Electronic Devices - 5bjt Chars

BJT Characteristics

BS P-III

Institute of Physics

BJT Characteristics Objects of the experiment 1. To determine transistor type (npn, pnp) , terminals, and material using a digital multimeter (DMM). 2. To graph the. collector characteristics of a transistor 3. To determine the value of the alpha and beta ratios of a transistor.

Introduction The word transistor has been derived from two words, "trans" means transfer of signal and "istor" as same general family resistor, so we can say transistor simply transfer the resistance. Transistor is a three terminal device, when these three terminals are used in biasing arrangement they form two p-n junctions, one junction is called emitter base junction and other is called collector base junction. Emitter base junction is forward biased and offer very low resistance and collector base junction is reverse biased and it offers a very high resistance. Therefore, the transistor transfers the signal from low resistance to high resistance. Types Of Transistors: There are two types of transistors. 1. Bipolar Junction Transistor (BJT) 2. Field effect Junction Transistor (FET). Bipolar Junction Transistor: BJT is a three terminal component, which is constructed with three doped semiconductor regions separated by two p-n junctions; the three regions are Emitter (E), Base (B) and collector(C). There are two types of BJT, such as a) NPN

b) PNP

NPN transistor has N-type emitter & collector and a P-type base, while PNP has P-type emitter & collector and a N-type base as shown in figure 1(a)and (b). Collector

iB

Base

Collector

iC

Base

NPN Transistor Emitter Holes

----- + + + --------- + + +

iB

B

Emitter

Electrons

Electrons ----- + + + E -----

PNP Transistor

iE

Holes - - - -

iC

+ VBE -

Fig.1 (a) NPN transistor

+ + + - - - + + + E +++ ++ - - - + + +

C + VCE -

iE

+ + + + +

iB

- - - +

B

+ +

+ VBE -

Fig.1 (b) PNP transistor

iC C + VCE -

BJT Characteristics

BS P-III

OUTPUT CHARACTERISTICS OF BJT: When dealing with the transistor configurations, characteristics curves are very important because they can predict the performance o1 a transistor. There are three curves, an input characteristic curve. a transfer characteristic curve and an output characteristic curve. Of these curves, the most useful for predicating the transistor performance is the output characteristics curve. The output characteristic curves for a BJT are a graph displaying the output voltages and currents for different input currents. It simply provides the V-I relationship at the output terminals, with either the input current or input voltage as parameters. For each transistor configuration, CE (Common Emitter), CC (Common Collector) and CB (Common Base), the output curves are slightly different. Common Emitter output characteristics curve: in common emitter configuration, the input is applied between base and emitter and output is taken from the collector and emitter as shown in figure 2. In CE input current is /B and output current is IC . In order to determine the output characteristics of CE configuration IB is maintained constant at several convenient levels. At each fixed level of base current ( /B ), collector emitter voltage (VCE) is adjusted in steps and corresponding values of collector-current (/C ) are recorded. Then for each level of IB. IC is plotted v/s VCE. A typical output

(mA)

characteristic for a BJT in CE mode is shown in figure 3.

(Volts)

Figure.2 Output characteristics of BJT in CE mode

EQUIPMENT & COMPONENTS EQUIPMENT: • Power supply COMPONENTS: • DMM (Digital multimeter) • Few connecting hard wires • 330 resistor • 1 K resistor

A bread board • NPN Transistor

•

• 1M

potentiometer

• 5K

potentiometer

Part I: To determine the type, terminals and material of given transistor a. Label the transistor terminals of Fig.3 as 1, 2, and 3. b. Set the selector switch of the multimeter to the diode scale. c. If the meter readings between two of the terminals will read high (O.L) regardless of the polarity of the meter leads connected then neither of these two terminals will be the base, hence you can identify the base terminanl. Terminal No.__________ is the base. 2

r

BJT Characteristics

BS P-III

Figure.3.Determination of the identities of BJT leads.

3

2

1

Terminal no.

Fill these boxes after compeleting all steps.

Terminal identity.

c. Connect the negative lead to the base terminal and the positive lead to either of the other terminals. If the meter reading is low (approximately 0.7 V for Si and 0.3 V for Ge or lower resistance), the transistor type is pnp . If the reading is high, the transistor type is npn. The transistor type is _______. d. For pnp type, connect the negative lead to the base terminal and the positive lead alternately to either of the other two terminals. The lower of the two readings obtained indicates that the base and collector are connected; thus the other terminal is the emitter. For npn type, connect the positive lead to the base terminal and the negative lead alternately to either of the other two terminals. The lower of the two readings obtained indicates that the base and collector are connected; thus the other terminal is the emitter. Terminal No.__________ is the emitter. Terminal No.__________ is the collector. If the readings in above step were approximately 700 mV, the transistor material is silicon. If the readings were approximately 300 mV, the material is germanium. If the meter does not have a diode testing scale, the material cannot be determined directly. The transistor Material is _______.

Part II: Collector Characteristics 1

Make the connections according to the fig:4.

+20V

+20V

1K C

1M

330 VRB V

B

NPN

VCE V

5K

VRC V

E

Figure 4.Common Emitter configuration. 2. 3. 4. 5.

Rotate both variable controls to minimum. Switch ON the power supply. Set 1 M potentiometer to give VRB = 3.3V. Starting from 0.5 V vary VCE according to table using 5K potentiometer and note down the corresponding values of VRC 6. Take similar readings with different values of VRB according to table.1. 7. Fill the following table and plot the graph representing the output characteristics of NPN transistor for CE mode, it will look like fig 2. 3

BJT Characteristics

BS P-III

TABLE.1. Data for Construction of Transistor Characteristic Curve and Calculations of Transisto r Parameters

VRB (V) (meas)

IB (uA) (calc)

VCE (V) (meas)

VRC (V) (meas)

IC

VBE

(mA) (calc)

(V) (meas)

IE (mA) (calc)

Alpha Beta (calc)

(calc)

2

4 6 3.3

10

8 10 12 14 16

2 4 6 6.6

20

8 10 12 14

2 4 9.9

30

6 8 10

2 13.2

40

4 6 8 2

16.5

50

4 6

Part III: Alpha and Beta ratios of transistor. a. b.

For each line of Table.1 calculat e the corresponding levels of Alpha and Beta using Alpha= I C /I E and Beta= IC /I B and complete the Table. Is there a significant variation in alpha an d beta from one region of the characteristics to another?

In which region are the largest values of the relative levels of VCE and IC. c.

Find the largest and smallest levels of on t h e plot of Fig. 8.3 using the notation

d. In general, did

found? Specify using

and m a r k their locations and

increase or decrease with increase in I C ?

e. In general, did increase or decrease with increase in V CE ? Was the effect of V CE on B greater or less t h a n the effect of I C ?

4

BJT Characteristics

BS P-III

REVIEW QUESTIONS: 1. 2. 3. 4. 5. 6.

How many regions of operation does a BJT have? Define saturation, cutoff and Active region? What is the behavior of device in active region? What is the relation between IC and/ B ? Give the BE and CB junction's condition in the three regions of operation. Does transistor follow ohm's law?

5

BJT Characteristics

BS P-III

Mode measurements VRB (V) (meas)

3.3

6.6

IB (uA) (calc)

10

20

VCE (V) (meas)

30

16.5

40

50

VBE (V) (meas)

IE (mA) (calc)

Alpha

Beta

(calc)

(calc)

0.24

0.238332

0.622 0.248332 0.959731 23.83317

4

0.243 0.241311

0.622 0.251311 0.960209 24.13108

6

0.245 0.243297

0.622 0.253297 0.960521 24.32969

8 10

0.247 0.245283 0.249 0.247269

0.622 0.255283 0.960828 24.5283 0.622 0.257269 0.96113 24.72691

12

0.251 0.249255

0.622 0.259255 0.961428 24.92552

14

0.251 0.249255

0.621 0.259255 0.961428 24.92552

16

0.254 0.252234

0.621 0.262234 0.961866 25.22344

2

0.753 0.747766

0.653 0.767766 0.97395

4 6

0.758 0.752731 0.764 0.758689

0.653 0.772731 0.974118 37.63654 0.652 0.778689 0.974316 37.93446

8

0.772 0.766634

0.652 0.786634 0.974575 38.33168

10

0.779 0.773585

0.651 0.793585 0.974798 38.67925

12

0.787 0.781529 0.794 0.788481

0.651 0.801529 0.975048 39.07646 0.65 0.808481 0.975262 39.42403

2

1.41

1.400199

0.669 1.430199 0.979024 46.67329

4

1.432 1.422046

0.669 1.452046 0.979339 47.40152

6

1.453 1.4429

0.668 1.4729

8

1.469 1.458788 1.482 1.471698

0.667 1.488788 0.979849 48.62628 0.667 1.501698 0.980023 49.0566

2

2.18

2.164846

0.681 2.204846 0.981858 54.12115

4

2.23

2.214499

0.68

6 8

2.26 2.32

2.24429 2.303873

0.678 2.28429 0.982489 56.10725 0.678 2.343873 0.982934 57.59682

2

3.08

3.05859

0.689 3.10859

4

3.14

3.118173

0.688 3.168173 0.984218 62.36346

6

3.19

3.167825

0.686 3.217825 0.984462 63.3565

10 13.2

IC (mA) (calc)

2

14

9.9

VRC (V) (meas)

37.38828

0.979632 48.09666

2.254499 0.982258 55.36246

0.983916 61.1718