Lab 3: BJT I-V Characteristics

1. Learning Outcomes Lab 3: BJT I-V Characteristics At the end of this lab, students should know how to theoretically determine the I-V (Current-Voltage) characteristics of both NPN and PNP Bipolar Junction Transistors (BJT), know how to experimentally determine the I-V characteristics, and have a good understanding of what they mean. 2. Health and Safety Any laboratory environment may contain conditions that are potentially hazardous to a person s health if not handled appropriately. The Electrical Engineering laboratories obviously have electrical potentials that may be lethal and must be treated with respect. In addition, there are also mechanical hazards, particularly when dealing with rotating machines, and chemical hazards because of the materials used in various components. Our LEARNING OUTCOME is to educate all laboratory users to be able to handle laboratory materials and situations safely and thereby ensure a safe and healthy experience for all. Watch for posted information in and around the laboratories, and on the class web site. 3. Lab Report Students work in groups of 2 with laboratories being on alternative week (in 2C80/82). Each student must have a lab book for the labs. The lab book is used for lab preparation, notes, record, and lab reports. The lab books must be handed before 5:00 pm on the due date (same day of the following week) into the box labeled for your section across from 2C94. The lab books are marked and returned before the next lab. 4. Background The BJT is best described as a current-controlled active element while the MOSFET and JFET are voltagecontrolled. The base current i B is usually considered the controlling quantity, controlling the collector current i C. Although i B in turn depends upon v BE (base-emitter voltage), the change in v BE for a given change in i B is so small because of the exponential relationship that is often neglected. This approximation (v BE =0.7 V) is often a source of confusion for the students who do not realize that a truly constant v BE implies no change in i B. 5. Material and Equipment Material (supplied by department) 2N2222A (NPN Transistor) 2N2907A (PNP Transistor) Resistors: 1 x 100 Ω, 1 x 22 kω Equipment (supplied by student) Analog Discovery Module Waveforms software Breadboard and wiring kit Rev B Copyright 2015 University of Saskatchewan Page 1 of 13

6. Prelab 1. For each of: 1.1. 2N2222A (NPN Transistor) 1.2. 2N2907A (PNP Transistor) Look up the characteristics of each device by doing a web search. Fill in the following table for each of the devices: Rating/Characteristic Maximum Collector-Emitter Voltage Maximum Continuous Collector Current DC Current Gain (h FE) Collector-Emitter Saturation Voltage Base-Emitter Saturation Voltage Small-Signal Current Gain (h fe) Value 2. A SPICE circuit file for the 2N2222A circuit (Figure 7-1) is given in Figure 6-1. Using LTspice (a guide to using LTspice can be found in Laboratory #1 Appendix A), include a screen capture of the plot of the I-V characteristics of the 2N2222A in your lab report. 2.1. The horizontal axis should be set to "V(vc)". To set the horizontal axis, mouse over any of the numbers listed in the horizontal axis and "click" to bring up the "Horizontal Axis" dialog. 2.2. The vertical axis should be set to "Ic(Q1)" (using "Plot Settings Add Trace"). 2.3. The plot should look similar to Figure 6-2. 3. The SPICE model for a 2N2907A Transistor is given in Figure 6-3. Modify the SPICE circuit file shown in Figure 6-1 to simulate the 2N2907A circuit shown in Figure 7-7. You will need to change the "sign" on the "Vcc" and "Vbb" input voltages. Help with the SPICE format can be found at: http://www.eecg.toronto.edu/~kphang/teaching/spice/ Include a screen capture of the plot of the I-V characteristics of the 2N2907A in your lab report. Rev B Copyright 2015 University of Saskatchewan Page 2 of 13

2N2222A BJT I-V Characteristics ** Circuit Description ** Vcc vcc 0 R2 vcc vc 100 Vbb vbb 0 R1 vbb vb 22k Q1 vc vb 0 2N2222A ** Analysis Requests **.DC Vcc 0V +4V 100mV Vbb 0.25V 4V 0.25V ****** *SRC=2n2222a;2n2222a;BJTs NPN; Si; 75.0V 0.800A 300MHz.MODEL 2N2222A NPN (IS=2.20f NF=1.00 BF=240 VAF=114 + IKF=0.293 ISE=2.73p NE=2.00 BR=4.00 NR=1.00 + VAR=24.0 IKR=0.600 RE=0.194 RB=0.777 RC=77.7m + XTB=1.5 CJE=24.9p VJE=1.10 MJE=0.500 CJC=12.4p VJC=0.300 + MJC=0.300 TF=371p TR=64.0n EG=1.12 ) ****** Central Semi Central Semi.end Figure 6-1: 2N2222A I-V Characteristics SPICE Circuit File Figure 6-2: 2N2222A I-V Characteristics SPICE Results Rev B Copyright 2015 University of Saskatchewan Page 3 of 13

****** * Model Generated by MODPEX.MODEL 2N2907A PNP +IS=3.02341e-12 BF=523.064 NF=1.16335 VAF=44.2994 +IKF=0.591421 ISE=3.31443e-11 NE=1.9954 BR=4.8572 +NR=1.18959 VAR=1.33092 IKR=5.91421 ISC=3.31443e-11 +NC=3.81262 RB=2.76209 IRB=0.1 RBM=0.880912 +RE=0.0001 RC=0.857407 XTB=0.119647 XTI=1 +EG=1.05 CJE=3.934e-11 VJE=0.680693 MJE=0.379312 +TF=2.75919e-10 XTF=0.674951 VTF=54426.6 ITF=0.067962 +CJC=2.40198e-11 VJC=0.4 MJC=0.462796 XCJC=1 +FC=0.570446 CJS=0 VJS=0.75 MJS=0.5 +TR=1e-07 PTF=0 KF=0 AF=1 ****** Figure 6-3: 2N2907A SPICE Model Rev B Copyright 2015 University of Saskatchewan Page 4 of 13

7. Lab Procedures Debugging (or What To Try When Things Aren't Working) There are a number of things/procedures you should use to debugging circuits when things are not working correctly. These include (but are not limited to): Check that all component pins are correctly inserted in the breadboard (sometimes they get bent underneath a component). Make sure that components are not "misaligned" in the breadboard (e.g. off by one row). Double check component values (you can measure resistors, capacitors, and inductors). Try a different section in the breadboard (in case there is a bad internal connection). Measure the source voltages to verify power input. Measure key points in the circuit for proper voltage/waveform (i.e. divide-and-conquer). 2N2222A I-V Characteristics 1. Construct the circuit shown in Figure 7-1 on your breadboard: 1.1. Use Arbitrary Waveform Generator 1 (W1, yellow wire, see Appendix A) to generate "Vcc". Set AWG1 to a 80 Hz, 0 to +4 V "sawtooth" (i.e. Amplitude = 2 V, Offset = 2 V). Also set "Phase" to "90 deg". The AWG1 setting dialog should look similar to Figure 7-2. R2 100 Ω Vbb R1 22 kω C B 2N2222A Q1 E Vcc Figure 7-1: 2N2222A Circuit Schematic Rev B Copyright 2015 University of Saskatchewan Page 5 of 13

Figure 7-2: AWG1 Settings 1.2. Use Arbitrary Waveform Generator 2 (W2, yellow-white wire) to generate "Vbb". Use "Select Channels" (highlighted in Figure 7-2) to enable AWG2. We want Vbb to be 16 voltage steps from 0.25 V to 4 V in 0.25 V steps: 1.2.1. Create a text file with the numbers 1, 2,, 16, one on each line. 1.2.2. Select the "Custom" waveform type. 1.2.3. Then select "File" and "Browse" to the text file you created. "Open" to create the step waveform. 1.2.4. Set Frequency to 10 Hz, Amplitude to 1.875 V, and Offset to 2.125 V. 1.2.5. Set "Mode" to "Auto sync" which synchronizes AWG1 and AWG2. 1.2.6. The AWG settings screen should look similar to Figure 7-3. Note how AWG1 (Vcc) varies from 0 to 4 V for each voltage step in AWG2 (Vbb). 1.3. Click "Run All" to start the output from AWG1 and AWG2. 2. Connect "Channel 1" (1+ and 1-) to measure the voltage V CE (voltage between Collector and Emitter of the transistor). 3. Connect "Channel 2" (2+ and 2-) to measure the voltage V R2 (voltage across Resistor R2) which we will use to calculate the collector current I C. 3.1. Open the Oscilloscope tool, use the "Settings Load Defaults" menu option to set the tool to a clean state, and hit "AutoSet" to get an initial view of the waveforms. If your circuit is working correctly, you should see something similar to Figure 7-2. Note that the waveforms shown do not make much sense in this mode. Rev B Copyright 2015 University of Saskatchewan Page 6 of 13

Figure 7-3: AWG 1 and AWG2 Settings 4. To calculate the collector current I C through the transistor: 4.1. "Add Channel" to "Add Mathematic Channel Custom" and set the function to "C2 / 100". 4.2. Note that there is now an "M1" channel shown in Red in the Oscilloscope window. Since this is really a current, change the units to "A". Set the Range on M1 to a more suitable value (e.g. "10 ma/div"). 5. To see the I-V Characteristics plot, use "Add XY" (set "X: Channel 1" (i.e. V CE) and "Y: Math 1" (i.e. I C)). Expand the XY window to fill the Oscilloscope window and adjust "C1" and "M1" to provide a better view of the I-V Characteristics. The screen should look similar to Figure 7-5 (which should be similar to the SPICE simulation). REQUIRED: Demonstrate to a lab instructor and make sure your demonstration is recorded by the lab instructor. 6. Include a screen capture of the I-V Characteristics and annotate on the graph the curves correspond to "Vbb" of 1.0, 2.0, 3.0, and 4.0 V. Also annotate the "Active", "Saturation", and "Cut-off" regions. 7. Change the Vcc waveform into sine wave and square wave to observe (and explain) if there any change in the I-V characteristic display. Rev B Copyright 2015 University of Saskatchewan Page 7 of 13

8. From the I-V Characteristics curves, what is the apparent value of V CE(saturation)? 9. To measure I B, move channel 1 to measure V R1. Add Math Channel 2 to calculate I B. Set Vcc to a constant 4 V. Turn off display of C1 and C2 to make the current waveforms easier to see. Use the "X Cursor" to measure the values for I C and I B (example Oscilloscope screen shown in Figure 7-6) and fill in Table 7-1. 10. Calculate β Fdc (i.e. II CC IIBB ) for each row in Table 7-1. How do these values compare to the value given in the 2N2222A datasheet? 11. To calculate β Fac, for each sample (e.g. "x") use the sample "before" ("x-1") as well as the sample "after" ("x+1"). How do these values compare to the values given in the 2N2222A datasheet? ββ FFFFFF (xx) = II CC (xx) II CC (xx 1) II BB (xx) II BB (xx 1) + II CC(xx + 1) II CC (xx) II BB (xx + 1) II BB (xx) 2 12. Is there any difference between β Fdc and β Fac? Why might there be a difference? Figure 7-6: I C and I B Oscilloscope Measurement Rev B Copyright 2015 University of Saskatchewan Page 9 of 13

Vbb (V) I C (ma) I B (µa) β Fdc (h FE) β Fac (h fe) 0.25 Every 0.25 V until 4 V Table 7-1: 2N2222A I C versus I B 13. Move Channel 2 to measure V BE. Set the Oscilloscope to display I B and V BE (include a screen capture of the Oscilloscope window). What appears to be the value of the Base-Emitter Saturation Voltage and how does that compare to the datasheet? Rev B Copyright 2015 University of Saskatchewan Page 10 of 13

2N2907A I-V Characteristics 1. Construct the circuit shown in Figure 7-7 on your breadboard: 1.1. Use Arbitrary Waveform Generator 1 (W1, yellow wire) to generate "Vcc". Set AWG1 to a 80 Hz, 0 to 4 V "sawtooth" (i.e. Amplitude = 2 V, Offset = -2 V). Also set "Phase" to "90 deg". 1.2. Use Arbitrary Waveform Generator 2 (W2, yellow-white wire) to generate "Vbb". We want Vbb to be 16 voltage steps from -0.25 V to -4 V in 0.25 V steps R2 100 Ω Vbb R1 22 kω C B 2N2907A Q1 E Vcc Figure 7-7: 2N2907A Circuit Schematic 2. Connect "Channel 1" (1+ and 1-) to measure the voltage V CE (voltage between Collector and Emitter of the transistor). 3. Connect "Channel 2" (2+ and 2-) to measure the voltage V R2 (voltage across Resistor R2) which we will use to calculate the collector current I C. 4. Similar to section 7.2, set up the Oscilloscope window to display the I-V Characteristic curves. The screen should look similar to Figure 7-8 (which should be similar to the SPICE simulation). REQUIRED: Demonstrate to a lab instructor and make sure your demonstration is recorded by the lab instructor. 5. Include a screen capture of the I-V Characteristics and annotate on the graph the curves correspond to "Vbb" of -1.0, -2.0, -3.0, and -4.0 V. Also annotate the "Active", "Saturation", and "Cut-off" regions. 6. From the I-V Characteristics curves, what is the apparent value of V CE(saturation)? 7. Similar to section 7.2, calculate β Fdc (i.e. II CC IIBB ) for each row in Table 7-2. How do these values compare to the values given in the 2N2907A datasheet? 8. Is there any difference between β Fdc and β Fac? Why might there be a difference? 9. Move the appropriate channels and set the Oscilloscope to display I B and V BE (include a screen capture of the Oscilloscope window). What appears to be the value of the Base-Emitter Saturation Voltage and how does that compare to the datasheet? Rev B Copyright 2015 University of Saskatchewan Page 11 of 13

Figure 7-8: 2N2907A I-V Characteristics Vbb (V) I C (ma) I B (µa) β Fdc (h FE) β Fac (h fe) -0.25 Every 0.25 V until -4 V Table 7-2: 2N2907A I C versus I B Rev B Copyright 2015 University of Saskatchewan Page 12 of 13