lecture 05 - nuu.edu.tw

Microelectronic Circuit by meiling CHEN

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Lecture 05Lecture 05BJTsBJTs CircuitsCircuits


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topicstopics

• Large-signal operation• BJT circuits at DC• BJT biasing schemes


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iBEBE

SccccCcco

vVveIRVRiVv TV

BEv

+=−=−=

Bias + signal

Large-signal Bias (DC) + signal (AC)


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BEBBBB VRIV +×=DC load line :


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CECCCC VRIV +×=


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CACB

BCECBCCC

ACECACCC

RRQVRIVQVRIV

>→+×=→+×=

Leg room (small)head room (small)


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BJT operate as a switch

Switch on saturation modeSwitch off cut-off mode

Switch off:

CCCCBI VviiVv =→=→=→< 005.0

Switch on:

VVvC 02.0 ≈=


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Example 5.3

15050 << β

BJT work in saturation mode

factoroverdriveII

mAmII

mAmII

mAk

I

VVV

BB

satCB

satCB

satC

satCEC

×=

===

===

=−

=

==

(max)

max

)((min)

min

)((max)

)(

)(

0653.0150

8.9

196.0508.9

8.91

2.010

2.0

β

β

kI

RB

B 2.296.13.47.05==

−=


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Example 5.4 (DC analysis)

VkIVmAIII

mAmAII

mAkR

VI

VVV

CC

CEB

EC

E

EE

E

3.57.41001.0

99.011100

100

13.33.3

3.37.04

=×−==−=

=×+

==

===

=−=

α

Reverse bias

forward bias100=β

Active mode check

Assume BJT in active mode :


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100=β

VkIVmAIII

mAmAII

mAkR

VI

VVV

CC

CEB

EC

E

EE

E

48.27.410016.0

584.16.11100

100

6.13.33.5

3.57.06

=×−==−=

=×+

==

===

=−=

α


JC : forward biasJE : forward bias

Not in active mode


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Assume BJT in saturation mode :

mAIII

mAI

mAmI

VI

VVVVVVV

CEB

C

E

EE

satCEEC

E

64.0

96.07.4

5.510

6.13.33.5

5.52.03.53.57.06

)(

=−=

=−

=

===

=+=+==−=


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100=β

VVVmAImAImAIVV

ccC

C

E

B

BE

100000

======


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100=β


mAIIIVkmVRIV

mAmII

mAk

I

VV

CEB

CCC

EC

E

E

05.04.51016.410

6.465.4101100

65.42

7.0107.0

=−=−=−×=−×=

=×==

=−

=

=

α

forward bias

reverse bias

Active mode check


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100=β


VkmRIVmAII

mAIIkVIkV

CCC

BC

B

BBEB

4.123.410103.4

043.07.01001005

=×−=−===

=⇒+×=+×=

β

Reverse bias

forward bias


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30=β


VVmAIimpossibleVVmkVVII

mAk

IVV

IelRVVV

CC

CEC

EE

BB

BEBE

05.0)(3853.4103.4

3.41

7.057.0

0arg

(max) =→==−×=→=≈

=−

=→≈

≈→+=


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100=β

Thevenin’s equivalent circuit

kkkR

Vkk

kVV

BB

BB

3.3350//100

550100

5015

==

=+

=

mAImAII

mAIRIIVRIV

RIVRIV

C

BE

B

EBBBEBBBBB

EEBEBBBBB

28.129.1101

0128.0)(

=⇒=×=⇒

=⇒+++=


++=β

Reverse bias

forward bias


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100=β

100=β

mAII

VkIVmAII

mAk

I

VVVVVV

VRIVRIIV

EB

CC

EC

E

CE

C

CCCCCBC

028.0101

62.77.282.2

85.22

3.9153.97.0

6.8)(15

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22

22

2

12

1

1111121

≈=

≈×=≈=

≈−

=

≈+=≈⇒

+≈++=

α

Find correct current by iteration

mAIwithstart B 028.02 =


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100=β

100=β

Exercise 5.30 (DC analysis)

2CV3CI


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100=β

100=β

Q1 and Q2 cannot be conducting at same time.If Q1 ON than Q2 OFF, and vice versa.Assume Q1 on and Q2 off :


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BJT’s biasing schemes1. self-bias2. Base fixed bias3. Collector-feedback bias4. Two power supply version bias5. Constant current bias


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BEv

Ci1T 2T 3T

1Ci

2Ci

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106.1)(1038.1

−

−

××

==

=

Kq

KTV

eIio

T

SCTV

BEV

1.Temperature change Collector biasing current change2.Device change biasing current change

Why we need good biasing scheme?


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1. Self-Bias

β+>>

>>

1B

E

BEBB

RR

VV

β++−

=1

BRE

BEBBE R

VVI

Constrains:

Insensitive to T and β

CCCBCE

CCCC

CCBB

VorVVVRI

VV

31

31

31

)( =

=

=

The rule of thumb :(經驗法則)

Voltage-divider :

↑→∴

+=

+>>

B

B

BE

IsmallRR

RRRRR

RR

21

21

21

,

1

Q

β

Trade-off

Suggestion:

CCE VIRR =××+ 1.0)( 21


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1. Self-Bias (emitter feedback bias)

CCV

EV

CRBR

ER

β++−

=1

BRE

BECCE R

VVI

The rule of thumb :

CCCBCE

CCCC

CCBB

VorVVVRI

VV

31

31

31

)( =

=

=


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Example 5.13 design the following self bias circuit

10012

1

===

βVV

mAIgiven

CC

E

β++−

=1

BRE

BEBBE R

VVI

The rule of thumb :

kmI

R

kmI

VR

VVVVV

EC

E

EE

BEE

B

4199.0

412

3.31

3.33.34

412

31

31

≈×

==

===

=−=

==

α

kRkR

ba

bVRR

RVV

aRRVIRR

CCB

CCE

4080

)(),(

)(4

)(1211.0)(1.0)(

2

1

21

2

21

21

==

⇒

+⇒=

=××+⇒=××+

L

L


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2. Base fixed bias

CCV

CRBR

Type 1 Type 2 Type 3

B

BEBBC R

VVI )( −=β

B

BECCC R

VVI )( −=β

B

BEBBC R

VVI )( −=β

CCBB

BBB

BBB

VRR

RV

RRR

21

2

21 //

+=

=


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3. Collector-feedback bias (a)

β++−

=+=

++=

1BR

C

BECCCBE

BEBBCECC

RVVIII

VRIRIV

β+>>

1B

CRR

Constrains:

↓↓⇒↓⇒⇒

↑↑⇒↑⇒

CBCE

CCC

IIV

RIIT

Good biasing scheme


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3. Collector-feedback bias (b)

CCV

CRBR

ER

β+++−

=+=

+++++=

1

)()(

BREC

BECCCBE

ECBBEBBCCBCC

RRVVIII

RIIVRIRIIV

↓↓⇒↓⇒⇒

↑↑⇒↑⇒

CBCE

EC

IIV

IIT

Good biasing scheme


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4. Two-power supply version

β++−

=⇒

=++

1BR

E

BEEEE

EEEEBEBB

RVVI

VRIVRI

β+>>

>>

1B

E

BEBB

RR

VV

Constrains:


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5. Constant current bias by Current mirror

RVVVII

RVVVI

BEEECCf

BEEECCf

−+==

−−−=

Re

Re)(

βββ

ββ

≈+

=

+===+=+=

==∴≡

++=

)2(

)2()2(2

12

1

21

21

211

REF

BCC

BBCREF

BBB

BBCREF

II

IIIIIIII

IIIQQ

IIIIQ

lecture 05 - nuu.edu.tw

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