Chapter 4

BIPOLAR JUNCTION TRANSISTORS (BJTs)

Chapter 4

INTRODUCTIONWhat is transistor?

A three-terminal device whose output current, voltage and/or power are controlled by its input.

Commonly used in audio application as an amplifier, in switching application as a switch and in power supply voltage and current regulator circuit.

2 basic transistor types: BJT and FETThese two transistor differ in their operating

characteristic and their internal construction.

Describe the basic structure of the bipolar junction transistor (BJT)

Explain and analyze basic transistor bias and operation

Discuss the parameters and characteristics of a transistor and how they apply to transistor circuits

OBJECTIVES

LECTURE OUTLINE

1. BJT structure2. Basic BJT operations3. BJT Characteristics and Parameters4. BJT as an amplifier5. BJT as a switch6. TroubleshootingSummary

1. BJT STRUCTURE

1. BJT STRUCTURE The BJT is constructed with three doped semiconductor

regions separated by two pn junctions.

The three region are called emitter (E),base (B) and collector (C)

The BJT have 2 types:

1. Two n region separate by a p region – called npn

2. Two p region separated by a n region – called pnp

The pn junction joining the base region and the emitter region is called the base-emiter junction

The pn junction joining the base region and the collector region is call base-collector junction

The base region is lightly doped and very thin compared to the heavily doped emitter and the moderately doped collector region

1. BJT STRUCTURE

1. BJT STRUCTURE

BJT schematic symbolThe arrow on schematic symbol is

important because:Identify the component terminal The arrow is always drawn on the emitter

terminal. The terminal opposite emitter is collector and the center terminal is base.

The arrow always points toward n-type material If the arrow point toward base, transistor is pnp

type. If it points toward emitter, transistor is npn type.

1. BJT STRUCTURE

Transistor terminal current

1. BJT STRUCTURE

Transistor Currents: The directions of the currents in npn transistor and pnp

transistor are shown in the figure. The emitter current (IE) is the sum of the collector current (IC)

and the base current (IB)

IB << IE or IC The capital letter – dc value Transistor is a current-controlled device - the value of

collector and emitter currents are determined by the value of base current.

An increase or decrease in value of IB causes similar change in values of IC and IE.

CBE III

BDCC II Current gain (β) factor by which current increases from base of transistor to its collector.

1. BJT STRUCTURETransistor Voltages: VCC – collector supply voltage. This is a power supply

voltage applied directly to collector of transistor. VBB – base supply voltage. this is dc voltage used to

bias base of transistor. VEE – emitter supply voltage. dc biasing voltage and in

many cases, VEE is simply a ground connection.

1. BJT STRUCTURE Transistor Voltages: VC – dc voltage measured from collector terminal of

component to ground VB – dc voltage measured from base terminal to

ground. VE – dc voltage measured from emitter terminal to

ground.

1. BJT STRUCTURE Transistor Voltages:VCE – dc voltage measured from collector to emitter

terminal of transistor.VBE – dc voltage measured from base to emitter

terminal of transistor.VCB – dc voltage measured from collector to base

terminal of transistor.

2. BJT OPERATION

2. BJT OPERATIONTo operate the transistor properly, the two

pn junction must be correctly biased with external dc voltages.

The figure shown the proper bias arrangement for both npn and pnp transistor for active operation as an amplifier.

2. BJT OPERATION

Transistor is made of 3 separate semiconductor materials that joined together to form two pn junction.

Point at which emitter and base are joined forms a single pn junction base-emitter junction

Collector-base junction point where base and collector meet.

2. BJT OPERATION

Cutoff regionBoth transistor

junctions are reverse biased.

With large depletion region between C-B and E-B, very small amount of reverse current, ICEO passes from emitter to collector and can be neglected.

So, VCE = VCC

2. BJT OPERATION Saturation regionBoth transistor junctions

are forward-biased. IC reaches its maximum

value as determined by VCC and total resistance in C-E circuit.

IC is independently from relationship of β and IB.

VBE is approximately 0.7V and VCE < VBE.

EC

CCC RR

VI

2. BJT OPERATION

Active regionBE junction is forward

biased and the BC junction is reverse biased.

All terminal currents have some measurable value.

The magnitude of IC depends on the values of β and IB.

VCE is approximately near to 0.7V and VCE falls in ranges VBE<VCE<VCC.

3. BJT CHARACTERISTICS & PARAMETERS

3. BJT CHARACTERISTICS & PARAMETERS

DC Beta ( ) and DC Alpha ( ): The ratio of the dc collector current (IC) to the dc base current

(IB) is the dc beta

( ) = dc current gain of transistor Range value : 20< <200 Usually designed as an equivalent hybrid (h) parameter,

on transistor data sheet –

The ratio of the dc collector current (IC) to the dc emitter current (IE) is the dc alpha ( ) – less used parameter in transistor circuits

Range value-> 0.95< <0.99 or greater , but << 1 (Ic< IE )

DCDC

DC

DC

FEhDCFEh

DCDC

B

CDC I

I

E

CDC I

I

3. BJT CHARACTERISTICS & PARAMETERS Current and Voltage Analysis: The current and voltage can be identified as follow: Current: Voltage:

dc base current, dc voltage at base with respect to emitter,

dc emitter current, dc voltage at collector with respect to base,

dc collector current, dc voltage at collector with respect to emitter,

BI

EI

CI CEVCBV

BEV

Transistor current & voltage

reverse-biased the base-collector junction

forward-biased the base-emitter junction

3. BJT CHARACTERISTICS & PARAMETERS Current and Voltage Analysis:

When the BE junction is forward-biased, like a forward biased diode and the voltage drop is

Since the emitter is at ground (0V), by Kirchhoff’s voltage law, the voltage across is: …….(1)

Also, by Ohm’s law: ……..(2)

From (1) ->(2) :

Therefore, the dc base current is:

VVBE 7.0

BR BEBBR VVVB

BBR RIVB

BBBEBB RIVV

B

BEBBB R

VVI

3. BJT CHARACTERISTICS & PARAMETERS Current and Voltage Analysis:

The voltage at the collector with respect to the grounded emitter is:

Since the drop across is:

The dc voltage at the collector with respect to the emitter is:

where

The dc voltage at the collector with respect to the base is:

CRCCCE VVV

CCCCCE RIVV

BECECB VVV

CR CCRC RIV

BDCC II

Example 1

Determine IB, IC, IE, VCE and VCB in the circuit below. The transistor has a βDC=150.

Solution Example 1

VmAVRIVV CCCCCE 55.3)100)(5.64(10

When BE junction is FB, act as normal diode. So, VBE=0.7V.

The base current,

Collector current,

Emitter current,

Solve for VCE and VCB.

VVVV BECECB 85.27.055.3

AkR

VVI

B

BEBBB 430

10

7.05

mAAII BDCC 5.64)430(150

mAAmAIII BCE 9.644305.64

3. BJT CHARACTERISTICS & PARAMETERS Collector Characteristic Curve:

Using a circuit as shown in below, we can generate a set of collector characteristic curve that show how the collector current, Ic varies with the VCE voltage for specified values of base current, IB.

variable voltage

Collector characteristic curve:3. BJT CHARACTERISTICS & PARAMETERS

3. BJT CHARACTERISTICS & PARAMETERS Collector Characteristic Curve:

Assume that VBB is set to produce a certain value of IB and VCC is zero.

At this condition, BE junction and BC junction are forward biased because the base is approximately 0.7V while the emitter and the collector are zero.

IB is through the BE junction because of the low impedance path to ground, therefore IC is zero.

When both junctions are forward biased – transistor operate in saturation region.

As VCC increase, VCE is increase gradually, IC increase – indicated by point A to B.

IC increase as VCC is increased because VCE remains less than 0.7V due to the forward biased BC junction.

When VCE exceeds 0.7V, the BC becomes reverse biased and the transistor goes into the active or linear region of its operation.

3. BJT CHARACTERISTICS & PARAMETERS Collector Characteristic Curve:

Once BC junction is RB, IC levels off and remains constant for given value of IB and VCE continues to increase.

Actually IC increases slightly as VCE increase due to widening of the BC depletion region

This result in fewer holes for recombination in the base region which effectively caused a slight increase in indicated in point B and C.

When VCE reached a sufficiently high voltage, the reverse biased BC junction goes into breakdown.

The collector current increase rapidly – as indicated at the right point C

The transistor cannot operate in the breakdown region.

When IB=0, the transistor is in the cutoff region although there is a very small collector leakage current as indicated – exaggerated on the graph for purpose of illustration.

BDCC II

DC Load Line: Cutoff and saturation can be illustrated in relation to

the collector characteristic curves by the use of a load line. DC load line drawn on the connecting cutoff and

saturation point. The bottom of load line is ideal

cutoff where IC=0 & VCE=VCC. The top of load line is saturation where IC=IC(sat) & VCE =VCE(sat)

In between cutoff and saturation

is the active region of transistor’s

operation.

3. BJT CHARACTERISTICS & PARAMETERS

Example 2

Determine whether or not the transistor in figure below is in saturation. Assume VCE(sat) = 0.2V

Solution Example 2

First, determine IC(sat),

Now, see if IB is large enough to produce IC(sat),

With specific βDC, this base current is capable of producing IC greater than IC(sat). Thus, transistor is saturated and IC = 11.5mA is never reached. If further increase IB, IC remains at its saturation value.

mAkR

VVI

C

satCECCsatC 8.9

0.1

2.010)()(

mAkR

VVI

B

BEBBB 23.0

10

7.03

mAII BDCC 5.11)23.0(50

More About beta, :

-Important parameter for BJT

-Varies both IC & temperature

-Keeping the junction temperature

constant, IC cause

-Further increase in IC beyond this

max. point cause to decrease

Maximum Transistor Ratings:

-Specified on manufacturer’s data sheet

-Given for VCE,VBE,VBC,IC & power dissipation

-The product of VCE and IC must not exceed the max. power dissipation

-Both VCE and IC cannot be max. at the same time.

CE

DC V

PI (max)

FEDC h,

DC

DC

3. BJT CHARACTERISTICS & PARAMETERS

Derating :

-Specified at 25°C, for higher temp, PD(max) is less.

-Data sheet often give derating factor for determining at > 25°C

-Example: derating factor of 2mW/°C indicates that the max. power

dissipation is reduced 2mW for each degree increase in temperature.

(max)DP

(max)DP

3. BJT CHARACTERISTICS & PARAMETERS

ON CharacteristicsDC 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

Data Sheets

Data sheets give manufacturer’s specifications for maximum operating conditions, thermal, and electrical characteristics. For example, an electrical characteristic is βDC, which is given as hFE. The 2N3904 shows a range of β’s on the data sheet from 100 to 300 for IC = 10 mA.

4. BJT AS AN AMPLIFIER

4. BJT AS AN AMPLIFIER

BDCC II • Transistor amplify

current because

• IB is very small, so IC ≈ IE.

• Amplification of a small ac voltage by placing the ac signal source in the base circuit.

• Vin is superimposed on the DC bias voltage VBB by connecting them in series with base resistor RB.

• Small changes in the base current circuit causes large changes in collector current circuit.

Voltage gain: resistance emitter ac internal 'r e

Cec RIV

b

c

V V

VA

Ccc RIV

ee

Ce

b

c

V rI

RI

V

VA

'

e

C

V r

RA

'

ec II

•Ac emitter current is Ie ≈ Ic = Vb / r’e.

•Ac collector voltage, Vc equals ac voltage drop across Rc.

•Since , ac collector voltage is

•Vb is considered as ac input voltage where Vb=Vin - IbRB. Vc as the transistorac output voltage. The ratio of Vc to Vb is ac voltage gain, Av of the circuit.

•Substituting IeRC for Vc and Ier’e for Vb, yields:

4. BJT AS AN AMPLIFIER

5. BJT AS A SWITCH

5. BJT AS A SWITCH

A transistor when used as a switch is simply being biased so that it is in:

1. cutoff (switched off)

2. saturation (switched on)

Conditions in Cutoff

CCcutoffCE VV )(

C

satCECCsatC

R

VVI

)()(

DC

satCB

II

)(

(min)

Conditions in Saturation

Since VCE(sat) is very small compared to VCC, it can be neglected.

Neglect leakage current and all currentsare zero. BE junction is reverse biased.

5. BJT AS A SWITCH

Example 3

a) For the transistor circuit in below figure, what is VCE when VIN=0v?

b) What minimum value of IB is required to saturate this transistor if βDC is 200?

c) Calculate the maximum value of RB when VIN=5V.

Solution Example 3

a) When VIN=0V, the transistor is in cutoff (act as open switch), so VCE(cutoff)=VCC = 10V.

b) Since VCE(sat) is neglected (assumed 0V),

This is the value of IB necessary to drive transistor to point of saturation.

c) When transistor is ON, VBE=0.7V. The voltage across RB is

VRB=VIN – VBE = 5 – 0.7 = 4.3VBy Ohm’s Law, the maximum value of RB is:

AmAI

I

mAk

V

R

VI

DC

satCB

C

CCsatC

50200

10

100.1

10

)((min)

)(

kI

VR

B

RBB 86

50

3.4

(min)(max)

6. TROUBLESHOOTING

6. Troubleshooting

Troubleshooting a live transistor circuit requires us to be familiar with known good voltages, but some general rules do apply. Certainly a solid fundamental understanding of Ohm’s law and Kirchhoff’s voltage and current laws is imperative. With live circuits it is most practical to troubleshoot with voltage measurements.

6. Troubleshooting

Voltage measurements that are typically low are caused by a point that not “electrically connected to ground”. This called a floating point. This is typically indicative of an open.

More in-depth discussion of typical failures are discussed within the textbook.

Possible faults are open bias resistors, open or resistive connections, shorted connections and open or short internal to the transistor itself.

Correct voltage measurement

6. Troubleshooting

Testing a transistor can be viewed more simply if you view it as testing two diode junctions. Forward bias having low resistance and reverse bias having high resistance.

6. TroubleshootingThe diode test function of a multimeter is more reliable than using an ohmmeter. Make sure to note whether it is an npn or pnp and polarize the test leads accordingly.

6. Troubleshooting

In addition to the traditional DMMs there are also transistor testers. Some of these have the ability to test other parameters of the transistor, such as leakage and gain. Curve tracers give us even more detailed information about a transistors characteristics.

Summary

The bipolar junction transistor (BJT) is constructed of three regions: base, collector, and emitter. The BJT has two p-n junctions, the base-emitter junction and the base-collector junction.

The two types of transistors are pnp and npn.

For the BJT to operate as an amplifier, the base-emitter junction is forward biased and the collector-base junction is reverse biased (transistor in active region). Of the three currents IB is very small in comparison to IE and IC. Beta is the current gain of a transistor. This the ratio of IC/IB.

Summary

A transistor can be operated as an electronics switch. When the transistor is off it is in cutoff condition (no current). When the transistor is on, it is in saturation condition (maximum current).

Beta can vary with temperature and also varies from transistor to transistor.