Chapter 5 Bipolar Junction Transistors (BJTs)simonfoucher.com/McGill/ECSE330 Electronics/Notes/5.1 BJT Part 1.pdf · Chapter 5 Bipolar Junction Transistors (BJTs) ... • 1- Introduction

Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I

BJTs 1

Chapter 5Bipolar Junction Transistors (BJTs)

Sedra/Smith, Sections 5.1-5.9


BJTs 2

Outline of Chapter 5• 1- Introduction to The Bipolar Junction Transistor• 2- Active Mode Operation of BJT• 3- DC Analysis of Active Mode BJT Circuits• 4- BJT as an Amplifier• 5- BJT Small Signal Models• 6- CEA, CEA with RE, CBA, & CCA • 7- Integrated Circuit Amplifiers


BJTs 3

Transistors

• A three terminal device is required to implement current switches and amplifiers.– need voltage control terminal– used to control current flow through

other two terminals

• All four ideal amplifier configurations (section 1) employ dependent sources.

• A small Control “voltage” can allow a large change in “current”.


BJTs 4

• 3 terminal device in which the voltage across 2 terminals controls the current flowing in/out of a 3rd terminal:

Bipolar Junction Transistor (BJT)

ICC

IB

B

IEEVBE

VCB

VCE

npn BJT

IEE

IB

B

ICCVBC

VEB

VEC

pnp BJT

No Body Terminal


BJTs 5

BJT Active Mode Terminal Equations

• Voltage across 2 terminals (base/emitter) controls current at the 3rd (collector):

npn pnp

• Additional observation (applies to both transistor types) – total input current equals total output current:

⎟⎟⎠

⎞⎜⎜⎝

⎛=

T

BESC V

vIi exp ⎟⎟⎠

⎞⎜⎜⎝

⎛=

T

EBSC V

vIi exp

BCE iii += In MOSFETs iD=iSand iG=0


BJTs 6

The npn BJT Operational Modes

B-E Junction B-C JunctionCutoff reverse reverseActive forward reverse

Saturation forward forwardReverse Active reverse forward

4 Modes of operation

EMITTER BASE COLLECTOR

Notes the difference with MOSFETs

Amplifier


BJTs 7

Active Mode: Base & Emitter Terminals

• Between the Base and the Emitter, have a pn junction; thus looks like a diode

• Across this junction in active mode, DC operating voltage is ~ 0.7V, just like a diode

• When the current begins to flow, a LARGE number of electrons from the emitter region enter the base region.


BJTs 8

Active Mode: Base & Collector Terminals

• Between the Collector and the Base is a pn junction as well,

• In active mode, this junction is either reversed biased, or zero bias (VCB = 0).

• Since the Base-region is so thin, the carriers from the emitter region are swept into the collector region.


BJTs 9

Active Mode: Collector & Emitter Terminals

• Between the Collector and the Emitter a LARGE current flows,

• Controlled by a small voltage: VBE.


BJTs 10

The pnp Bipolar Junction Transistor






BJTs 11

Collector Current• Based on device operational

principles, we write a diode-like equation for the current flowing in the collector:

⎟⎟⎠

⎞⎜⎜⎝

⎛=

T

BESC V

vIi exp

• IS is the current-scale factor or saturation current– Proportional to area of the base-emitter junction– Inversely proportional to base region width– Inversely proportional to doping level in base– Between 10-12 and 10-18A, typically, in ICs– Strongly dependent on temperature

(doubles every 5°C increase)


BJTs 12

Base Current

• Base current has two components– hole diffusion current from base to emitter– electron recombination in base

• The base current also has a diode-like expression for current, but is a fraction of the collector current. It is given in terms of a parameter called the common-emitter current gain, β, and the collector current:

βC

BII =


BJTs 13

Transistor β

• Transistor Beta (β)– Always treated as a fixed constant in EC1– In reality, dependent on IC, VCB, temperature, and operating

frequency

– For large IC, recombination increases (β decreases)

– β is typically between 100 and 200– A large β represents an efficient BJT

• Variations in β

– For elevated temperatures, number of free holes in base region increases (β decreases)

– As VCB increases, effective base width (W) decreases (βincreases)


BJTs 14

Emitter Current

• Emitter current: BCE III +=

• In terms of IC and the common-base current gain, α:

CC

CBCE IIIIII ⎟⎟⎠

⎞⎜⎜⎝

⎛ +=+=+=

ββ

β1

11

; ≈+

==ββαα EC II

• In terms of IB: ( ) BBBBCE IIIIII 1+=+=+= ββ

( ) BE II 1+= β


BJTs 15

Active Mode Biasing• Conceptual biasing

arrangement: • VBE = 0.7V (forward bias B-E junction)

• B-C junction kept from forward-bias conduction

VVVVV BEEBC 7.0 ; ≈>≥

– In principle, VCB ≥ -0.5V when cut-in voltage of 0.5 is assumed

– In simplified cases VCB ≥ 0V is sometimes assumed

ICC

IB

B

IEE

VCB

VBE

+VCE

_


BJTs 16

IC vs VBE for Constant VCE

ICC

IB

B

IEE

VCE

VBE

Constant VCE

+VCE

_

For constant VCE, IC vs. VBE follows a diode I-V curve, consistent with IC vs VBE relationship:

⎟⎟⎠

⎞⎜⎜⎝

⎛=

T

BESC V

VII exp


BJTs 17

Ideal IC vs VBE/VCE Curve

VCE

IC

VBE1

Ideal IC vs. VCE curve indicates no dependence on VCE

VBE2

VBE3

VBE3 > VBE2 > VBE1


BJTs 18

Real IC vs VBE/VCE Curve

VBE1

Real IC vs. VCE curve indicates dependence on VCE

VBE2

VBE3

VBE3 > VBE2 > VBE1

VCE

IC


BJTs 19

npn BJT Family of Curves

• Family of curves describes IC vs. VBE/VCE.

• 3 Modes of operation:– Active– Saturation– Cutoff

• Active mode for analog applications

• Saturation/cutoff for digital applications

VBE1

VBE2

VBE3

VBE3 > VBE2 > VBE1

VCE

IC

ActiveSaturation

Cut-Off


BJTs 20

IC vs VCE for Constant VBE

• For constant VBE and small VCE, BJT in saturation:

ICC

IB

B

IEE

VCE

VBE

+VCE

_

Linear dependence on IC vs. VCE for constant VBEdefined as the Early Effect

Active Mode; Early Effect


BJTs 21

Modeling the Early Effect• Extrapolated curves

intersect at common point (-VA, 0)

• VA is the Early voltage– typically 50 to 100V

⎟⎟⎠

⎞⎜⎜⎝

⎛+⎟⎟

⎠

⎞⎜⎜⎝

⎛=

A

CE

T

BESC V

VVVII 1expIC

VCE


BJTs 22

BJT Output Resistance

C

A

constvCE

Co I

VVIr

BE

=⎥⎥⎦

⎤

⎢⎢⎣

⎡

∂∂

≡

−

=

1

.

- Nonzero slope of IC vsVCE is a measure of the output resistance looking into the collector.- Defined ro as the BJT output resistance:

IC

VCE

VBE

C

Ao I

Vr =


BJTs 23

Summary of npn Active Mode Characteristics

⎟⎟⎠

⎞⎜⎜⎝

⎛+⎟⎟

⎠

⎞⎜⎜⎝

⎛=

A

CE

T

BESC V

VVVII 1exp

βC

BII =

( ) BE II 1+= β VVVVV BEEBC 7.0 ; ≈>≥

ICC

IB

B

IEE11

; ≈+

==ββαα EC II

BCE III +=

C

Ao I

Vr =


BJTs 24

• 3 terminal device in which the voltage across 2 terminals controls the current flowing in/out of a 3rd terminal:

Bipolar Junction Transistor (BJT)

ICC

IB

B

IEEVBE

VCB

VCE

npn BJT

IEE

IB

B

ICCVBC

VEB

VEC

pnp BJT


BJTs 25

The pnp BJT Operational Modes






BJTs 26

Active Mode Biasing

• VEB = 0.7V (forward bias E-B junction)

• C-B junction kept from forward-bias conduction

VVVVV EBCBE 7.0 ; ≈>≥

– In principal, VBC ≥ -0.5V

– VBC ≥ 0V in simplified cases

• Conceptual biasing arrangement:


BJTs 27

Summary of pnp Active Mode Characteristics

⎟⎟⎠

⎞⎜⎜⎝

⎛+⎟⎟

⎠

⎞⎜⎜⎝

⎛=

A

EC

T

EBSC V

VVVII 1exp

VVVVV EBCBE 7.0 ; ≈>≥

IEE

IB

B

ICCβC

BII =

( ) BE II 1+= β

11

; ≈+

==ββαα EC II

BCE III +=

pnp

C

Ao I

Vr =


BJTs 28

pnp BJT Family of Curves

• Family of curves describes IC vs. VEB/VCE.

• 3 Modes of operation:– Active– Saturation– Cutoff

• Active mode for analog applications

• Saturation/cutoff for digital applications

VEB1

VEB2

VEB3

VEB3 > VEB2 > VEB1

VEC

IC

ActiveSaturation

Cut-Off

Documents

Chapter 5 Bipolar Junction Transistors (BJTs)simonfoucher.com/McGill/ECSE330 Electronics/Notes/5.1 BJT Part 1.pdf · Chapter 5 Bipolar Junction Transistors (BJTs) ... • 1- Introduction