BB

CC

EE

Transistors

• They are unidirectional current carrying devices like diodes with capability to control the current flowing through them

• Bipolar Junction Transistors (BJT) control current by current

• Field Effect Transistors (FET) control current by voltage

• They can be used either as switches or as amplifiers

• A transistor allows you to control the current, not just block it in one direction.

• A good analogy for a transistor is a pipe with an adjustable gate.

• A transistor has three terminals.

• The main path for current is between the collector and emitter.

• The base controls how much current flows, just like the gate controls the flow of water in the pipe.

BIPOLAR JUNCTION TRANSISTOR

• Two back to back P-N junctions• Emitter

– Heavily doped– Main function is to supply majority carriers to base

• Base– Lightly doped as compared to emitter– Thickness 10-6 m

• Collector– Collect majority carriers from emitter through base– Physically larger than the emitter region

EE

BB

NN PP NN CC EE

BB

PP NN PP CC

The BJT – Bipolar Junction TransistorThe BJT – Bipolar Junction Transistor

The Two Types of BJT TransistorsThe Two Types of BJT Transistors

npnnpn pnppnp

nn pp nnEE

BB

CC pp nn ppEE

BB

CC

Cross SectionCross Section Cross SectionCross Section

BB

CC

EE

Schematic Schematic SymbolSymbol

BB

CC

EE

Schematic Schematic SymbolSymbol

LP4 6

NPN Bipolar Junction Transistor

LP4 7

PNP Bipolar Junction Transistor

• The collector surrounds the emitter region, making it almost impossible for the electrons injected into the base region to escape being collected, thus making the resulting value of α very close to unity, and so, giving the transistor a large β

STRUCTURE

• Energy Band diagram of an unbiased transistor– N-region moves down and P-region moves up due to

diffusion of majority carriers across junction.– The displacement of band and carrier migration

stops when Fermi levels in the three regions are equalized

Biasing of Transistor– Base and emitter current when collector is open

• EB is forward biased- electron diffusion from emitter to base and hole diffusion from base to emitter

• Hence IB will be large and is equal to IE

• Collector is open so no current flows into collector

• Base and Collector current when the Emitter is open (ICBO)

• CB is reverse biased- electron from base flow into collector region and holes from collector flow into base

• This current is known as reverse saturation current

• The base current IB will be small and is equal to ICBO

• Four Ways of Transistor biasing– Both EB and CB junctions are fwd biased- Huge

current flows through base. The transistor is said to be operating in Saturation region (mode)

– Both EB and CB junctions are reverse biased- The transistor is said to be operating in cut off region (mode)

– EB junction is fwd biased and CB junction is reverse biased. The collector current is controlled by emitter current or base current- The transistor is said to operate in Active region (mode)

– EB junction in reverse biased and CB junction in fwd biased- inverted region (mode)

Transistor Biasing-Active RegionWhen both Emitter and Collector are closed

• Emitter-base junction is forward biased• Collector-base junction is reverse biased

• DC emitter supply voltage (VEE)- Negative terminal of VEE is connected to emitter

• DC collector supply voltage (VCC)- Positive terminal of VCC is connected to collector

• IB becomes very small and IC will be as large as IE

NN PP NN

VVEEEE VVCCCC

IIEE IICC

IIBB

Transistor currents• Forward biasing from base to emitter narrows the BE

depletion region• Reverse biasing from base to collector widens the

depletion CB region• Conduction electrons diffuse into p-type base region• Base is lightly doped and also very thin- so very few

electron combine with available hole and flow out of the base as valence electrons (small base electron current)

N P N

VEE VCC

IE IC

IB

• Sufficient holes are not avail in base – remote possibility of joining of electrons with holes

• Electron concentration is large on emitter side and nil on collector

• Electrons swiftly move towards collector• At CB junction they are acted upon by strong electric

field due to reverse bias and are swept into collector

Transistor currents• Most of the electrons diffuse into CB depletion region

• These electrons are pulled across the reverse biased CB junction by the attraction of the collector supply voltage and form the collector electron current. Therefore

IE= IC + IB

• 1-2% of emitter current goes to supply base current and 98-99% goes to supply collector current

• Moreover, IE flows into the transistor and IB & IC flow out of transistor

• Current flowing in is taken as positive and currents flowing out are taken as negative

• The ratio of the number of electrons arriving at collector to the number of electrons emitted by the emitter is called base transportation factor

Important Biasing Rule• Both collector and base are positive with respect to

emitter• But collector is more positive than base• Different potentials have been designated by double

subscripts as shown in the figure

• VCB (Collector is more positive than base) and VBE (base is more positive than emitter)

++ C

- E

+B

VCB

VBE

E C

+B

- ++

VBE VCB

Transistor circuit configuration• There are of three types

– Common base (CB) OR grounded base– Common emitter (CE) OR grounded emitter– Common collector (CC) OR grounded collector

• Common is the term used to denote the electrode that is common to the input and output circuits and it is generally grounded

• Common-Base Biasing (CB) : input = VBE & IE

• output = VCB & IC

• Common-Emitter Biasing (CE): input = VBE & IB

• output = VCE & IC

• Common-Collector Biasing (CC): input = VBC & IB

• output = VEC & IE

Common Base Configuration• Input signal applied between emitter & base• Output is taken from collector & base• Ratio of collector current to emitter current

is called dc alpha (dc) of a transistor

E C

+

B

- ++

VBE VCB

• The subscript dc on signifies that this ratio is defined from dc values of IC and IE

• There is also an ac which refers to the ratio of change in collector current to the change in emitter current

• For all practical purposes dc= ac=

• IE is taken as positive (flowing into transistor) and IC is taken as negative (flowing out of transistor)

• is the measure of quality of a transistor- higher its values, better is the transistor

• Value ranges from 0.95 to 0.999

EdcC

E

Cdc

II

OR

I

I

Common Emitter Configuration• The input signal is applied between the base and emitter

and the output signal is taken out from the collector and the emitter

• Ratio of collector current to base current is called dc beta (dc) of a transistor

C

E

+B

-

BC

B

C

II

OR

I

I

Relation between and

E

C

I

I

B

C

I

Iand

B

E

I

I

CEB III usingB

C

I

Ithen becomes

CE

C

II

I

or

1//

/

ECEE

EC

IIII

II

1 or 1 or 1/

Common Collector Configuration• The input signal is applied between the base and

collector and the output signal is taken out from the emitter-collector circuit

• Ratio of emitter current to base current is

11/

.B

C

C

E

B

E

I

I

I

I

I

I

From the figureC

E

+B- BBBCBE IIIIII 1

Output current=(1+) x Input current

Relation between transistor currents

CBE III ::We know

EBC III and

EEEEC

B IIIII

I

11

1/

1

1

and

because

1

11We get

• This shows that emitter current initiated by the forward biased emitter base junction is split into two parts

• (1-)IE which becomes base current in the external circuit

IE which becomes collector current in the external circuit

Therefore

:)1(:1

:)1(:

EEE III

Static Characteristics• Common Base Static characteristics

–Input characteristics. IE varies with VBE when voltage VCB is held constant

• VCB is adjusted with the help of R1

• VBE is increased and corresponding values of IE are noted

• The plot gives input characteristics• Similar to the forward characteristics of P-N diode• This characteristics is used to find the input

resistance of the transistor. Its value is given by the reciprocal of its slope

Rin= VBE / IE

BJT Input CharacteristicsBJT Input Characteristics

VVBEBE

IIEE

2 2 mAmA

4 4 mAmA

6 6 mAmA

8 8 mAmA

0.7 V0.7 V

VCC

E C

+

B

VBE

VCB

ICIE

VEE

R1R2

Static Characteristics• Common Base Static characteristics

– Output characteristics. IC varies with VCB when IE is held constant

• VBE is adjusted with the help of R2 and IE is held constant

• VCB is increased and corresponding values of IC are noted

• The plot gives output characteristics

• Then IE is increased to a value little higher and whole process is repeated

• The output resistance of the transistor is given by

Rout= VCB / IC

VCC

E C

+

B

VBEVCB

ICIE

VEE

R1R2

IICC flows even when V flows even when VCBCB=0 for different values of =0 for different values of

IIEE(due to internal junction voltage at CB junction)(due to internal junction voltage at CB junction)

IICC flows even when I flows even when IEE=0 (Collector leakage current or =0 (Collector leakage current or

reverse saturation current Ireverse saturation current ICBOCBO))

The output resistance is very high (500k)

Sa

tura

tio

n R

egio

nS

atu

rati

on

Reg

ion

IIEE

IICC

VVCBCB

Active Active RegionRegion

CutoffCutoff

IIEE = 0 = 0

BJT Output CharacteristicsBJT Output Characteristics

Static Characteristics– It can be seen that IC flows even when VCB is zero

– It is due to the fact that electrons are being injected into base due to forward biased E-B junction and are collected by collector due to action of internal junction voltage at C-B junction

– Another important feature is that a small amount of collector current flows even when the emitter current IE

is zero called collector leakage current (IICBOCBO)

– When VCB is permitted to increase beyond a certain value, IC increases rapidly due to avalanche breakdown

• This characteristics may be used to find ac

ac =IC/ IE

LP4 30Common Emitter(CE) Connection

Common Emitter Configuration• Transistor is biased in active region• Called CE because emitter is common to both VBB and VCC• VBB forward biases the EB junction and VCC reverse biases the CB

VCC

BC

B

E

VBE

VCE

ICIB

VBB

R1R2

Static Characteristics

• Common Emitter Static characteristics– Input characteristics. IB varies with VBE when

voltage VCE is held constant

• VCE is adjusted with the help of R1

• VBE is increased and corresponding values of IB are noted

• The plot gives input characteristics

• Procedure is repeated for different (constant) values of VCE

• This characteristics is used to find the input resistance of the transistor. Its value is given by the reciprocal of its slope

Rin= VBE / IB

VVBEBE

IIBB

2 mA2 mA

4 mA4 mA

6 mA6 mA

8 mA8 mA

0.7 V0.7 V

Static Characteristics• Common Emitter Static characteristics

– Output characteristics. IC varies with VCE when IB is held constant

• IB is held constant

• VCE is increased and corresponding values of IC are noted

• The plot gives output characteristics

• Then IB is increased to a value little higher and whole process is repeated

• The output resistance in this case is very less as compared to CB circuit and is given by

Rout= VCE / IC

As VAs VCECE increases from zero, I increases from zero, ICC rapidly increases to saturation level for a fixed rapidly increases to saturation level for a fixed

value of Ivalue of IBB

IICC flows even when I flows even when IBB=0 (Collector leakage current or reverse saturation =0 (Collector leakage current or reverse saturation

current Icurrent ICEOCEO), the transistor is said to be cutoff), the transistor is said to be cutoff

When VCB is permitted to increase beyond a certain value, IC increases rapidly due to avalanche breakdown

This characteristics may be used to find ac ac =IC/ IB

VVCECE

IICC

Active Active RegionRegion

IIBB

Saturation RegionSaturation RegionCutoff RegionCutoff Region

IIBB = 0 = 0

Region of Operation

Description

Active Small base current controls a large collector current

Saturation VCE(sat) ~ 0.2V, VCE increases with IC

Cutoff Achieved by reducing IB to 0, Ideally, IC will also equal 0.

BCE III

EdcCE

Cdc II

I

I

CBOCE III ,0where

Therefore, in general CBOEdcC III

Common Base

Common Emitter

BCE III

BdcCB

Cdc II

I

I

(Reverse saturation current)

1

where CEOCB III ,0

CEOBdcC III

(Reverse saturation current)

Relationship between dc and dc

1

and

Common Base Formulas

VCC

E C

B

VBE

VCB

ICIE

IB

VEE

RLRE

E

BEEEEBEEEEE R

VVIVRIV

0

Where VBE=0.3 V for Ge and 0.7 V for Si

Generally VEE>>VBE so IE=VEE/RE

LCCCCBCBLCCC RIVVVRIV 0

and

Common Emitter Formulas

B

BEBBEBEBBBB R

VVIVRIV

0

LCCCCECELCCC RIVVVRIV 0

and

VCC

E C

B

VBE

VCE

ICIB

IE

VBB

RLRB

DC DC and DC and DC

= Common-emitter current gain= Common-emitter current gain

= Common-base current gain= Common-base current gain

= I= ICC = I = ICC

IIBB I IEE

The relationships between the two parameters are:The relationships between the two parameters are:

= = = =

+ 1+ 1 1 - 1 -

Note: Note: and and are sometimes referred to as are sometimes referred to as dcdc and and dcdc

because the relationships being dealt with in the BJT because the relationships being dealt with in the BJT are DC.are DC.

BJT ExampleBJT ExampleUsing Common-Base NPN Circuit ConfigurationUsing Common-Base NPN Circuit Configuration

++__

++__

Given: IGiven: IBB = 50 = 50 A , I A , ICC = 1 mA = 1 mA

Find: IFind: IEE , , , and , and

Solution:Solution:

IIEE = I = IBB + I + ICC = 0.05 mA + 1 mA = 1.05 mA = 0.05 mA + 1 mA = 1.05 mA

= I= ICC / I / IBB = 1 mA / 0.05 mA = 20 = 1 mA / 0.05 mA = 20

= I= ICC / I / IEE = 1 mA / 1.05 mA = 0.95238 = 1 mA / 1.05 mA = 0.95238

could also be calculated using the value of could also be calculated using the value of with the formula from the previous slide. with the formula from the previous slide.

= = = 20 = 0.95238 = 20 = 0.95238

+ 1 21+ 1 21

IICC

IIEE

IIBB

VVCBCB

VVBEBE

EE

CC

BB

Transistor as an amplifier

Transistor as an amplifier• An electronic circuit that causes an increase in the

voltage or power level of a signal• It is defined as the ratio of the output signal voltage to

the input signal voltage

i

o

v

v

geInputVolta

ageOutputVoltG

VVEEEE VVCCCC

IIEE IICC

IIBB RRLL

i

iE r

vI

In the figure we see that an output voltage is developed across RL

The dc voltage VEE is a fixed voltage and causes a dc current IE to flow through EB junction

When the ac voltage Vi is super-imposed on VEE, the emitter base voltage varies with time

Say if VEE =10V and the peak voltage of Vi is is 1V, the EB voltage swings from 9V to 11V

The causes corresponding variations in IE and IC which gives Vo

The emitter variation due to EB voltage variation can be expressed as

The collector current IC changes by

EdcC II This current IC flows through RL causing a voltage drop

Liidco

LEdco

LCo

Rrvv

RIv

RIv

/

iLdcio rRvvG //

Hence

as 1dc

Where ri is very small (100 ) and RL is of the order of kilo-ohms. It means Vo is larger than Vi indicating that the transistor has amplified small Vi to a larger Vo

Problems• In the CE Transistor circuit VBB= 5V, RBB=

107.5 k, RCC = 1 k, VCC = 10V. Find IB, IC, VCE, and the transistor power dissipation

BB

BEBBB R

VVI

In the CE Transistor circuit shown earlier VBB= 5V, RBB= 107.5 k, RCC = 1 k, VCC = 10V. Find IB, IC, VCE, and the transistor power dissipation using the characteristics as shown belowBy Applying KVL to the base emitter circuit

By using this equation along with the iB / vBE characteristics of the base

emitter junction, IB = 40 A

By Applying KVL to the collector emitter circuit

CC

CECCC R

VVI

By using this equation along with the iC / vCE characteristics

of the base collector junction, iC = 4 mA, VCE = 6V

10040

4

A

mA

I

I

B

C

Transistor power dissipation = VCEIC = 24 mW

We can also solve the problem without using the characteristics

if and VBE values are known

iB

100 A

0

5V vBE

Input Characteristics Output Characteristics

iC

10 mA

0

vCE

100 A

80 A

60 A

40 A

20 A