EMT 1255 LAB 6 - Laboratory PDF

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EMT 1255

Lab 6: Bipolar Junction Transistor Chracteristics

DATE: 10/14/17

TABLE OF CONTENTS:

·

Objective

·

Theory

·

Procedure/ Data Tables

·

Conclusion

·

Questions

Objective of the experiment: After performing this experiment, you will be able to:

1) Measure and graph the collector characteristics curves for a bipolar junction transistor. 2) Use the characteristics curves to determine the

β

DC

of the transistor at a given

point.

Theoretical Background: A bipolar junction transistor (BJT) is a three-terminal device capable of amplifying an ac signal. The three terminals are called the base, emitter and the collector. BJTs consist of a very thin base material sandwiched in between two of the opposite type materials. They are available in two forms, either npn or pnp. The middle letter indicates the type of material used for the base, while the outer letters indicate the emitter and collector material. The sandwiched materials produce two pn junctions. These two junctions form two diodes - the emitter-base diode and the base-collector diode. BJTs are current amplifiers. A small base current is amplified to a larger current in the collector-emitter circuit. An important characteristics is the dc current gain, which is the ratio of collector current to base current. This is called the dc beta ( β

) of the

DC

transistor. Another useful characteristics is the dc alpha. The dc alpha is the ratio of the collector current to the emitter current and is always less than 1. For a transistor to amplify, power is required from dc sources. The dc voltage required for proper operation are referred to as bias voltages. The produce of bias is to establish and maintain the required operating conditions despite variations between transistors or changes in circuit parameters. For normal operation, the base-emitter junction is forward-biased and the base-collector junction is reversed-biased. Since the

base-emitter junction is forward-biased, it has characteristics of a forward-biased diode. A silicon bipolar transistor requires approximately 0.7 V of voltage across the baseemitter junction to cause base current. Materials Needed: Resistor: One 100 Ω resistor, one 33 kΩ resistor One 2N3904 npn transistors (or equivalent) Procedure: 1) Measure and record the resistance of the resistors listed in Table 6-1. Resistor

Listed Value

Measured Value

R1

33K Ω

32.8k Ω

R2

100 Ω

100 Ω

2) Connect the common-emitter configuration illustrated in FIgure 6-1. Start with both power supplies set to 0 V.

3) Without disturbing the setting of VBB, slowly increase VCC until +2.0 V is measured between the transistor’s collector and emitter. This voltage is VCE. Measure and record VR2 for this setting. Record VR2 in Table 6-2 in the column labeled Base Current = 50 μA.

Table 6-2 VCE

Base Current = 100 mA

Base Current = 50 mA

(measu

VR2

VR2

IC

Base Current = 150

IC

red)

mA VR2

2V

1.7 V

4V

1.8 V

0.017 A

3.15V 3.5V

0.018 A 6V

1.9 V

0.051 A 5.65 V

0.038 A 4V

0.02 A

0.045 A

5.1 V

3.8 V

2V

4.5 V

0.035 A

0.019 A 8V

0.0315 A

IC

0.0565 A 6.3 V

0.04 A

0.063 A

4) Compute the collector current, IC, by applying Ohm’s law to R2. Use the measured voltage, VR2, and the measured resistance, R2, to determine the current. 5) Without disturbing the setting of VBB, increase VCC until 4.0V is measured across the transistor’s collector to emitter. Measure and record VR2 for this setting. 6) Reset VCC for 0 V and adjust VBB until VR1 is 3.3 V. The base current is now 100 μA. 7) Without disturbing the setting of VBB, slowly increase VCC until VCE is 2.0V. Measure and record VR2 for this setting in Table 6-2 in the column labeled Base Current = 100 μA. 8) Increase VCC until VCE is equal to 4.0 V.

9) Reset VCC for 0 V and adjust VBB until VR1 is 4.95 V. The base current is now 150 μA. 10) Complete Table 6-2 by repeating steps 7 and 8 for 150 μA of the base current. 11) Plot three collector characteristics curves using the data tabulated in Table 6-2.

12) Use the characteristics curve you plotted to determine the β

DC

for the

transistor at a VCE of 3.0 V and a base current of 50 μA, 100μA, 150μA. At 50 μA it would be around 25 V. At 100μA it would be around 30 V. And at 150μA it would be around 35 V.

CONCLUSION From this experiment, we measured and graphed the collector characteristic curves for a bipolar junction transistor. Using the plot we were also able to determine β

DC

of the transistor at any given point.

Questions: 1) Does the experimental data indicate the β The experimental data shows that 2) What effect would a higher

β

DC

β

DC

DC

is a constant at all points?

is not a constant at all points.

have on the characteristics curves you

measured?. A higher

β

DC

would increase the voltage on the characteristic curve

3) What is the maximum power dissipated in the transistor for the data taken in the experiment?

0.3969 W

4) (a) The dc alpha of a bipolar transistor is the collector current, IC, divided by the emitter current, IE. Using this definition and IE = IC + IB, show that dc alpha can be written as:

αDC =

β DC β DC +1

(b) Compute dc alpha for your transistor VCE = 4.0 V and IB = 100 μA. 5) What value of VCE would you expect if the base terminal of a transistor were open? Explain your answer. The DC voltage would get added directly to the input giving it higher V CE....