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Course Notes for EE 0257 Analysis and Design of Electronic Circuits

CV

v

SB

V

v

bnpD

A

n

p

SBBB

ie

Ii

eD

W

L

W

N

N

D

DIiii

T

BE

T

BE

=

=

+=+=

2

212

1

Chapter 5: Bipolar Junction Transistors (BJTs) 3

is called common-emitter current gain, in range 50 to 200. The emitter current

EEC

CBCE

iii

iiii

=+

=

+=+=

1

1

is called common-base current gain

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Course Notes for EE 0257 Analysis and Design of Electronic Circuits

Using this model, we can related the scale current Is, and to thecomplete I-V characteristics.

Chapter 5: Bipolar Junction Transistors (BJTs) 4

+

+

RF

S

V

v

F

S

B

R

S

V

v

SC

F

S

V

v

F

S

E

IeI

i

IeIi

IeIi

T

BE

T

BE

T

BE

11

11

11

Here is derivation:

Operation of pnpin the active mode

Similar to the operation of npn transistor, the pnp transistor alsorely on ONE type of carriers to function.

Unlike the npntransistor, current in thepnpis conducted by holesinjected from the emitter into the base.

CBJ is reversed biased to attract holes in the n-based into the p-collector.

Their large signal model of pnpdevice is similar to the npn typetoo, however the biased voltage are different.

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Course Notes for EE 0257 Analysis and Design of Electronic Circuits

The IV Characteristics

Chapter 5: Bipolar Junction Transistors (BJTs) 5

)300(251

1KmV

q

kTV

eIi

ieIi

ieIi

T

Vv

SCE

Vv

SCB

Vv

SCT

BE

T

BE

T

BE

==

=+

=

==

===

In the last lecture and from above equations, we learn that the collector

current only depends on vBEin forward active mode, the mode most ofBJTs operates. The only requirement for VCBis that VCBshould NOT

be forwarded bias for more than 0.4 to 0.5 V.

Example: a npn BJT has =100 and exhibits a vBE of 0.7 V at iC=1mA. Design a circuit so that a current of 2 mA flow through the

collector and a voltage +5V appears at the collector.

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Course Notes for EE 0257 Analysis and Design of Electronic Circuits

The Common-Emitter CharacteristicsAs the common-emitter configuration is the most used configuration,lets take a deeper look of its performance.

The Common-emitter Current Gain : If we feed the based with acurrent source, the iC~ vCEcharacteristics is shown below:

Chapter 5: Bipolar Junction Transistors (BJTs) 7

In the active region, given an operational point Q, the DC or largesignal current gain is defined as:

BQ

CQ

I

I=

The small signal gain or AC gain is defined as:tconsvB

C

CE

i

i

tan=

=

The magnitude of acand dcdiffer by ~ 10% to 20%. The smallsignal acis also known as hfe, or short-circuit common-emitter

current gain.

The Saturation voltage VCEsatand Saturation ResistanceRCEsat:

When a transistor is operated in the saturation region, the current gain

will become a lot of smaller due to the inefficient carrier transportationfrom the emitter to the collector.

The current gain of a saturated transistor is referred as forced .