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Morgan State University
Department of Electrical and Computer Engineering
EEGR 317 Microelectronics
Credits – 4
LAB 3 : NPN I-V & DC Characteristics
Lab Due: March 15, 2017
Objective:
To Study NPN transistor I-V curves by:
• Simulating a transistor to investigate the collector current versus the collector-to-emitter voltage.
• Implementing a circuit and taking measurements of the IC vs. VCE curves.
Materials:
1. Laboratory setup, including breadboard
2. 1 NPN transistor (e.g., 2N2222)
3. Several wires and resistors of varying sizes
Part 1:Simulation:
Consider the circuit shown below.
Enter the circuit into your simulator’s schematic editor, applying DC voltage supplies to the base and to the collector of the
transistor.
IC vs. VBE
• While setting VCE to a constant value of 5V , sweep the base voltage from 0V to 0.8V in increments of 0.1V . Plot a curve
of IC vs. VBE and place the plot in your lab writeup.
• At what value of VBE does current begin to conduct?
• What are the values of IB and IC when VBE = 0.7V ?
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• Based on these numbers, what is your estimate of β?
IC vs. VCE
• For three values of VBE (0.6 V, 0.7 V, and 0.8 V), sweep the collector voltage from 0 V to 2 V in increments of O.1 V. Plot the
curves for IC vs. VCE using a graphing program indicating the value of VBE next to each curve.
• What observations can be made about the curves?
Part 2:Measurements:
Assemble the circuit from the igure above, using a power supply to generate the DC voltages.
IC vs. VBE
• While setting VCE to a constant of 5 V, sweep the base voltage from 0V to 0.8V in increments of 0.1V, and measure the
collector current using the power supply. (Note that not all power supplies allow you to accurately measure current. If
this is the case for your lab setup, you place a small resistor in series with the collector and measure the voltage drop
across the resistor.)
• Plot a curve of IC vs VBE
• At what value of VBE does the current turn on?
• Using a small resistor placed in series with the base and collector terminals, measure IB and IC for VBE = 0.7V
• Based on these numbers, what is your β?
IC vs. VCE
• For three values of VBE (0.6 V, 0.7 V, and 0.8 V), sweep the VCE from 0 V to 1 V in increments of 0.1 V, and measure the
collector current using the power supply.
• Plot the curves for IC vs. VCE using a graphing program and clearly indicating the value of VBE next to each curve.
Part 3:Post-Measurement Exercise:
Simulation vs. Measurement
• What are the main differences between your simulated and measured curves? Can you explain the differences?
Early voltage, VA
• Based on your simulated IC vs. VCE curves for an active transistor, extract the Early voltage VA .
• Does VA change signi icantly for each value of VBE ?
• What is the average value of VA
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Part 4:Design and Simulation
Consider the circuit shown below.
Design the circuit such that IC = 1mA, VB = 0, and VC = +5V . Use supplies of V+ = −V_ = 15V . Although there will be
variations from transistor to transistor, you may assume a value 100 for β in your calculations. Also assume that VBE = 0.7V .
In your lab writeup, perform the following.
Hand Calculations
• What are IB and IE ? Based on these numbers, what is VE ?
• You now have enough information to calculate RC and RC . Are the calculated values available in your kit? Can you achieve
these values by combining several resistors? Comment.
• Derive the Thevenin equivalent of R1 and R2 . What values of R1 and R2 do you need to use to achieve VB = 0 V? Remember
that IB 6= 0. Is the problem completely speci ied? If not, what needs to be speci ied?
Simulations
• Simulate your circuit using values of RE , RC , R1 , and R2 based on you r calculations.
• Report the values of VE , VC , VB , IE , IC , and IB . How closely do they match your calculations? (Remember: The simulator
has its own, more complex model of the real transistor, so there should be some small variations.)
Prototyping and Measurement
• Assemble the circuit onto a breadboard.
• Using a digital multimeter, measure VE , VC , and VB .
• Using a digital multimeter, measure all resistors to three signi icant digits.
Post-Measurement Exercise
• What are the measured values of VBE and VCE ? How do they compare to your pre-lab calculations? Explain any discrepancies.
• Based on the measured values of VC and VE and your measured resistor values, what are the measured values of IB , IC
and IE based on your lab measurements?
Part 5:NPN in Saturation Mode:
Redesign the circuit in the igure above such that IC = 1 mA, IE = 1.2 mA, VC = +2 V, and VCE = 0.2 V. Use supplies of V+ =
−V− = 15 V. Note that you must use the saturation model.
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Hand Calculations
• Based on the speci ications , calculate VE and VB .
• You now have enough information to calculate RC and RE . Are the calculated values available in your kit? Can you achieve
this value by combining several resistors? Comment.
• What is βf orced ?
• What values of R1 and R2 do you need to use? Is the problem completely speci ied?
Simulation
• Simulate your circuit using values of RE , RC , R1 , and R2 based on your calculations.
• Report the values of VE , VC , VB , IE , IC , and IB · How closely do they match your calculations?
Part 6:Post-Measurement Exercise:
Prototyping and Measurement
• Assemble the circuit onto a breadboard.
• Using your volt multimeter, measure VE , VC , and VB · Report them in your lab writeup.
• Using a digital multimeter, measure all resistors to three signi icant digits.
Post-Measurement Exercise
• What are the measured values of VBE and VCE ? How do they compare to your pre-lab calculations? Explain any discrepancies.
• Based on the measured voltages and resistor values, what are the measured values of IB , IC and IE based on your lab
measurements? What is βf orced ?
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