Dr. Nasim ZafarElectronics 1

EEE 231 – BS Electrical EngineeringFall Semester – 2012

COMSATS Institute of Information TechnologyVirtual campus

Islamabad

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Bipolar Junction Transistors-BJTs

Lecture No: 14

Contents:

Introduction

Bipolar Transistor Currents

Bipolar Transistor Characteristics and Parameter

Early Effect

 

 

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References: Microelectronic Circuits:

Adel S. Sedra and Kenneth C. Smith.

Electronic Devices :

Thomas L. Floyd ( Prentice Hall ).

Integrated Electronics Jacob Millman and Christos Halkias (McGraw-Hill).

Electronic Devices and Circuit Theory:

Robert Boylestad & Louis Nashelsky ( Prentice Hall ).

Introductory Electronic Devices and Circuits:

Robert T. Paynter.

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Reference:

Chapter 4 – Bipolar Junction Transistors:

Figures are redrawn (with some modifications) from

Electronic Devices By

Thomas L. Floyd

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Bipolar Junction Transistors

BJTs-Circuits

B

C

E

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Transistor Types

MOS - Metal Oxide Semiconductor

FET - Field Effect Transistor

BJT - Bipolar Junction Transistor

◄

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Transistor Current Characteristics

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An Overview of Bipolar Transistors:

While control in a FET is due to an electric field. Control in a bipolar transistor is generally considered to be due

to an electric current.– current into one terminal

determines the currentbetween two others

– as with an FET, abipolar transistorcan be used as a‘control device’

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Transistor Biasing Configurations:

1. Common-Base Configuration (CB) : input = VEB & IE ; output = VCB & IC

2. Common-Emitter Configuration (CE): input = VBE & IB ; output = VCE & IC

3. Common-Collector Configuration (CC): input = VBC & IB ; output = VEC & IE

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Operation Modes:

Active: – Most importance mode, e.g. for amplifier operation.– The region where current curves are practically flat.

Saturation:– Barrier potential of the junctions cancel each other out

causing a virtual short.– Ideal transistor behaves like a closed switch.

Cutoff:– Current reduced to zero– Ideal transistor behaves like an open switch.

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VCE (V)

IC(mA)

IB = 50 A

IB = 0

30

5 10 15 20 0

0

IB = 100 A

IB = 150 A

IB = 200 A

22.5

15

7.5

Saturation Region

Active Region

Cutoff Region

Operation Modes:

Active: BJT acts like an amplifier (most common use).

Saturation: BJT acts like a short circuit.

Cutoff: BJT acts like an open circuit.

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Common Emitter Characteristics:

We consider DC behaviour and assume that we are

working in the normal linear amplifier regime with

the BE junction forward biased and the CB junction

reverse biased.

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Common-Emitter Output Characteristics

VCE

IC

Active Region

IB

Saturation RegionCutoff RegionIB = 0

Region of Operation

Description

Active Small base current controls a large collector current

Saturation VCE(sat) ~ 0.2V, VCE increases with IC

Cutoff Achieved by reducing IB to 0, Ideally, IC will also equal 0.

Output Characteristic Curves - (Vc- Ic

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Common-Base-Configuration (CBC) NPN Transistor

Circuit Diagram: NPN Transistor

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Common-Base Output Characteristics:

Although the Common-Base configuration is not the most common configuration, it is often helpful in understanding the operation of BJT

Output Characteristic Curves - (Vc- Ic

Satu

ratio

n R

egio

n

IE

IC

VCB

Active Region

CutoffIE = 0

0.8V 2V 4V 6V 8V

mA

2

4

6

IE=1mA

IE=2mA

Breakdown Region

)

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Transistor Currents - Output characteristics:

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Common-Collector Output Characteristics:

Emitter-Current Curves

VCE

IE

Active Region

IB

Saturation RegionCutoff Region IB = 0

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Bipolar Transistor Characteristics

• Behaviour can be described by the current gain, hfe or by the transconductance, gm of the device

21.4

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Conventional View & Current Components:NPN Transistor-CEC

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Current Components: NPN Transistor-CEC

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BJT Characteristics and Parameters

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BJT-Current Gain Parameters:

Two quantities of great importance in the characterization of transistors are the so-called common-base current gain ..

and the so-called common-emitter gain .

DC and DC

= Common-emitter current gain

= Common-base current gain

Note: and are sometimes referred to as dc and dc

because the relationships being dealt within the BJT are DC.

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BJT-Current Gain Parameters:

Common-base current gain , is also referred to as hFB and is defined by:

= hFB = IC / IE Common-emitter current gain β, is also referred as hFE and

is defined by:

= IC/IB

Thus: BC IβI

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Beta () or amplification factor: The ratio of dc collector current (IC) to the dc base current

(IB) is dc beta (dc ) which is dc current gain where IC and IB are determined at a particular operating point, Q-point (quiescent point).

It’s define by the following equation:

30 < dc < 300 2N3904

On data sheet, dc=hFE with h is derived from ac hybrid equivalent circuit. FE are derived from forward-current amplification and common-emitter configuration respectively.

In the dc mode the level of IC and IE due to the majority carriers are related by a quantity called alpha:

=

IC = IE + ICBO

It can then be summarize to IC = IE (ignore ICBO due to small value)

For a.c situations where the point of operation moves on the characteristics curve, an a.c alpha defined by

Alpha a common base current gain factor that shows the efficiency by calculating the current percent from current flow from emitter to collector. The value of is typical from 0.9 ~ 0.998.

E

C

II

E

C

II

25Nasim Zafar.

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BJT-Current Gain Parameters:

TBEV

V

SC eIi

TBEV

VS

E eIi

E

C

ii

= Common-Base Current Gain (typical 0.99)

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BJT-Current Gain Parameters:

TBEV

V

SC eIi

TBEV

VS

B eIi

B

C

ii

= Common-emitter current gain (10-1000; typical 50-200)

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DC and DC

11

= Common-emitter current gain (10-1000; typical 50-200)

= Common-base current gain (0.9-0.999; typical 0.99)

The relationship between the two parameters are:

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Performance Parameters for PNP:

Common emitter dc current gain, dc:

BdcC II

Bdc

dcC

BCdcEdcC

1

)(

II

IIII

But,

T

T

dc

dcdc 11

Note that is large (e.g. = 100)

For NPN transistor, similar analysis can be carried out. However,the emitter current is mainly carried by electrons.

Example: .etcENEP

EPII

I

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Performance Parameters for PNP:

Emitter efficiency:

E

EP

ENEP

EPII

III

Fraction of emitter current carried by holes.We want close to 1.

Base transport factor:

Ep

CT I

Iα Fraction of holes collected by the collector.We want T close to 1.

Common base dc current gain:

EdcETEPTC IIII

Tdc

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Example: NPN Common-Base Configuration:

+_

+_

Given: IB = 50 A , IC = 1 mA

Find: IE , , and

Solution:

IE = IB + IC = 0.05 mA + 1 mA = 1.05 mA

b = IC / IB = 1 mA / 0.05 mA = 20

= IC / IE = 1 mA / 1.05 mA = 0.95238

IC

IEIB

VCB

VBE

E

C

B

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