Example Lab Report
This is an example of how your formal lab reports should look. The horizontal bars on this web page represent page breaks. Notice that each of the major sections -- Cover Sheet, Introduction, Schematics, Experiment Data, and Conclusion -- begins on a new page.
Notes about specific sections:
The purposes of the lab experiment are always given in the lab assignment. These may simply be copied into your report.
The equipment list must be determined, and generated by you.
The procedure is an expanded version of that given in the lab assignment. In this example, the only instructions were these: Design a circuit with a 2N3904 NPN transistor that has Ic = 10 mA and Vce = 10 V. The allowable tolerance of this circuit is + or - 10%. The circuit must use a single 20 V power source. Prove that the Q point is independent of beta by testing the circuit with three different transistors.
The procedure section of the report explains the details of how this is done. A good rule of thumb is to write it for someone who has the same background you have at the beginning of the lab. Write the directions specific enough so that he or she can exactly duplicate your work. Notice the procedure given in this report is in bullet format. This makes the steps easier to follow.
The schematic diagram of the circuit has been generated on a computer. If you do not have software that can do this, you may neatly hand-draw your schematic on plain white paper. The heading, however, must be typed.
The expected (calculated) results are shown, along with the method used to derive them. The actual (measured) results are also clearly labeled.
The conclusion requires some thought to write properly. Notice that it is not simply a summary of what was done or the obtained results. It compares the expected results with the actual results and describes what it all means. You will find words like, "This demonstrates . . .", "This means . . .", "This is because . . .", etc. The last paragraph goes on to compare this circuit with others that have been studied, and outlines its advantages, disadvantages, and possible applications.
The actual lab report follows:
Laboratory # 5
Design for a Specific Q Point
Rick Sparks
CoE 255
Introduction
Purpose
1. Practice designing a transistor circuit that meets certain specifications.
2. Verify that transistor circuits with negative feedback are not sensitive to
changes in beta.
3. Troubleshoot problems, without the teacher's help.
Equipment
1. Three 2N3904 NPN transistors
2. One 1800 ohm resistor
3. One 330 ohm resistor
4. One 820 ohm resistor
5. One 270 ohm resistor
6. One +20 V DC power supply
7. One prototyping board
8. 22 AWG wires
Procedure
Design a circuit with a 2N3904 NPN transistor that has Ic = 10 mA and Vce = 10 V. The allowable tolerance of this circuit is ±10%. The circuit must use a single 20 V power source. Prove that the Q point is independent of beta by testing the circuit with three different transistors.
1. Use the "Approximation Method" to calculate the resistor values, starting with the desired Ic and Vce values.
2. Construct the circuit and measure the values of Ic and Vce, using three different transistors. Record the values as "measured data".
Schematics
Experiment Data
Expected Results (Calculations)
Calculations for voltage-divider network:
Assume Vcc = 20 V; Ivd = 10 mA; Vb = 3 V
R2 = 3 V / 10 mA = 300 ; use 330
R1 = 17 V / 10 mA = 1.7 k; use 1.8 k
Expected Ivd = 20 V / (330 + 1800 ) = 9.85 mA
Expected Vb = 330 x 9.85 mA = 3.25 V
Calculate Ve, Re, Ie, and Ic:
Assume Ie = 10 mA
Ve = 3.25 V - 0.7 V = 2.55 V
Re = 2.55 V / 10 mA = 255 ; use 270
Expected Ie = 2.55 V / 270 = 9.44 mA
Expected Ic = Ie = 9.44 mA (It meets spec: Ic = 10 mA ± 10%)
Calculate Rc, Vc, and Vce:
Assume Vcc = 20 V; Vce = 10 V; Ve = 2.55 V; Ic = 9.44 mA
Vrc = 20 V - 2.55 V - 10 V = 7.45 V
Rc = 7.45 V / 9.44 mA = 789 ; use 820
Expected Vrc = 9.44 mA x 820 = 7.74 V
Expected Vce = 20 V - 2.55 V - 7.74 V = 9.71 V (It meets spec: Vce = 10 V ± 10%)
Actual Results (Measurements)
Q Vce Ic
A 9.8 V (Vce = 10 V ± 10%) 9.9 mA (Ic = 10 mA ± 10%)
B 9.6 V (Vce = 10 V ± 10%) 9.7 mA (Ic = 10 mA ± 10%)
C 9.7 V (Vce = 10 V ± 10%) 9.8 mA (Ic = 10 mA ± 10%)
Conclusion
At first, I could not get any measurements that made sense. One of the other
students showed me that I was reading the pin diagram in the data book backwards! After
connecting the transistor correctly, the circuit performed as expected.
The measured values were well within the 10% tolerance allowed by the design specifications. They were also within 2% of the calculated values. This demonstrates that the method used to design this circuit works well. The closeness of the calculated and actual values means that the actual resistor values were very close to their nominal values.
There was a slight difference in measured values for the three transistors. This is because the beta of each one is quite different. This shows that a large difference in beta corresponds to a very small difference in the Q point for this circuit configuration. Although this voltage-divider-biased circuit (a form of emitter-biasing) is very stable over a wide range of beta, the difference in beta does have some effect.
This circuit uses the emitter resistor as a source of negative feedback. This results in good Q point stability. Increasing the amount of current through the voltage-divider network makes it even more stable, because the percentage of current lost to Ib becomes smaller, and there is less of an effect on Vb. (Beta affects Vb by changing Ib, which changes the amount of current through the voltage-divider.) This circuit also requires only one power supply, while some other emitter-biased circuits require two voltage sources. It does waste a lot of power in the voltage-divider network, however. It would probably not be a good choice if the power source was a battery or solar cell.