ECE 332 Lab 3 -update 3.1

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ECE332 Lab Project III page 1 ECE 332 – Electronics-I Laboratory Project III Bipolar Junction Transistor Characterization 1. Objectives: The BJT was invented in 1948 by William Shockley at Bell Labs, and became the first mass-produced transistor. Having a good grasp of the physics of the BJT is key to understanding its operation and applications. In this lab, we will explore the BJT’s four regions of operation and also determine its characteristic values, such as DC current gain β and Early voltage VA. The transistor used in this lab is the 2N3904, an NPN device. It is strongly recommended that you read and understand the section on BJT physics before beginning this experiment. 2. Materials and Equipment: You will need the components listed in Table 1. Note: Be sure to answer the questions on the report as you proceed through this lab. The report questions are labeled according to the sections in the experiment. CAUTION: FOR THIS EXPERIMENT, THE TRANSISTORS CAN BECOME EXTREMELY HOT!!! Component Quantity 1 M Ω resistor 1 5 k Ω resistor (choose close one) 1 100 Ω resistor 4 2N3904 NPN BJT 2 Equipment: Direct-Current Power Supply, 0-20 V, Digital Multimeter. Breadboard and electronics parts kit, Oscilloscope (optional) PART ONE – PRE-LABORATORY 3 ASSIGNMENT 1. For the NPN device shown in Figure 1, label I C , I B , and I E next to their respective current arrows. Figure 1: A simple NPN device 2. What is β in terms of I C and I B ? What is α in terms of I C and I E ? Express α in terms of β . β (I C , I B ) = α (IC, IE) = α ( β ) =

ECE332 Lab Project III page 2 3. Given that you are a circuit designer working with an NPN device and you want the device to have a g m of 1 mS (= 0.001 Ω -1 ), what V BE value would you need to properly bias the NPN? Assume I S is 5 x 10 −16 A. 4. Consider the following NPN BJT circuit shown in Figure 2. 4.a. Determine R C and R B such that the operating point of DC is V CE = 5 V, and I C = 4 mA. Typically, β = 225 and V BE(ON) = 0.7 V for a 2N3904 transistor. 4.b. Select standard resistor values (as supplied in your electronics parts kit) for R C and R B that are close to your calculated numbers in part a. Recalculate the quiescent operating point. V CC = + 10 V R B R C 2N3904 Figure 2. Single-resistor bias circuit for Prelab Problems 4. 5. PSPICE (Optional, Please try it) • Write a SPICE netlist for the BJT test circuit shown in Figure 3. Refer to the HSPICE Tutorial if you have trouble with SPICE. • Use the 2N4401 PSPICE model provided on the course website. ( You can use any possible NPN BJT, if you cannot find 2N4401 from the library) • Using the .dc command, sweep V CC from 0 V to 5 V in 0.01 V increments and step V BB from 0.6 V to 0.7 V in 0.025 V increments. • Run the simulation and check the output file for any errors. • If there are no errors, plot I C versus V CC and print out a copy of the plot. Note: If your I C is negative, use Awaves to plot the absolute value of I C . I C appears to be negative because SPICE defines I C to be going out of the BJT. Figure 3: Circuit to simulate in SPICE + - + - V BB V CC

ECE332 Lab Project III page 3 6. Complete the blanks in the table below according to their appropriate fields by searching them in their respective datasheets. Some of the table is given as an example and for guidance. For the last row, please pick your own transistor and complete the rest of the information for it. Transistors Maximum DC Current Gain (Hfe) Maximum Continuous Collector current (Ic) mA Minimum VBE (on) mV Maximum VBE (sat) mV Application 2N3904 200 LED driver 2N222 2N3906 650 BC547 800 100 900 Audio Amplifier

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ECE332 Lab Project III page 4 PART TWO: LABORATORY PROJECT 1. Procedure 1.1. Determining the Region of Operation 1. Set up the circuit shown in Figure 4, with R B = 1 M Ω , R C = 5 k Ω , R E = 100 Ω , and V CC = 5V. If you need help identifying the terminals of the discrete BJT, please refer to Figure 5. Figure 4: BJT measurement setup for this lab. Figure 5: For your reference, this is an illustration of the mapping between the terminals of a discrete NPN BJT and those of its circuit symbol counterpart. 2. Set V BB to 10 V, then measure V BE and V BC . The transistor is in which region of operation? Warning: Never set V BE higher than 5 V for any of the transistors used in the labs. Doing so will permanently damage the transistor and may cause personal injury. 3. Now measure I B . What is the value of β ? 4. From the value found above, calculate α and use it to calculate I E . Afterwards, measure I E and check if the calculated and measured values agree. 5. Now take a look at the 2N3904 datasheet. Does your calculated β value agree with the value given in the datasheet? Hint: β is called h FE in the datasheet, and there is a plot of h FE versus I C under “Typical Characteristics” . If the values do not agree, explain why there is a discrepancy. 6. On another note, let us examine the temperature dependence of the collector and base currents: Put two fingers around Q 1 to heat it up, and then, measure I B and I C (have your partner heat the BJT while you measure the currents if you are having trouble doing both at the same time). How + - + - R B R C R E V BB V CC I C I E I B

ECE332 Lab Project III page 5 do the values of I C and I B compare to the values you measured before you heated the transistor (i.e. do I C and I B increase or decrease)? 7. Explain, using the equation for the collector current, how you would expect I C to vary with temperature. Does this agree with your experimental results? If not, explain why this might be the case. ( Hint: I S depends on the intrinsic carrier concentration n i and the diffusion coefficients D n and D p. Intuitively, how would n i , D n , and D p change with temperature? How would I S change with temperature as a result? Note: You do not need to explicitly answer the questions in the hint but do think about them. ) 8. Let us now explore the different operation regions of a BJT. Set V BB to 4V and V CC to 2V. Measure I B , I C , V BE , and V BC . What is the region of operation for the BJT? 9. Set V BB to −3V and V CC to 5V. Measure I B , I C , V BE , and V BC . What is the region of operation for the BJT? 10. Now swap the emitter and the collector connections of the BJT in the circuit (you can do this by physically rearranging the BJT to face the opposite direction). Set V BB to 4V and keep V CC at 5V. Measure I B , I C , V BE , and V BC . What is the region of operation? 1.2 Single-Resistor-Bias Circuit Measurements Before making the following connections to your breadboard, verify that the power supply is off and the voltage outputs are set to zero . Connect your breadboarded circuit shown in Figure 2 (Pre-Lab) to the dc power supply. Use the completed circuit to perform the following. The objective is to find the dc operating point. Adjust the power supply for a V CC Voltage of + 10 V, and verify this voltage with your DMM. Measure V CE , V BE , and the voltages across R C and R B . Perform calculations to determine I C , I B , and β . Record the results of your measurements and calculations using a table similar to Table 1 (on last page).

ECE332 Lab Project III page 6 TABLE 1 Example Table for Recording Data V BE V CE I B I C β α Post Lab Assignment Include your answers in the report. Use any method to find information on similar BJT applications. You can use the answers from any sources you use. Given below is an example of a transistor as a switch with sensors in two circuits. Fill in the blanks in the tables. LDR, light-dependent resistors, are light-sensitive devices most often used to indicate the presence or absence of light or to measure the light intensity. LDR sensor input cases are given below to determine if LED output is LED ON or LED OFF. Circuit 1 LDR Sensor Input LED OUTPUT LDR Bright LDR dark

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ECE332 Lab Project III page 7 Circuit 2 LED output cases are given below to determine if the LDR Sensor input is LDR Bright or LDR dark. LDR Sensor Input LED OUTPUT LED ON LED OFF