Project-3 Transistor

Objective

The objective of this project is to find β of a particular transistor by setting several values of base current and measuring the corresponding values of collector current. Next, you divide the values of collector current by the values of the base current to determine β. The value of β will be almost the same for all the measured values of current. This demonstrates that β is a constant for a transistor.
 

General Instructions

While the circuit is set up, measure the collector voltage for each current value. This demonstrates (experimentally) some points that are covered in future problems. As you perform the project, observe how the collector voltage VC drops as the collector current increases.
 

Parts List

• One 9 V battery (or a lab power supply)

•  One multimeter set to μA

• One multimeter set to mA

• One multimeter set to measure DC voltage

• One 10 kΩ resistor

• One 510 ohm resistor

• One 2N3904 transistor

• One breadboard

• One 1 MΩ potentiometer

• Step-by-Step Instructions 

Set up the circuit shown in Figure 3.19 on a breadboard. If you have some experience in building circuits, this schematic (along with the previous parts list) should provide all the information you need to build the circuit. If you need a bit more help building the circuit, look at the photos of the completed circuit in the “Expected Results” section.

Fig 3.19

Follow these steps, recording your measurements in the blank table following the steps.

1. Set the potentiometer to its highest value; this sets IB to its lowest possible value.

2. Measure and record IB.

3. Measure and record IC.

4. Measure and record VC. This voltage is sometimes referred to as the collector-emitter voltage (VCE), because it is taken across the

collector-emitter leads if the emitter is connected to ground or the negative of the power supply.

5. Adjust the potentiometer to give the next targeted value of IB. You do not need to hit these values exactly. For example, for a target of

20 μA, a measured value of 20.4 μA is fine.

6. Measure and record the new values for IB, IC, and VC.

7. Adjust the potentiometer to give the next targeted value of IB.

8. Measure and record the new values for IB, IC, and VC again.

9. Repeat steps 7 and 8 for each of the targeted values of IB.

10. For each value of IB and its corresponding value of IC, calculate the value of β (β = IC/IB). The values will vary slightly but will be close

to an average. Did you get a consistent β?

 

Expected Results

Figure 3.20 shows the breadboarded circuit for this project.

 

Figure 3.21 shows the test setup for this project.

Fig 3.21

 

Compare your measurements with the ones shown in the following table.

Don't worry if your results give a different value of β. The manufacturing process that produces transistors can allow variation of the base thickness and doping levels, which causes variation of β in the finished transistors.