

Figure 3.6 (a) Input characteristics for CB amplifier; (b) Output characteristics for CB amplifier

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BJT DC Biasing

Introduction• BJT is used for different purposes like amplifier,

oscillator, etc.• BJT must be DC biased for the most purposes • DC biasing is that process of introducing a fixed level

of IDC and VDC at the output circuit of BJT

• For transistor amplifiers, IDC and VDC establish an operating point (Q- point) on the characteristic curve

• Q-point define the region to be used for amplification of input signal

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BJT DC bias continue…

Biasing Circuits (CE)• Most commonly used biasing circuits are

Collector to base bias circuit Fixed bias circuit Emitter stabilized bias circuit Voltage divider bias circuit

• Fixed Bias Circuit Fig. 3.7 shows the fixed bias circuit Applying KVL to the input loop

VCC - IBRB - VBE = 0 ………………………………………(3.6)

Applying KVL to the collector-emitter loopVCE + ICRC – VCC = 0 …………………………………(3.7)

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BJT DC bias continue…

Figure 3.7 Fixed Bias Circuit Figure 3.8 Emitter stabilized bias circuit

• Emitter stabilized bias circuitIt is a modified version of a fixed bias circuit as shown in Fig.

3.8 Consider base-emitter loop

IB(RB + ( β +1)RE) = VCC – VBE ………………………(3.8)

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BJT DC bias continue… Consider collector-emitter loop

VCE – VCC + IC(RC + RE) = 0 ………………………(3.9)

• Voltage divider bias circuit The name voltage divider comes from voltage divider formed

by the resistor R1 and R2 in Fig. 3.9

Figure 3.9 Voltage divider bias circuit

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BJT DC bias continue…Consider the input side of Fig. 3.9

The output side of Fig. 3.9 will make use of an Eqn. 3.9

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THE END OF LECTURE 3