Determination of Bipolar Junction Transistor (BJT)

Output characteristic (VCE Vs IC) curve for C-E configuration And observing it as Controlled Switch

OBJECTIVE The purpose of this experiment is to

• Measure and calculate the beta and alpha of a BJT. • Study data sheet of BJT

COMPONENTS REQUIRED • Transistor 2N3904 (NPN) • Resistors 100KΩ, 1KΩ PRELAB Refer to the specifications for the 2N3904 and find the following information:

a. transistor type b. maximum power it can dissipate at 250C c. maximum collector current rating d. maximum collector to emitter voltage rating e. operating temperature range f. minimum and maximum hFE (β) g. the emitter to base breakdown voltage

SUMMARY OF THEORY

A Bipolar junction transistor (BJT) is a three terminal device capable of amplifying an ac signal. The three terminals are called base (B), emitter (E), collector(C), and come in two flavors NPN and PNP. The middle letter indicates the type of material used for the base, while outer letters indicate the emitter and collector material.

BJTs are current amplifiers. They are used as switch as well. A small base current controls a larger current in the collector-emitter circuit. Consider the NPN transistor connected in Common-Emitter configuration shown in figure (a). If the base is at higher (≈ 0.7 volt) potential than the emitter then a current iB will flow into the base. The current into the collector is β times larger than the base current.

β is a ratio of collector current to base current and it specify the current gain for a transistor.

Β (Beta) = Collector current (IC) / Base Current (IB)

The quantity β (usually called hFE in transistor data sheets) is a characteristic of the individual transistor and is typically in the range from 100-500 for the types of transistors we will be using. The transistor can be thought of as a current amplifier device -- the current at the

output (collector or emitter) is β times larger than the current at the input (base). Another useful characteristic is the dc alpha (α) which is a ratio of collector current to emitter current.

For a transistor to amplify, power is required from dc sources. The dc voltages required for proper operation are referred to as bias voltages. The purpose of bias is to establish and maintain the required operating conditions despite variations between transistors or changes in the circuit parameters. For normal operation, the base emitter junction is forward-biased and base-collector junction reverse-biased. Since the base emitter junction is forward-biased, it has characteristics of a forward-biased diode. OUTPUT (VCE VS IC) CHARACTERISTIC CURVE FOR C-E CONFIGURATION:

Connect the given NPN transistor (2N3904) in the common emitter configuration as shown in figure (a).

Figure (a)

Find the values of Collector current (IC) and Collector-to-Emitter voltages (VCE) for

different values of Base current (IB) and Vcc.

By applying KVL we find the following formula to set the value of IB.

VBB = IB * RB + VBE VBB = IB * RB + 0.7

IB = (VBB - 0.7)/ RB

Fix the value of RB resistance to 100 KΩ and vary VBB to get different values of IB. Note down the value of β (also written as hfe value) from the datasheet of 2N3904 NPN

transistor. Find collector current for each value of base current using formula (IC = β * IB ).

IB = 0.02mA OR 20µA IC = β * IB = VBB= ………………………… Vary Vcc from 0V to 9v from power supply to get following values of VCE

VCC

0

1V

2V

3V

4V

5V

6V

7V

8V

9V

VCE

IC

IB = 0.04mA OR 40µA IC = β * IB = VBB= ………………………… Vary Vcc from 0V to 9v from power supply to get following values of VCE

VCC

0

1V

2V

3V

4V

5V

6V

7V

8V

9V

VCE

IC

IB = 0.06mA OR 60µA IC = β * IB = VBB= ………………………… Vary Vcc from 0V to 9v from power supply to get following values of VCE

VCC

0

1V

2V

3V

4V

5V

6V

7V

8V

9V

VCE

IC

IB = 0.08mA OR 80µA IC = β * Iβ = VBB= ………………………… Vary Vcc from 0V to 30v from power supply to get following values of VCE

VCC

0

1V

2V

3V

4V

5V

6V

7V

8V

9V

VCE

IC

After completing first table, draw the points on paper to get graph. It can be seen that for

a fixed value of IB we get a value of collector current IC which is β times amplified of Base current ( IB) i.e IC = β * IB.

It can also be seen that when VCC=0 no collector current flows. As we start increasing VCC voltage, the collector current starts to increase and reach its determined value when C-B junction is properly reverse biased. Further increase in VCC will not make a significant increase in collector current (IC).

From table values we also conclude that its base current (IB) that controls the collector current (IC). And VCC has very minute affect in changing collector current (IC). This behavior is called “switching behavior” of BJT.

Complete the graph for all tables values. IC (mA)

0 VCE (V)