rec101 unit ii (part 1) bjt characteristics

Bipolar Junction Transistor (BJT) Characteristics

Unit II

Bipolar Junction Transistor: Transistor Construction, Operation, Amplification action.Common Base, Common Emitter, Common Collector Configuration DC Biasing BJTs:Operating Point, Fixed-Bias, Emitter Bias, Voltage-Divider Bias Configuration. CollectorFeedback, Emitter-Follower Configuration. Bias Stabilization. CE, CB, CC amplifiers and ACanalysis of single stage CE amplifier (re Model ). Field Effect Transistor: Construction andCharacteristic of JFETs. AC analysis of CS amplifier, MOSFET (Depletion andEnhancement)Type, Transfer Characteristic

10/31/2017 1REC 101 Unit II by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad

Bipolar Junction Transistor (BJT): Construction

• Transistor is 3-layer semiconductor device consisting of either n-p-nlayers (npn transistor) or p-n-p layers (pnp transistor).


Q

EEmitter

CollectorC

Base BQ

EEmitter

CollectorC

Base B

n npE C

B

p pnE C

B

npn transistor pnp transistor

Substrate

Collector

base

Emitter

Metal ContactOxide layer

Basic epitaxial planar structure

Bipolar Junction Transistor (BJT): Construction

• The arrow in the symbol defines the direction of emitter current.

• The emitter layer is heavily doped in comparison to collector.

• The outer layers have widths much greater than the sandwiched p-or n -type material. (typically 150:1)

• Doping of sandwiched layer is considerably less than outer layers(typically, 1:10 or less) resulting in low conductance (high resistance)

• The transistor is called bipolar junction transistor (BJT).Term bipolarreflects the fact that both polarity of charges (holes and electrons)participate in current flow.


BJT: Operation


• EB junction is forward biased and CBjunction is reverse biased

• Large number of majority carriersdiffuse across forward biased p-njunction into n material.

• In thin base with low conductivity, few carrier go to base terminalwhile most carriers diffuse across reverse biased junction. IB istypically in microamperes, while IC & IE are in milliamperes.

• Using KCL IE= IC + IB

• IC is mainly due to majority carriers. minority carriers (leakagecurrent: IC with emitter terminal open) also contribute in IC

IC = IC majority + ICO minority

E C

B

p n p

Majority current component

Minority current component

VEE VCC

Depletion layer

IE IC

IB

BJT: Common Base configuration

• Behaviour of BJT is described by two sets of characteristics; drivingpoint or input parameters and output parameters.

• The input set for the CB amplifier relates an input current (IE) to aninput voltage (VBE) for various levels of output voltage (VCB).

• The output set for the CB amplifier relates an output current (IC) toan output voltage (VCB) for various levels of input current (IE).


E C

B

VEEVCC

IE IC

IB

Q

VBE VCB

E C

B

VEE VCC

IE IC

IB

Q

VBE VCB

Common base configuration pnp transistor Common base configuration npn transistor



VCB (V)IE =0 mA

IE =1 mA

2 mA

3 mA

4 mA

5 mA

IC (mA)

6 mA

Cutoff region

Satu

rati

on

regi

on

Active region

ICO = ICBO BVCBO

0 10 20 30 40

6

5

4

3

2

1

0

Output characteristics of CB configuration

VBE (V)

VCB =1 V

IE (mA)

0 0.2 0.4 0.6 0.8

7

6

5

4

3

2

1

VCB =10 V

VCB =20 V

Input characteristics of CB configuration



Input characteristics:

• For a VCB, input characteristic is like diode. If transistor ON, VBE0.7 V

output characteristics: has 3 regions : active, cut-off, and saturation

Active region: normally employed for amplification

• EB junction is forward-biased, & CB junction is reverse-biased.

• For IE=0, collector current IC =ICO=ICBO while for higher IE, IC IE

Cut-off region

• EB junction & CB junction both reverse-biased & no Collector current

Saturation region

• EB junction & CB junction both is forward biased

• IC increases exponentially as the voltage VCB increases toward 0 V


Alpha ()

• Where IC and IE are the currents at the point of operation

• for practical devices alpha typically extends from 0.90 to 0.998

• Since alpha is defined solely for the majority carriers

IC = IE + ICBO

• Where ICBO is very small and can be ignored in some approximations

Breakdown voltage

• In active region VCB increases beyond a point, due to avalanche effect, IC rises suddenly.

• The largest permissible VCB before breakdown is labelled as VCBO


E

C

I

Igain current basecommon

BJT: Common Emitter Configuration

• The input set for the CE amplifier relates an input current (IB) to aninput voltage (VBE) for various levels of output voltage (VCE).

• The output set for the CE amplifier relates an output current (IC) toan output voltage (VCE) for various levels of input current (IB).


E

C

B

VBB

VCC

IE

IC

IB

Q

VBE

VCE

E

C

B

VBB

VCC

IE

IC

IB

Q

VBE

VCE

Common emitter configuration pnp transistor Common emitter configuration npn transistor



Output characteristics of CE configuration

VBE (V)

VCE =20 V

IB (mA)

0 0.2 0.4 0.6 0.8

70

60

50

40

30

20

10

VCE =10 V

VCE =1 V

Input characteristics of CE configuration

VCE (V)

IB =0 A

10 A

20 A

40 A

50 A

IC (mA)60 A

Satu

rati

on

regi

on

Active region

ICEO = ICBOVCE Saturation

0 5 10 15 20

6

5

4

3

2

1

30 A

Cutoff region



Input characteristics:

• For a VCE, input characteristic is like diode. If transistor ON, VBE0.7 V

output characteristics: has 3 regions : active, cut-off, and saturation

Active region: normally employed for amplification

• BE junction is forward-biased, & CB junction is reverse-biased.

• exists to the right of the vertical line at VCE saturation and above the curve for IB =0. For IB=0, collector current IC =ICEO=ICBO /(1-)

• IC curves are not as horizontal as in CB configuration, VCE affects IC .

Cut-off region: Region below the curve for IB=0,

Saturation region: region to the left of VCE saturation

• IC increases exponentially as VCB increases toward VCE saturation


Beta ()

• Where IC and IB are the currents at the point of operation

• for practical devices Beta typically ranges in between 50 to 400

Relation between Alpha and Beta


B

C

I

Igain current emitter Common

CC

CBCE

II

IIII

1

1

large] very is [as )1(1

CBOCBOCBO

CEO III

I

BJT: Common Collector Configuration

• CC configuration is also known as emitter follower configuration asthe emitter voltage follows the base voltage.

• This configuration is mostly used as a buffer.• CC configuration is used mainly for impedance-matching, as it has

high-input & low-output impedance, opposite to CB/CE Configuration


C

E

B

VBB

VEE

IC

IE

IB

Q

VBE

VCE

C

E

B

VBB

VEE

IC

IE

IB

Q

VBE

VCE

Common emitter configuration pnp transistor Common emitter configuration npn transistor

BJT: Common collector Configuration


Input characteristics of CC configuration

VCB (V)

VEC =2 V

IB (mA)

0 1 2 3 4 5 6

70

60

50

40

30

20

10

VEC =4 V

Output characteristics of CC configuration

VEC (V)

IB =0 A

10 A

20 A

40 A

50 A

IE (mA)

Satu

rati

on

regi

on

Active region

0 5 10 15 20

5

4

3

2

1

30 A

Cutoff region

• Input characteristics of the CC configuration is different as current IB

decreases with increase in base collector voltage• Output characteristics of the CC configuration are the same as of CE

configuration.

BJT: Amplification

• BJT can be used in two modes– As a switch (operates in cut-off / saturation region of characteristics)

– As an amplifier (operates in active region of characteristics)

• BJT can be employed as an amplifying device in active region (BEJunction forward biased and CB junction reverse biased).

• In amplification output ac signal power is greater than the input acsignal power

• Increased ac power is contributed by dc power of transistor biasing

• Amplification factor is ratio of output to input parameter


i

OV

V

VA gain Voltage

i

Oi

I

IA gain Current

ii

OO

i

OP

IV

IV

P

PA gain Power

rec101 unit ii (part 1) bjt characteristics

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