EET-129 LAB 8 TRANSISTOR BIAS 23W

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Centennial College *

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Electrical Engineering

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Apr 3, 2024

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1 Centennial College ELECTRICAL ENGINEERING TECHNICIAN Course: EET-129 Electronics 1 Name (Please Print Student Number Date:_____________________________________________________________ LAB # 8 TRANSISTOR BIAS Based on Laboratory Exercises for Electronic Devices by David Buchla For the transistor to operate in the linear region, it is necessary to forward-bias the base-emitter junction and reverse-bias the base-collector junction. The purpose of bias is to provide DC voltages to set up the quiescent conditions which establish the operating point on the characteristic curve for the device. In this lab we will investigate two common bias circuits: the base bias and the voltage-divider bias. For each bias circuit, we will use three transistors 2N3904 to verify the stability of the operating point. Materials required : Resistors: one 680 Ω, one 2 kΩ; one 6.8 kΩ; one 33 kΩ, 1 MΩ (all ½ W) Three NPN transistors 2N3904 Procedure Ensure your work area is clean and free of hazards while you are in the lab and left in a safe state after the lab period is over. 1- Measure and record the values of the resistors listed in Table 1. Nominal Value Measured Value 680Ω 679 2 KΩ 1.98 6.8 KΩ 6.7k 33 kΩ 32.6k 1 MΩ 0.99

2 Table 1 2. Fixed Base Bias Fixed base bias is the most sensitive to differences in β DC . Transistors of the same type can have wide differences in the value of β DC , which restricts the base bias to specialized applications such as switching circuits. The 2N3904 Data Sheet shows β DC can range from 100 to 300 (a factor of 3), which implies that the collector current can also vary by a factor of 3. For our computations, we will assume a β DC value in the middle of the specified range (200). Compute the parameters listed in Table 2 for the fixed base bias circuit shown in Fig. 1. I B = V CC − V BE R B I C = β DC I B V RC = I C R C V C = V CC − I C R C Fig. 1 Fixed Base Bias Start by computing V RB , and the current in this resistor I B . Using β DC , find the collector current, Ic, the voltage across the collector resistor, V RC , and the voltage from collector to ground, Vc. Show calculations below: I B = ¿ 12 – 0.7 / 1.0M = 11.3uA I C = ¿ 200 x 11.3u =2.26mA

3 V RC = ¿ 4.52V V C = ¿ 7.48 V DC Parameter Computed Value Measured Value Q1 Q2 Q3 (heated) V RB 11.3V 11.2 11.2 11.4 I B 11.3uA --------- --------- --------- Ic 2.26mA Ic = V Rc /Rc =7.6mA Ic = V Rc /Rc =9.44mA Ic = V Rc /Rc =9.40mA V RC 4.52V 3.8 4.72 4.7 Vc 7.48V 8.08 7.28 7.27 Table 2 (Fixed Base Bias) 11.3 rb 4.82rc 7.18c 3. Label each of the three NPN transistors as Q1, Q2, and Q3 in a manner that allows you to keep track of each transistor. Construct the circuit shown in Fig. 1 using Q1. Measure the voltages listed in Table 2 for Q1. Then remove Q1 from the circuit and test the other two transistors in the same circuit. For Q3 apply some heat (e.g. hear dryer) Record the data in Table 2. 4. Voltage-Divider Bias A more stable form of bias is the voltage-divider bias, which you will test using the same three transistors. The circuit meets the condition that R1//R2 ‹ 0.1 β DC R E so the simplified analysis formulas discussed in theory apply. 5. Compute the parameters listed in Table 3 for the circuit shown in Fig. 2 using the simplified analysis formulas.

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4 V B = R 2 R 1 + R 2 V CC V E = V B − 0.7 I C ≈ I E = V E R E I B = I C β DC V C = V CC − I C R C Fig. 2 Voltage-Divider Bias Show calculations below: V B=2.05 V E=1.35 I E ≈Ic=1.98mA V RC= 3.96 Vc= 8.04 DC Parameter Computed Value Measured Value Q1 Q2 Q3 V B 2.05V 2.00 1.98 1.99 V E 1.35V 1.33 1.32 1.32 I E ≈Ic 1.98mA Ic = V Rc /Rc =1.94mA Ic = V Rc /Rc =1.92mA Ic = V Rc /Rc =1.925mA V RC 3.96V 3.88 3.84 3.85 Vc 8.04V 8.08 8.13 8.12 Table 3 (Voltage-Divider Bias)

5 6. Substitute the same three transistors into the voltage-divider bias circuit. Construct the circuit and measure/record the parameters listed in Table 3. 7. By referring to Table 2 and Table 3 measured values. Calculate the biggest change in collector current and biggest change in collector voltage among three transistors (you will find 2 extreme values for each parameter (e.g. I Ca ∧ I Cb to find  I C / I C ) and V Ca ∧ V Cb to find  V C /V C ), show calculations and write in down in Table 4. Table 4 Base bias Voltage divider Max △ I C I C △ I C I C = I Ca − I Cb I Cb × 100% = -19.1 △ I C I C = I Ca − I Cb I Cb × 100% =0.78 Max △ V C V C △ V C V C = V Ca − V Cb V Cb × 100% = ¿ 11.1 △ V C V C = V Ca − V Cb V Cb × 100% = ¿ -0.49

6 8. Compare your observations of the two bias methods tested in the experiment. Which one showed the least variation of collector current and collector voltage among the transistors? Why? The transistor with the least variation of collector current and voltage current were those using the voltage divider bias technique. Voltage divider bias as the voltage is divided and delivered into the transitor which makes use of the few resistors present. As the transistor has no effect, temperature increases therefore offering better stabilization. 9. Recreate the circuit from Fig.2 in the Multisim (individually), and place measured results in table 5. DC Parameter Measured Values (Multisim) V B 1.981V V E 1.3V I E 1.91mA Ic 1.9mA V RC 3.8V Vc 8.21V Table 5 Compare DC parameters in Table 5 to Table 3 and describe your observations below: Values are similar with the experimental and simulated values proving the accuracy of the analysis.

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7 10. Save your Multisim File as EET-129 Lab 8 and submit it to the Lab folder (individually) 11. What is the value of the collector voltage V C (collector to ground) in Fig. 2 (Voltage divider bias) for each of these troubles (use Multisim)? a. R1 open 12V. b. R2 open 3.24V c. R E open 12V d. Rc open 172.85mV e. Collector-emitter open 1.73V

8 Conclusion: The conclusion summarizes important points of the laboratory work. You must analyze the examples to add emphasis to significant points. You must also include features and-or things you did/benefits of a particular procedure, instrument, component or circuit directly related to the experiment This lab has broadened my understanding the use of transistors in which an operating point is given within the circuit. Understanding base bias and voltage-divider bias had also broadened my knowledge with creating stability for the operating point. Rubric-Grading: Criteria Max. Grade Materials, Tools and Equipment Set-up 2 Following Procedure 2 Data Collection and Analysis 3 Conclusion 2 Spelling, Grammar, Sentence Structure 1

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