UNIT-3

Bipolar junction Transistor (BJT)

A bipolar transistor is a semiconductor device in which electric current flows due to electrons and holes BOTH, simultaneously. Thus both types of charges take part in the conduction of current through it. Hence it is called bipolar transistor.

A bipolar junction transistor (BJT) consists of two PN junctions formed by sandwiching either p-type or n-type semiconductor between a pair of opposite types.

These are two types 1. NPN 2. PNP transistorThe outer layers of the npn or pnp sandwich are called the

emitter and the collector, and the centre layer is termed the base.

Two pn junctions are formed with depletion regions and barrier voltages at each junction.

Contd…A transistor (PNP or NPN) has three

sections of doped semiconductors.

It consists of 3 terminals

1.emitter 2. Base 3. collector

1. section on one side that supplies charge carriers (electrons of holes) is called emitter. Which is heavily doped .

2. The middle section which form two pn-junctions between the emitter and collector is called the base

Contd…3. The section on the other side that collects the charges is

called the collector . Collector junction is always reverse biased. This section is moderately doped.

Where as emitter is heavily doped so that it can inject a large number of charge carriers into the base.

The base is lightly doped and very thin.The resistance of emitter diode is small and collector diode is

large.Therefore the transistor transfers the input signal current from

a low-resistance circuit to a high-resistance circuit.

Understanding of BJT

force – voltage/currentwater flow – current - amplification

Physical structure of BJT

The barrier voltages are negative on the p-side and positive on the n-side.

The base-emitter junction is forward, so that charge carriers are emitted into the base.

The collector-base junction is reverse-biased, and its depletion region penetrates deep into the base.

The base section is made as narrow as possible so that charge carriers can easily move across from emitter to collector.

The base is lightly doped, so that few charge carriers are available to recombine with the majority charge carriers from the emitter.

Most charge carriers from the emitter flow to the collector, a few flow out through the base terminal.

Operation of NPN and PNP

Contd …

NPN and PNP Transistors

Current components in BJT

Contd..It can be noted from the diagram the battery VEB forward biases

the EB junction while the battery VCB reverse biases the CB junction.

As the EB junction is forward biased the holes from emitter region flow towards the base causing a hole current IPE.

At the same time, the electrons from base region flow towards the emitter causing an electron current INE. Sum of these two currents constitute an emitter current IE = IPE +INE.

The ratio of hole current IPE to electron current INE is directly proportional to the ratio of the conductivity of the p-type material to that of n-type material.

Contd,..Not all the holes, crossing EB junction reach the CB junction

because some of the them combine with the electrons in the n-type base.

If IPC is the hole current at (Jc) CB junction. There will be a recombination current IPE - IPC leaving the base as shown in fig above

If emitter is open circuited, no charge carriers are injected from emitter into the base and hence emitter current IE =0.

Under this condition CB junction acts a a reverse biased diode and therefore the collector current ( IC = ICO) will be equal to te reverse saturation current.

Therefore when EB junction is forward biased and collector base junction is reverse biased the total collector current IC = IPC +ICO.

Transistor operating regions.

Contd...A transistor can be operated in three different regions as a) active regionb) saturation regionc) cut-off regionActive region: The transistor is said to be operated in active

region when the emitter-base junction is forward biased and collector –base junction is reverse biased.

The collector current is said to have two current components one is due to the forward biasing of EB junction and the other is due to reverse biasing of CB junction.The collector current component due to the reverse biasing of

thecollector junction is called reverse saturation current (ICO or ICBO)

andit is very small in magnitude.

Contd…Saturation Region:Transistor is said to be operated in saturation region when both

EB junction and CB junction are forward biased as shown. When transistor is operated in saturation region IC increases rapidly for a very small change in VC.

Cut-off region:When both EB junction and CB junction are reverse biased, the

transistor is said to be operated in cut-off region. In this region, the current in the transistor is very small and thus when a transistor in this region it is assumed to be in off state.

Transistor Model

Large signal Equivalent model of NPN BJT

BJT is used as a current controlled current source

BJT is current controlled current sourceFor a BJT to amplify we give input signal if suppose we use BJT in CE

configuration input is given at Emitter-Base junction and output is taken at Collector base junction, the input voltage increases or decreases the forward bias of the E-B junction affecting a change in the base current and we know that collector current is a function of base current collector current also varies so by selectively changing the base current we can effectively change the collector current

Because the operation of the transistor is determined by the current at the base. the principle equations of BJT operation are: Ic = h*Ib ,and Ie=Ib+Ic. Thus device operation is controlled by the input current.

Hence BJT is also called as current controlled current source.

α is common base current gain α dc = IC / IE

β is common emitter current gain β = IC / IB

IC = α dc ( IC + IB) IC = α dc IB / 1- α dc IC = β dc IB

β = α dc / 1- α dc

COMMON BASE CHARACTERISTICS

Common base characteristicsA pnp transistor with its base terminal

common to both the input (emitter-base) terminal and the output (collector-base) terminal.

The transistor is said to be connected in common-base configuration.

Voltmeters and ammeters are used to measure the input and output voltages and currents

Common-Base input characteristicsTo investigate the input characteristics, the

output voltage (VCB) is kept constant, and the input voltage (VEB) is set at several convenient levels.

At each input voltage, the corresponding input current (IE) is recorded.

The IE and VEB levels are then plotted to give the common-base input Characteristics

Input Characteristics

Common base Output characteristics

To investigate output characteristics, the input current (IE) is kept constant, and VCB is adjusted in convenient steps, and the corresponding values of IC are recorded.

The corresponding IC and VCB levels obtained when IE was held constant at 1 m A are potted, and the resultant characteristic is identified as IE=1mA. Similarly other characteristics are potted for IE equal to 2mA, 3mA, and so on.

Output Characteristics

COMMON EMITTER CHARACTERISTICS

COMMON-EMITTER CHARACTERISTCS

A pnp transistor with its Emitter terminal common to both the input (base-emitter) terminal and the output (collector-emitter) terminal.

The transistor is said to be connected in common-emitter configuration.

Voltmeters and ammeters are used to measure the input and output voltages and currents

Relation between transistor currents

Common-Emitter Input CharacteristicsTo investigate the input characteristics, the

output voltage (VCE) is kept constant, and the input voltage (VBE) is set at several convenient levels.

At each input voltage, the corresponding input current (IB) is recorded.

The IB and VBE levels are then plotted to give the common-emitter input Characteristics

Common EmitterOutput characteristicsTo investigate output characteristics, the

input current (IB) is kept constant, and VCE is adjusted in convenient steps, and the corresponding values of IC are recorded.

The corresponding IC and VCE levels obtained when IB was held constant at 10 µ A are potted, and the resultant characteristic is identified as IB=10 µ A. Similarly other characteristics are potted for IB equal to 20 µ A, 30 µ A, and so on.

Common emitter output characteristics of NPN-BJT

Early EffectAn increase in magnitude of collector voltage

increases (reverse bias) the space charge width increase at the output junction diode (C-B junction). Such action causes the effective base width ‘W’ to decreases. A phenomenon know as ‘Early Effect’

Punch-through (or) Reach -through

If an excessive reverse-bias voltage is applied to the collector-base junction, the device breakdown may occur

Breakdown can also result from the collector-base depletion region penetrating into the base (as the reverse bias increases) until it makes contact with emitter-base depletion region

This condition is known as punch through or reach through

A Very large currents can flow when it occurs, possible destroying the device

Common collector input characteristics

i/p characteristics

o/p characteristics

AMPLIFICATION in Bipolar junction transistors (BJTs)

i i B

o L C

v R i

v R i

Discussion of an amplification effect

CEBEi L

B C

vvR R

i i

B Ci iWith i ov v

DC Load Line

DC Load Line (Contd..) Load line:To draw load line, we have to find saturation current and the cutoff

voltage. After plotting these values on the vertical and the horizontal axes, a line is drawn

joining these two points, which represents DC load line. It represents all possible combinations of the collector current Ic and the

collector .voltage Vc (or Vce) for the given load resistor Rc. Saturation point

The point at which the load line intersects the characteristic curve near the collector current axis is referred to as the saturation point. At this point of time, the current through the transistor is maximum and the voltage across collector is minimum for a given value of load.

Therefore saturation current for the fixed bias circuit,Ic (sat) =Vcc/Rc

Cutoff pointThe point where the load line intersects the cutoff region of the collector curves is referred as the cutoff point (i.e. end of load line). At this point, collector current is approximately zero and emitter is grounded for fixed bias circuit. Therefore,       Vce (cut) =  Vc = Vcc

Q-Point (Static Operation Point)/Quiescent point

The intersection of the dc bias value of IB with the dc load line determines

the Q-point.When a circuit is designed to have a centered Q-point, the amplifier is said

to be midpoint biased.Midpoint biasing allows optimum ac operation of the amplifier.When the Q-point is centered, IC and VCE can both make the maximum

possible transitions above and below their initial dc values

Q-Point (contd..) When the Q-point is below

midpoint on the load line, the input signal may cause the transistor to cutoff. This can also cause a portion of the output signal to be clipped.

When the Q-point is above the center on the load line, the input signal may cause the transistor to saturate. When this happens, a part of the output signal will be clipped off

Cutoff and Saturation Clipping

DC Biasing Purpose of the DC biasing circuit is to turn the device “ON” .To place it in operation in the region of its characteristic where the device

operates most linearly, i.e. to set up the initial dc values of IB, IC, and VCE .

DC biasing is a static operation since it deals with setting a fixed (steady) level of current (through the device) with a desired fixed voltage drop across the device.

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WHY BIASING?

If the transistor is not biased properly, it would work inefficiently and produce distortion in output signal.

HOW A TRANSISTOR CAN BE BIASED? A transistor is biased either with the help of battery or associating a circuit with the transistor. The later method is more efficient and is frequently used. The circuit used for transistor biasing is called the biasing circuit.

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The Thermal Stability of Operating Point (SIco)

Stability Factor S:- The stability factor S, as the change of collector current with respect to the reverse saturation current, keeping β and VBE constant. This can be written as:

The Thermal Stability Factor : SThe Thermal Stability Factor : SIcoIco

SSIcoIco = = ∂∂IIcc

∂∂IIcoco

This equation signifies that IThis equation signifies that Icc Changes S Changes SIcoIco times as fast as I times as fast as Icoco

Differentiating the equation of Collector Current IDifferentiating the equation of Collector Current IC C = (1+= (1+ββ)I)Ico+ co+ ββIIbb & rearranging the terms we can write & rearranging the terms we can write

SSIco Ico ═ 1+═ 1+ββ

1- 1- ββ ( (∂∂IIbb//∂∂IICC))

It may be noted that Lower is the value of SIt may be noted that Lower is the value of S IcoIco better is the better is the stabilitystability

VVbebe,, ββ

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Various Biasing Circuits

Fixed Bias Circuit Fixed Bias with Emitter Resistor Collector to Base Bias Circuit Potential Divider Bias Circuit

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The Fixed Bias Circuit

15 V

C

E

B

15 V

200 k 1 k

The Thermal Stability Factor : SThe Thermal Stability Factor : SIcoIco

SSIcoIco = = ∂∂IIcc

∂∂IIcoco

General Equation of General Equation of SSIco Ico Comes out to Comes out to bebe

SSIcoIco ═ 1 + ═ 1 + ββ

1- 1- ββ ( (∂∂IIbb//∂∂IICC))

VVbebe, , ββ

Applying KVL through Base Circuit we Applying KVL through Base Circuit we can write, can write, IIb b RRbb+ V+ Vbebe= V= Vcccc

Diff w. r. t. IDiff w. r. t. ICC, we get (, we get (∂∂IIbb / ∂I / ∂Icc) = 0) = 0

SSIcoIco= (1+= (1+ββ) is very large) is very large

Indicating high un-stabilityIndicating high un-stability

IIbb

RRbb

RRCC

RRCC

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                                The fixed bias circuit is modified by attaching an external resistor to the emitter. This resistor introduces negative feedback that stabilizes the Q-point.

Fixed bias with emitter resistor

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The Collector to Base Bias Circuit

VCC

RC

C

E

B

RF

IIcc

IIbb

VVBEBE++

-- IIEE

This configuration employs negative feedback to prevent thermal runaway and stabilize the operating point. In this form of biasing, the base resistor RF is connected to the collector instead of connecting it to the DC source Vcc. So any thermal runaway will induce a voltage drop across the Rc resistor that will throttle the transistor's base current.

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Applying KVL through base circuit Applying KVL through base circuit

we can write (Iwe can write (Ibb+ I+ ICC) R) RCC + I + Ib b RRff+ V+ Vbebe= V= Vcccc

Diff. w. r. t. IDiff. w. r. t. ICC we get we get

((∂∂IIbb / ∂I / ∂Icc) = - R) = - RC C // (R (Rff + R + RCC))

Therefore, Therefore, SSIcoIco ═ (1+ ═ (1+ ββ) )

1+ 1+ [[ββRRCC//(R(RCC+ R+ Rff))]]

Which is less than (1+Which is less than (1+ββ), signifying better thermal ), signifying better thermal stabilitystability

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This is the most commonly used arrangement for biasing as it provide good bias stability. In this arrangement the emitter resistance ‘RRE’E’ provides stabilization. The resistance ‘RRE’E’ cause a voltage drop in a direction so as to reverse bias the emitter junction. Since the emitter-base junction is to be forward biased, the base voltage is obtained from R1-R2 network. The net forward bias across the emitter base junction is equal to VB- dc voltage drop across ‘RRE’E’. The base voltage is set by Vcc and R1 and R2. The dc bias circuit is independent of transistor current gain. In case of amplifier, to avoid the loss of ac signal, a capacitor of large capacitance is connected across RE. The capacitor offers a very small reactance to ac signal and so it passes through the condensor.

The Potential Divider Bias Circuit

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VCC

RC

C

E

B

VCC

R1

RE R2 IIEE

IICC

IIbb

The Potential Divider Bias Circuit

To find the stability of this circuit we To find the stability of this circuit we have to convert this circuit into its have to convert this circuit into its Thevenin’s Equivalent circuitThevenin’s Equivalent circuit

RRthth == R R11*R*R2 2 && VVthth == Vcc R Vcc R22

RR11+R+R2 2 RR11+R+R22

VCC

RC

C

E

B

VCC

R1

RE R2

RRthth == R R11*R*R2 2 && VVthth == Vcc R Vcc R22

RR11+R+R2 2 RR11+R+R22

RRthth == R R11*R*R2 2 && VVthth == Vcc R Vcc R22

RR11+R+R2 2 RR11+R+R22

RRthth == R R11*R*R2 2 && VVthth == Vcc R Vcc R22

RR11+R+R2 2 RR11+R+R22

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Applying KVL through input base circuit Applying KVL through input base circuit

we can write Iwe can write IbbRRThTh + I + IE E RREE+ V+ Vbebe= V= VThTh

Therefore, ITherefore, IbbRRThTh + (I + (ICC+ I+ Ibb) R) REE+ V+ VBEBE= V= VThTh

Diff. w. r. t. IDiff. w. r. t. ICC & rearranging we get & rearranging we get

((∂∂IIbb / ∂I / ∂Icc) = - R) = - RE E // (R (RThTh + R + REE))

Therefore, Therefore,

This shows that SThis shows that SIIcoco is inversely proportional is inversely proportional to Rto RE E and It is less than (1+and It is less than (1+ββ), signifying ), signifying better thermal stabilitybetter thermal stability

Thevenin Thevenin Equivalent Equivalent

CktCkt

ThRR

R

E

EIcoS

1

1

The Potential Divider Bias Circuit

VCC

RC

C

E

B

RE

RTh

VTh _ +

Thevenin Thevenin Equivalent Equivalent

VoltageVoltage

Self-bias ResistorSelf-bias Resistor

IIEE

IIbb

IICC

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This is the most commonly used arrangement for biasing as it provide good bias stability. In this arrangement the emitter resistance ‘RRE’E’ provides stabilization. The resistance ‘RRE’E’ cause a voltage drop in a direction so as to reverse bias the emitter junction. Since the emitter-base junction is to be forward biased, the base voltage is obtained from R1-R2 network. The net forward bias across the emitter base junction is equal to VB- dc voltage drop across ‘RRE’E’. The base voltage is set by Vcc and R1 and R2. The dc bias circuit is independent of transistor current gain. In case of amplifier, to avoid the loss of ac signal, a capacitor of large capacitance is connected across RE. The capacitor offers a very small reactance to ac signal and so it passes through the condensor.

The Potential Divider Bias Circuit

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Merits:• Operating point is almost independent of β variation. • Operating point stabilized against shift in temperature. Demerits: • As β-value is fixed for a given transistor, this relation can be satisfied either by keeping RE fairly large, or making R1||R2 very low.

If RE is of large value, high VCC is necessary. This increases cost as well as precautions necessary while handling.

If R1 || R2 is low, either R1 is low, or R2 is low, or both are low. A low R1 raises VB closer to VC, reducing the available swing in collector voltage, and limiting how large RC can be made without driving the transistor out of active mode. A low R2 lowers Vbe, reducing the allowed collector current. Lowering both resistor values draws more current from the power supply and lowers the input resistance of the amplifier as seen from the base. AC as well as DC feedback is caused by RE, which reduces the AC voltage gain of the amplifier. A method to avoid AC feedback while retaining DC feedback is discussed below.

Usage:The circuit's stability and merits as above make it widely used for linear circuits.

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SummaryThe Q-point is the best point for operation

of a transistor for a given collector current.The purpose of biasing is to establish a

stable operating point (Q-point).The linear region of a transistor is the

region of operation within saturation and cutoff.

Out of all the biasing circuits, potential divider bias circuit provides highest stability to operating point.