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MAK302-Elektronik

MAK302

ELEKTRONİK

Chapter 4

MAK302-Elektronik

The BJT has three regions called the emitter, base, and collector.

Between the regions are junctions as indicated. The term bipolar

refers to the use of both holes and electrons as current carriers in

the transistor structure

BJT Structure

B

(base)

C (collector)

n

p

n

Base-Collector

junction

Base-Emitter

junction

E (emitter)

B

C

p

n

E

p

npn pnp

The base is a thin lightly doped region compared to the heavily doped

emitter and moderately doped collector regions.

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MAK302-Elektronik

In normal operation, the base-emitter is

forward-biased and the base-collector is

reverse-biased.

BJT Operation

npn

For the pnp type, the voltages

are reversed to maintain the

forward-reverse bias.

–

+ –

+

–

+

+

BC reverse-

biased

–

BE forward-

biased

–

+

+

–

–

BC reverse-

biased

+

BE forward-

biased

–

+

pnp

For the npn type shown, the collector is more

positive than the base, which is more positive than

the emitter.

MAK302-Elektronik

BJT Currents

IE IE

IC

IB

IC

IBn

p

n

p

n

p

+

– +

–

–+

IE

IC

IB

+

–

+

IE

IC

IB

+

–

–

npn pnp

The direction of conventional current is in the direction of the arrow

on the emitter terminal. The emitter current is the sum of the

collector current and the small base current. That is, IE = IC + IB.

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MAK302-Elektronik

BJT Characteristics

The collector characteristic curves show the relationship

of the three transistor currents.

The curve shown is for a fixed

based current. The first region is

the saturation region.

When both junctions are forward-

biased, the transistor is in the

saturation region of its operation.

Saturation is the state of a BJT

in which the collector

current has reached a maximum

and is independent of the base

current.

IC

B

C

A

0 0.7 V VCE(max)

VCE

Saturation

region

Active region

Breakdown

region

MAK302-Elektronik

BJT Characteristics

IC

B

C

A

0 0.7 V VCE(max)

VCE

Saturation

region

Active region

Breakdown

region

Once the basecollector junction is

reverse-biased, IC levels off and

remains essentially constant for a

given value of IB as VCE continues

to increase.

As VCE is increased, IC increases until B. Then it flattens in

region between points B and C, which is the active region.

For this portion of the characteristic

curve, the value of IC is determined

only by the relationship expressed

as

Ideally, when VCE exceeds 0.7 V, the base-collector junction becomes

reverse-biased and the transistor goes into the active, or linear, region

of its operation.

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MAK302-Elektronik

BJT Characteristics

IC

B

C

A

0 0.7 V VCE(max)

VCE

Saturation

region

Active region

Breakdown

region

After C, is the breakdown region.

When VCE reaches a

sufficiently high voltage, the

reverse-biased base-collector

junction goes into breakdown;

and the collector current

increases rapidly as indicated

by the part of the curve to the

right of point C.

A transistor should never

be operated

in this breakdown region

MAK302-Elektronik

BJT Characteristics

0

IC

VCE

IB6

IB5

IB4

IB3

IB2

IB1

IB = 0Cutoff region

By setting up other values of base current, a family of

collector curves is developed.

bDC is the ratio of collector

current to base current.

It can be read from the curves.

The value of bDC is nearly the

same wherever it is read.

CDC

B

I

Ib

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MAK302-Elektronik

BJT Characteristics

What is the bDC for the transistor shown?

Choose a base current near the

center of the range – in this

case IB3 which is 30 mA.

IC

VCE

IB6

IB5

IB4

B3I

IB2

B1I

IB = 0

= 10 Am

= 20 Am

= 30 Am

= 0

= 60 Am

= 40 Am

= 50 Am

10.0

8.0

6.0

4.0

2.0

0

(mA)

Read the corresponding

collector current – in this case,

5.0 mA. Calculate the ratio:

CDC

B

5.0 mA

30 A

I

Ib

m 167

BJT Circuit Analysis

• When the base-emitter junction is forward-biased, it is like a forward-biased diode and has a nominal forward voltagedrop of.

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MAK302-Elektronik

Cutoff

In a BJT, cutoff is the condition in which there is no base

current, which results in only an extremely small leakage

current (ICEO) in the collector circuit. For practical work, this

current is assumed to be zero.

IB = 0 –

+

–

+ICEO

RC

VCCVCE ≅VCC

RB

In cutoff, neither the base-emitter

junction, nor the base-collector

junction are forward-biased.

MAK302-Elektronik

Saturation

In a BJT, saturation is the condition in which there is

maximum collector current. The saturation current is

determined by the external circuit (VCC and RC in this case)

because the collector-emitter voltage is minimum (≈ 0.2 V)

In saturation, an

increase of base

current has no effect

on the collector

circuit and the

relation IC = bDCIB is

no longer valid. –

+ –

+

VCC

VBB

VCE = VCC – IC RC

RB

RC

IB

IC

–

+

– +

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MAK302-Elektronik

DC Load Line

the transistor. It is drawn by

connecting the saturation

and cutoff points.

The transistor

characteristic curves are

shown superimposed on

the load line. The region

between the saturation

and cutoff points is called

the active region.

The DC load line represents the circuit that is external to

0

IC

VCE

IB = 0 Cutoff

VCE(sat) VCC

IC(sat)

Saturation

MAK302-Elektronik

DC Load Line

What is the saturation current and

the cutoff voltage for the circuit?

Assume VCE = 0.2 V in saturation.

–

+ –

+VCC

15 V

VBB

3 V

RC

RB

βDC = 200

220 kW

3.3 kW

CCSAT

C

0.2 V 15 V 0.2 V

3.3 k

VI

R

W4.48 mA CO CCV V 15 V

Is the transistor saturated? B

3.0 V 0.7 V10.45 A

220 kI m

W

IC = b IB = 200 (10.45 mA) = 2.09 mA Since IC < ISAT, it is not saturated.

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MAK302-Elektronik

MAK302-Elektronik

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Maximum Transistor Ratings

• The product of VCE and IC must not exceed the maximum power dissipation.

• Both VCE and IC cannot be maximum at the same time. If VCE is maximum, IC can be calculated as

• If IC is maximum, VCE can be calculated by rearranging the previous equation as follows:

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Power Derating

MAK302-Elektronik

Data Sheets

Data sheets give manufacturer’s specifications for maximum operating

conditions, thermal, and electrical characteristics. For example, an

electrical characteristic is bDC, which is given as hFE. The 2N3904 shows

a range of b’s on the data sheet from 100 to 300 for IC = 10 mA.

ON Characteristics

DC current gain ( IC = 0.1 mA dc, VCE = 1.0 V dc)

( IC = 1.0 mA dc, VCE = 1.0 V dc)

( IC = 10 mA dc, VCE = 1.0 V dc)

( IC = 50 mA dc, VCE = 1.0 V dc)

( IC = 100 mA dc, VCE = 1.0 V dc)

2N39032N3904

2N39032N3904

2N39032N3904

2N39032N3904

2N39032N3904

hFE2040

3570

50100

3060

1530

––

––

150300

––

––

–

Characteristic Symbol Max UnitMin

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MAK302-Elektronik

MAK302-Elektronik

End