BJT operation modeling - ITESOBJT Operation and Modeling Dr. José Ernesto Rayas Sánchez February 8, 2007 2 Dr. J. E. Rayas Sánchez 3 Simplified Structure of a BJT and Symbol NPN

BJT Operation and ModelingDr. José Ernesto Rayas Sánchez

February 8, 2007

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1

BJT Operation and Modeling

Dr. José Ernesto Rayas Sánchez

Some figures of this presentation were taken from the instructional resources of the following textbooks:

A. S. Sedra and K. C. Smith, Microelectronic Circuits. New York, NY: Oxford University Press, 2003.

A. R. Hambley, Electronics: A Top-Down Approach to Computer-Aided Circuit Design. Englewood Cliffs, NJ: Prentice Hall, 2000.

R. C. Jaeger, Microelectronic Circuits Design. New York, NY: McGraw Hill, 1997.

2Dr. J. E. Rayas Sánchez

Outline

Simplified physical structure and symbols

Regions of operation

Operation in the active region

Models for the active region

I-V characteristics: iC – vBE , iC – vCE , iC – vCB

Early effect

Models for the cutoff region

Models for the saturation region

Variation of β with IC and temperature


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Simplified Structure of a BJT and Symbol

NPN Transistor

PNP Transistor


Regions of Operation of the BJT

Collector-Base Junction (CB)

Forward-biased Reversed-biased

Forw

ard-

bias

ed

Saturation Region (switch on)

Active Region (good amplifier)

Bas

e-Em

itter

Junc

tion

(BE)

Rev

erse

d-bi

ased

Inverted Active Region (poor amplifier)

Cutoff Region (switch off)


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Operation in the Active Region


Operation in the Active Region (cont)

EC ii α=

Junction BE behaves as a forward biased diode

Junction CB behaves as a current controlled current source

T

BEV

V

SBB eIi η≈

Typically, 0.970 ≤ α ≤ 0.997

Since BCE iii += then BBC iii βα

α =−

=1

Typically, 32.3 ≤ β ≤ 332.3)1( += βBE ii


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Models for the Active Region (NPN)

TBE VvSE eIi /

α=

TBE VvSB eIi /

β=

TBE VvSC eIi /=




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Models for the Active Region (PNP)

TEB VvSE eIi /

α=

TEB VvSB eIi /

β=

TEB VvSC eIi /=




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I-V Characteristics (iC – vBE)

iB

B C

E

TBE VvS eI /

β

iB iC

iE Considering temperature effects:


I-V Characteristics (iC – vCE Ideal)

iB

B C

E

TBE VvS eI /

β

iB iC

iE


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I-V Characteristics (iC – vCE Real)

iB

B C

E

TBE VvS eI /

β

iB iC

iE

iC increases with vCE due to the Early effect


Early Effect and Early Voltage (VA)


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NPN BJT Models in the Active Region - Summary

Considering the Early effect:

Neglecting the Early effect:

B C

E

iB iC

iE

⎟⎟⎠

⎞⎜⎜⎝

⎛+

A

CEB V

vi 1βT

BE

Vv

S eI η

β

B C

E

iB iC

iE

0.7V ⎟⎟⎠

⎞⎜⎜⎝

⎛+

A

CEB V

vi 1β

B C

E

T

BE

Vv

S eI η

β

iB iC

iE

BiβBiβ


PNP BJT Models in the Active Region - Summary

Considering the Early effect:

Neglecting the Early effect:

BiβT

EB

Vv

S eI η

βBiβ

⎟⎟⎠

⎞⎜⎜⎝

⎛+

A

ECB V

vi 1β

B

C

E

T

EB

Vv

S eI η

β

iB

iC

iE

⎟⎟⎠

⎞⎜⎜⎝

⎛+

A

ECB V

vi 1β


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BJT Models for the Cutoff Region


BJT Models for the Saturation Region


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iC-vCE Characteristics


iC-vCE Characteristics Including Breakdown

Zooming the saturation region


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Active Region vs Inverted Active Region)

-5V 0V 5V 10V

V

3.0mA

2.0mA

1.0mA

0.0mA

-1.0mA

Collector

Current

RegionSaturation

Cutoff

RegionSaturation

RegionForward-Active

RegionReverse-Active β = 25 β = 5

I = 0 uA

I = 100 uA

I = 80 uA

I = 60 uA

I = 40 uA

I = 20 uA

CE

B

B

B

B

B

B

F R


iC-vCB Characteristics


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Typical Variation of β with IC and Temperature


Variation of β with IC – An Example

Documents

BJT operation modeling - ITESOBJT Operation and Modeling Dr. José Ernesto Rayas Sánchez February 8, 2007 2 Dr. J. E. Rayas Sánchez 3 Simplified Structure of a BJT and Symbol NPN