Chapter 13Bipolar Junction Transistors

1. Bipolar Junction Transistor Operation in amplifier circuits.

2. Load-line Analysis & Nonlinear Distortion.

3. Large-signal equivalent circuits to analyze BJT circuits.

4. Bias circuits.

5. Small-signal equivalent circuits to analyze BJT amplifiers.

6. Several Important Amplifier Configurations.

Goal

1st Transistor

Brattain and Bardeen's pnp point-contact germanium transistor operatedas a speech amplifier with a power gain of 18 on December 23, 1947

This picture shows the workbench of John Bardeen and Walter Brattainat Bell Laboratories. They were supposed to be doing fundamentalresearch about crystal surfaces. The experimental results hadn't been very good, though, and there's a rumor that their boss, William Shockley, came near to canceling the project. But in 1947, working alone, they switched to using tremendously pure materials. It dawned on them that they could build the circuit in the picture. It was a working amplifier! John and Walter submitted a patent for the first working point contact transistor. Shockley was furious and took their work and invented the junction transistor and submitted a patent for it 9 days later. The three shared a Nobel Prize. Bardeen and Brattain continued in research (and Bardeen later won another Nobel). Shockley quit to start a semiconductor company in Palo Alto. It folded, but its staff went on to invent the integrated circuit (the "chip") and to found Intel Corporation. By 1960, all important computers used transistors for logic, and ferrite cores for memory. Memory chips replaced core in the 1970's

Explanation of 1st Transistor

1948 - POINT CONTACT TRANSISTOR 1950 - SINGLE-CRYSTAL GERMANIUM 1951 - GROWN JUNCTION TRANSISTOR1952 - ALLOY JUNCTION TRANSISTOR1952 - ZONE MELTING AND REFINING 1952 - SINGLE-CRYSTAL SILICON1955 - DIFFUSED -BASE TRANSISTORS 1957 - OXIDE MASKING 1960 - PLANAR TRANSISTOR 1960 - MOS TRANSISTOR 1960 - EPITAXIAL TRANSISTOR 1961 - INTEGRATED CIRCUITS

- from Bell Laboratories Record magazine January 1975, p.74

Major Milestones in Transistor Electronics

Close-Up of 2N23 transistor by Western Electric in 1954

Transistor

The Most Basic & Important Active Component- Power Amplification- IC uses more recently : interfacing required- if no proper IC, only transistor can do something.

1948년미국 Bell Lab 의Walter Houser Brattain, John Bardeen및William Bradford Shockley는반도체격자구조의시편(試片)에가는도체선을접촉시켜주면전기신호의증폭작용을나타내는것을발견하여이를트랜지스터라고명명하였다. 이것이그동안신호증폭의구실을해오던진공관(眞空管)과대치되는트랜지스터의시초가된것이다. 트랜지스터그자체가소형이어서이를사용하는기기(機器)는진공관을사용할때에비하여소형이되며, 가볍고소비전력이적어편리하다. 초기에는잡음·주파수특성이나쁘고, 증폭도도충분하지못하였으나, 그후많이개량되어아주대전력을다룰수있는등특수한경우를제외하고는진공관에대치되었다.

-두산백과사전 -

Function: Current AmplificationType : PNP , NPN Type Code : PNP High Frequency (2SA×××)

PNP Low Frequency (2SB×××)NPN High Frequency (2SC×××)NPN Low Frequency (2SD×××)

마이너스전압측을접지로, 플러스전압측을전원으로하는회로의경우, NPN 타입쪽이사용하기쉽다.

Transistor

Lead in TransistorNPN : 2SC1815 NPN : 2SD880

품명이 인쇄되어 있는면을 바라 보았을 때,오른쪽 리드가 베이스중앙의 리드가 컬렉터왼쪽의 리드가 이미터

트랜지스터의 종류에 따라 리드의 내용이 다르기 때문에매뉴얼 등을 참조하여 확인할 필요가있다

품명이 인쇄되어 있는면을 바라 보았을 때,오른쪽 리드가 이미터중앙의 리드가 컬렉터왼쪽의 리드가 베이스

Ohmmeter’s View of Transistor’s Terminal

같은 npn 인데 리드가 다름

Transistor ManTransistor Man

“Transistor Man” observes the base current and adjusts the output rheostat in an attempt to maintain the output current hFE times larger

Basic Transistor ModelIn npn Transistor, for example1. Collector must be more positive than Emitter2. Base-Emitter and Base-Collector behave like diode3. Maximum Parameter

VCE : Voltage between Collector and Emitter at open BaseVBE : Voltage between Base and Emitter IC : Collector CurrentIB : Base CurrentPC : Power Dissipation at Collector at 25oC fT : frequency at IC = IB

4. Collector Current is proportional to Base CurrentIC = hFEIBhFE : Current Gain ≈ 100 ~ 300

Note : hFE is not a good parameter range : 100 - 300“Circuit based on a particular hFE value is a bad circuit”

Schematic of BJT Transistor

Small fraction of Emitter Current flows into Base(C-B junction : Reverse Bias , B-E Junction : Forward Bias)

Typical Junction Connection

Equations of Operation

−

= 1exp

T

BEESE V

vIi BCE iii +=

E

C

ii

=α ESs II α=

≅

T

BEsC V

vIi expα

αβ−

==1B

C

ii

BC ii β=

Schokley Equation KCLAt B-E Junction

Let

−

= 1exp

T

BEESC V

vIi α: Scale Current

Collector Current is Amplified Version of Base Current

: Typically 100

Common-Emitter Characteristics

CEBEBC vvv −= CEBEBC vvwithv << 0

B-C junction : Reverse Bias , B-E Junction : Forward Bias

NP

N

Input Output

BBC iii 100== βCEBEBC vvwithv << 0

vBE must be larger than 0.6 ~ 0.7V

Analysis of Common-Emitter Amplifier

( ) ( ) ( )tvtiRtvV BEBBBB +=+ in CECCCC viRV +=

KVL in CircuitsInput Output

DC AC

DC

AC

iB = 0

vBE = 0

Max iB

Min iB

Min vin Max vin

Load-Line Analysis of Input( ) ( ) ( )tvtiRtvV BEBBBB +=+ in

vin = 0 : Q point AC swing around Q Point

From Analysis, iB can be obtained

( ) BBBB Rtvvti /1),(, in∝DC AC Bias

CECCCC viRV +=

Load-Line Analysis of Output

From Input AnalysisiB can be SelectedMax iB

Min iB

Min vCE Max vCE

Positive vB Negative vCE : Inverting Amplifier

BC ii β=

10V

1.6V

40kΩ

2kΩ( ) )2000sin(4.0in ttv π=

1.6VA

Rvi

B

BBB µ4010

406.1 3 =×== −

Q point : only DC component

VvBB 6.1=

40µA

AiA B µµ 3515 ≤≤

vIN : 1.2 ~ 2.0VvCE : 7 ~ 3 VAv = 7/1.2 = 5

AiA B µµ 3515 ≤≤

( ) )2000sin(2.1in ttv π=

Nonlinear Distortion

Output Signal is not Exactly Sine Curve ( ) )2000sin(4.0in ttv π=

Clipping Occur : Cutoff

PNP Bipolar Junction TransistorsExcept for reversal of current directions and voltage polarities, pnp BJT is almost identical to the npn BJT.

−

= 1exp

T

BEESE V

vIi

BCE iii +=

BC ii β=

Very Typical Transistor Connection Common-Emitter Amplifier

When iC becomes zero, we say that the transistor is cutoff. When vCE≈ 0.2 V, we say that the transistor is in saturation.

≅

Operating Region of Transistors

Large-Signal DC Circuit

DC Bias

Active RegionSaturationCut-off

C

CC

Rv

CCvB

BB

Rv

BBv

Large-Signal DC Circuit Model

Active-Region Saturation-Region Cutoff-Region

TR : Dependant Current Source with Base Voltage ≈ 0.7V

TR : Constant Voltage Sourceof -0.2V Collector Voltage

TR : Open Circuit

Input Output

Cut-off Saturation Active

B

BB

Rv

BBv

C

CC

Rv

CCv

IB

Ωk200Ωk1

V15Analysis of Fixed Base Bias Circuit

Ω= k200BR Ω= k1CR V15=CCV

100=β

AR

viB

CCB µ5.717.0

=−

= mAR

viC

CCC 8.142.0

=−

=

mAAii BC 15.75.71100 =×== µβ

VIRVV CCCCCE 85.7=−=

: iB > 0 & iC > βiB Active Region

003=β

AR

viB

CCB µ5.717.0

=−

= mAR

viC

CCC 8.142.0

=−

=

mAAii BC 45.215.71300 =×== µβIB

Ωk200Ωk1

V15

iB > 0 & iC < βiB Saturation Region

VVCE 2.0=

EEBB RIv += 7.0

( ) EB

BEBB RR

VVI1++

−=

β

15V

5V

2kΩ

2kΩ

mAR

VIE

BE 15.27.0

=−

=

BCE iii += BC ii β=

EECECCCC RIvRIv ++=

6.422.147.143006.442.1321.3100

VCE(V)IC(mA)IB(µA)β

Analysis of Four-Resistor Bias Circuit

2121 11

1 RRRR

RB =+

=

21

2

RRRVV CCB +

=

Thévenin Equivalent :

Two Resistor connected to Base Set vB,RB,iB

EECCCCCE IRIRVV −−=

BC IβI =( ) EB

BEBB RR

VVI1++

−=

β

EEBEBBB IRVIRV ++=

BE II )1( += β

KVL around B-E Loop

KCL around C-B-E Node

KVL around C-E Loop

vBE≈ 0.7V

vBE≈ 0.7VvCC≈ 15V

Ω=+

= kRR

RB 33.311

1

21

VRR

RVV CCB 521

2 =+

=

( ) ARR

VVIEB

BEBB µ

β2.41

1=

++−

= mAII BC 12.4== β

VIRIRVV EECCCCCE 72.6=−−=

100=β

Example of Four-Resistor Bias Circuit

IB

IC

iB swings around Q point

vBE swings around Q point

Typical BJT Circuit (AC Coupled)

Q point is set by Bias Circuit with vBB, RB

To Describe BJT with Characteristic Model 1. Variation around Q point is smaller than that of Q point itself

Small Signal Circuit Model is Required2. iC is dependant on iB or vBE as Described in above Figure

Dependant Current Source Model can be used

Small Signal Model for BJT

Hybrid-π Model

Base : Resistor rπCollector : Voltage Controlled

Current Source

Base : Resistor rπCollector : Current Controlled

Current Source

No Passive Element along Emitter

T Model

Base : Resistor reCollector : Voltage Controlled

Current Source

Base : Resistor reCollector : Current Controlled

Current Source

No Element along Base

ib(t) : Signal current flowing into BaseIBQ : DC current that flows with Zero Signal iB(t) : Total Base current.

( ) ( )tiiti bBQB +=

( ) ( )tvVtv beBEQBE +=

Small-Signal Equivalent Circuit

IBQ

vb(t) : Signal current flowing into BasevBEQ : DC current that flows with Zero SignvBE(t) : Total Base current.

vBEQ

( )

+−=+

T

beBEQESBBQ V

tvvItiI

)(exp)1( α

−

−= 1exp)1(

T

BEESB V

vIi α

−=

T

BEQESBQ V

vII exp)1( α

( )

+≅

=+

T

beBQ

T

beBQBBQ V

tvIV

tvItiI )(1)(exp

BQ

T

IVr =π

( )πr

tvti beB

)(=

( ) ( )titi bC β=CQ

T

IVr β

π =

Define

From Previous Definition of Q point

( ) ( )tiIti bBQB +=

Small-Signal Equivalent Circuit for the BJT

CQ

T

IVr β

π = ( ) ( )titi bC β=

1. Common Emitter

2. Emitter Follower(Common Collector)

Typical BJT Amplification Mode

Output Terminal Choice

B C

E

3. Common Base

Common-Emitter Amplifier

C1, C2 : Coupling Capacitorwithout Affecting DC bias from Input & Output (High Pass Filter)AC signal only Pass

CE : Bypass CapacitorLow Impedance path for AC IEto Ground (Same as C1,C2)

for Midband Frequency

Small Signal Equivalent Circuit

C1 : Coupling Capacitorwithout Affecting DC bias from Input (High Pass Filter) : AC signal only Pass

R1 , R2 : See Four Resistor BiasParallel to Base - Emitter

2121 11

1RR

RRRB +==

21

2

RRRVV CCB +

=

C2,CE: Coupling Capacitorwithout Affecting DC bias from Input (High Pass Filter) : AC only Pass CE : Short Circuit to Ground

RC , RL : See Four Resistor BiasParallel to Collector - Emitter

LCLC RR

RRRTotal 11

1'

+==

npn Transistor Hybrid π Model

Small-Signal ac Equivalent Circuit

CLCLL RR

RRR11

1+

==′21

21 111

RRRRRB +

==

π

βr

RvvA Lo

v′

−==in π

βr

RvvA Co

vo −==in

birvv π== bein bL iRv β'o −=

Open Circuit : RL = 0

Input Impedance & Gains

πrRivZ

B 111

in

inin +

==

Lv

in

Looi R

ZAZvRv

iiA in

inin //

===vi AAG =

Output Impedance

Co RZ =

Using Thevnin Equivalent Circuit without LoadZeroing Voltage Source

Common-Emitter Amplifier

C1, C2 : Coupling CapacitorCE : Bypass Capacitor

Co RZ =πrR

ZB 11

1in +=

Lvi R

ZAA in=

vi AAG =π

βr

RA Lv

′−=

π

βr

RA Cvo −=CLL RRR =′

21 RRRB =

CQ

T

IVr β

π =

Ω== kRZ Co 1

Ω=+

= 53111

1in

πrRZ

B

1.28in −==L

vi RZAA

2980== vi AAG106−=′

−=π

βr

RA Lv

158−=−=π

βr

RA Cvo

Ω==′ 667CLL RRRΩ== kRRRB 33.321Ω== 631

CQ

T

IVr β

π

ssin

insin v

RZZvv 515.0=+

= mVtvvAv sinvo )sin(6.546.54 ω=−==

mVtvin )sin(0.1 ω=

Output Source

Wave form in Common-Emitter Amp

Emitter Followers

C1, C2 : Coupling Capacitor

2121 11

1RR

RRRB +==

ELELL RR

RRR11

1+

==′

( )( ) L

Lv Rr

RA′++

′+=

ββ

π 11

itBitB

i ZRZR

Z =+

=11

1 ( ) Lb

it RrivZ ′++== βπ 1in

Lvi R

ZAA i= vi AAG =

Even though the voltage gain of the emitter follower is less than unity, the current gain and power gain can be large.

Output Impedance

21 1111

RRRR

ss ++=′

( ) ( ) Esx

xo RrRi

vZ11

1++′+

==πβ

Parallel Connection

Darlington Configuration