Lecture 3 BJT2

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Electronic DevicesKEEE 2224

Lecture 3

BJT

Modes of Operation, Current Amplifications,I & V Characteristics

Dr. Ghafour Amouzad Mahdiraji

September 2012



BJT Operation

• In the forward active operation

mode, the B-E junction is forward

biased so majority carriers from

emitter are injected across the B-E

junction into the base.

• The B-C junction is reverse biased,

so the minority carrier concentration

at the edge of the B-C junction is

ideally zero (like shown in the fig).• The large gradient in the minority

carrier concentration means that

majority carriers injected from the

emitter into the base will diffuseacross the base region into the B-C

space charge region, where the E-

field will sweep the minority carrier

into the collector.



Transistor Configurations

There are three possible configurations for BJT transistor:

• Common-base

• Common-collector

• Common-emitter



Common-Base Configuration

The base is common to both input (emitter–base) and

output (collector–base) of the transistor.

The arrow in the

graphic symbol defines

the direction of emitter

current through the

device.

npn : not pointing inpnp : pointing in



Common–Emitter Configuration

The emitter is common to both input

(base-emitter) and output (collector-

emitter).

The input is on the base and the

output is on the collector.



Common–Collector Configuration

The input is on the base and the output is on the emitter.



Transistor Modes of Operation

There are also four modes of operation for BJT transistor:

• Forward active

• Cutoff

• Saturation

• Inverse active



Cutoff Region of Operation

• If the B-E voltage is zero or reverse biased (V BE ≤

0), the majority carrier from the emitter will not be

injected into the base. Since the B-C junction is

also reversed biased; thus, the emitter and collectorcurrent ideally will be zero for this case. This

condition is referred to as cutoff, where all current

in the transistor are zero or I C = 0.

• In the cutoff region the C-B junctions is

reverse-biased and the B-E voltage is

either zero or reverse-biased.



Forward Active Region of Operation

• When the B-E junction is forward biased, an emitter current will be generated,

and the injection of electrons into the base results in a collector current. This

condition is referred as the forward-active operating mode.

• In this mode of operation, the majority

carriers from the emitter are injected

across the B-E junction into the base.

• These injected carriers create an excess

concentration of minority carriers in the

base, where the E-field will sweep the

minority carriers into the collector and

results in a collector current.

• The forward active or just simply active

region normally employed for linear

(undistorted) properties.



Saturation Region of Operation

• As the forward biased B-E voltage increases, the collector

current and hence V R will also increase. The increase in V R

means that the reverse biased C-B voltage decreases.

• A slight increase in I C beyond this point will

cause a slight increase in V R and the B-C

junction will become forward biased (V CB <

0). This condition is called saturation.

• In the saturation region the B-E and B-C

junctions are both forward biased and the

collector current is no longer controlled by

the B-E voltage.

• At some point, the collector current may become large

enough that the combination of V R and V CC produces 0

V across the B-C junction.

CE RCC V V V KVL: )(

BE CBC C V V R I



Inverse Active Region of Operation

• Inverse active region is opposite of the Forward Active Region, when B-E

junction is reversed biased and the B-C junction is forward biased.

• In this case, transistor is operating "upside down", the role of emitter andcollector are reversed.

• Since the transistor is not a symmetrical device; therefore, the inverse-

active characteristics will not be the same as the forward-activecharacteristics.



BJTAmplifications

in Different Configurations



• Ideally, the minority

carrier concentration in

the base is a linear

function of distance,

which implies norecombination.

• Considering the ideal

case, the collector

current can be writtenas a diffusion current:

• where e is magnitude of electronic charge (C), Dn is minority carrier electron diffusion coefficient

(cm2/s), A BE is the cross sectional area of the B-E junction (cm2), n B0 is the thermal-equilibrium

electron concentration in the base, and V t is the thermal voltage.

Minority carrier distribution and basic

current in a forward-biased npn BJT.

Collector Current

t

BE B

B

BE n

B

B BE n BE nC

V V n

x AeD

xn AeD

dx xdn AeD I exp

00)0()( 0



Collector Current

• Considering only the magnitude:

t

BE

sC V

V

I I exp

• The collector current is controlled by the B-E voltage; it means, the current

at one terminal of the device is controlled by the voltage applied to the other

two terminals of the device. This is the basic transistor action.

• where I s is the ideal reverse-biased saturation current (A)

CO I

E BI

C I

1

• The emitter injection efficiency γ isHole

e- flow

I C

I B

I E

p+ pn

12

345

I Ep

or I E 1

I En

or I E 2

I C

• The collector current is made up entirely of those injected at the emitter,

which are not lost to recombination in the base.

• Thus, I C is proportional to a fraction ( B) of the holes component of the

emitter current I E 2 , plus the saturation current

21

1

E I E I

E I

I CO : Collector current when emitter is O pen

and B is the base transport factor .



Emitter Current

• I E 2 is only for B-E junction current so this component of emitter current is

not part of the collector current.

21MinorityMajority E E E E E I I I I I

Hole

e- flow

I C

I B

I E

p+ pn

1

2

345

I Ep

or I E 1

I En

or I E 2

I C



Emitter Current

Input Characteristics

This curve shows the relationship between

input current ( I E

) to input voltage (V BE

) for

three output voltage (V CB) levels.

At a fixed V CB, by increasing V BE , I E will be

increase in a manner like diode

characteristics, where after around 0.7 V, thetransistor goes to the 'On' state.

On the other hand, increasing V CB have very

small effect on characteristics of transistor,

which can be ignored.

Approximation: once a transistor is in the'on' state, the base-to-emitter voltage will be

assumed to beSilicon)(forV0.7

E V

Input or driving point characteristics for acommon-base silicon BJT amplifier.



Common-Base Characteristics I

• The common-base current gain is referred to the ratio of I C to I E , assuming

I CO = 0.

• Since I C < I E , therefore α < 1.

• We would then like the α to be as close to unity as possible.• Note that the emitter current is an exponential function of the B-E voltage

and the collector current is I C = α I E .

B

E I

E I

E BI

E I

C I

21

1

• Approximately, we can say, the

collector current is independent of theB-C voltage as long as the B-C

junction is reverse biased.

• In ideal case, the bipolar transistor in

the common-base configuration actslike a constant current source.

• Thus, the bipolar transistor in the

common-base configuration does not

have any current amplification.

Ideally: α = 1

In reality: α is between 0.9 and 0.998



Common-Base Characteristics II

This graph demonstrates the output current (IC) to an output

voltage (VCB) for various levels of input current (IE).

Output Characteristics

Practical output or collector characteristics for a common-base BJT amplifier.



Common-Base Characteristics III

• Active – Operating range of

the amplifier.

• Cutoff – The amplifier isbasically off . There is voltage,

but little current.

• Saturation – The amplifier is

fully on. There is current, butlittle voltage.

As the emitter current increases above zero, the collector current increases to a

magnitude essentially equal to that of the emitter current.

Note also the almost negligible effect of V CB on the collector current for the active

region.

Approximation: E I C I



Example

a) using the characteristics curves, determine the resulting collector current if

I E = 3 mA and V CB = 10 V.

b) using the characteristics curves, determine the resulting collector current if

I E

remains at 3 mA but V CB

is reduced to 2 V.

c) using the characteristics curves, determine V BE if I C = 4 mA and V CB = 20 V.

d) Repeat part (c) using the approximation of V BE = 0.7 V in on state.

Answer: I C ≈ I E = 3 mA

Answer: I C = 3 mA

Answer:

V BE = 0.74 V

Answer: V BE = 0.7 V



Example: Transistor Voltage Amplification

Voltage Gain:

V50k Ω5ma10

mA10

10mA20Ω

200mV

))(( R L

I L

V

i I

L I

E I

C I

i R

i V

i I

E I

Currents and Voltages:

250200mV

50V

i V

LV v A

Typical values of amplification for the common-base configuration vary from 50 to 300.

The current amplification ( I C / I E ) is always less than 1 for common-base configuration.

+

I i

I C

I B R L

vi



Base Current• If B is the base transport factor or represents the carriers transferred to

collector with out recombination, then (1- B) means the fraction of the

carriers that are recombined in the base.

• Ideally ( I CO = 0), there are two current components related to the base,

1. the majority carriers from the base diffuse into the emitter, I E 2

2. the majority carriers (electron here) in the base recombine with the

minority carriers (holes) diffused into the base from emitter, (1- B) I E 1

12 )1( E E B I B I I



Beta ( β )• The ratio of collector current to base current is a constant since both

currents are directly proportional to exp(v BE /V t ). This ratio is the base-to-

collector current amplification factor, which usually obtained in common-

emitter configuration:

211

2111

1)1(

2

1

E I E I E I E I E I E I

B

B

E I B

E I

E BI

B I

C I

• Normally, the base current will berelatively small so that, in general,

the common-emitter current gain is

much larger than unity (on the

order of 100 or larger).

11 B

B

B I

C I



Common-Emitter Characteristics

Collector Characteristics Base Characteristics



Example: Beta ( )

108

A25

mA2.77.5VCE

β

Determining from a

Graph at V CE

= 7.5 and

I B

= 25 μA



Common–Collector Characteristics

The characteristics are

similar to those of the

common-emitter

configuration, except the

vertical axis is I E

.

Opposite to common-base

and common-emitter

configuration, the common-

collector is used primarily for

impedance-matching

purposes since it has a highinput impedance and low

output impedance.

I E (mA)



Operating Limits for Each Configuration

VCE is at maximum and

IC is at minimum

(ICmax= ICEO) in the

cutoff region.

IC is at maximum and

VCE is at minimum

(VCE max = VCEsat =

VCEO) in the saturationregion.

The transistor operates

in the active region

between saturation andcutoff.



Power Dissipation

Common-collector:

CCBCmax IVP

CCECmax IVP

ECECmax IVP

Common-base:

Common-emitter:



Transistor Specification Sheet



Transistor Specification Sheet



Transistor Testing

• Curve Tracer

Provides a graph of the characteristic curves.

• DMM

Some DMMs measure DC.

• Ohmmeter



Transistor Terminal Identification

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Lecture 3 BJT2