Using National Instruments LabVIEW Software in an
Introductory Electronics Course – MOSFET Transistor
Parameter Estimation and Bias Circuit Design
John T. Oldenburg, Ph.D.
Department of Electrical & Electronic Engineering
California State University, Sacramento, CA
Abstract
Student understanding of the link between theory and application has always been a critical
objective in electronics education. National Instruments LabVIEW virtual instrument software is
a tool that can be used very effectively to promote student understanding of that link. It can be
employed to compare device theory with the actual operation of an electronic device, using an
effective visual format. It can also facilitate the implementation of computer-aided circuit design
and verification. This paper presents an example of the application of the software to MOSFET
parameter estimation and bias circuit design in an introductory electronics course. Four
LabVIEW virtual instruments (VIs) have been developed for this application. The first allows
the student to perform a direct measurement of the transconductance (Kn), gate threshold voltage
(VTO), and channel length modulation parameter (λ) of an NMOSFET transistor. The second
helps the student calculate resistor values for a simple 4-R bias circuit for the transistor, when the
supply voltage and Q-point have been specified. The third allows the student to verify and, if
necessary, modify the bias circuit design through graphical illustration of the design Q-point.
The last VI is used to directly measure the Q-point parameters of the actual prototype transistor
bias circuit that the student has constructed in the laboratory. Student evaluation of these VIs has
been very positive with respect to their effectiveness in helping them to understand the link
between transistor theory and application.
Introduction
The B.S. in Computer Engineering Program at California State University, Sacramento requires
the course EEE 102 Analog/Digital Electronics and the associated laboratory EEE 102L
Analog/Digital Electronics Laboratory. This introduction to electronics “service” course
endeavors to provide the junior-level student with a sound understanding of the electrical
characteristics of semiconductor devices as well as their use in basic electronic circuits such as
logic gates, power supplies, wave generators, and amplifiers. The laboratory is designed to be
taken concurrently with the lecture course and to reinforce lecture concepts with practical
application. For the past five years, National Instruments LabVIEW virtual instrument software
has been used as a principal tool in laboratory exercises for electronic device modeling, device
testing, circuit design, circuit simulation, and circuit testing [1]. The unique capabilities of this
software allow for development of a very cost-effective “workbench” of analysis, design, and
testing tools that can be custom configured for each laboratory exercise.
Proceedings of the 2004 American Society for Engineering Education Pacific Southwest Section Conference
April 1-2, 2004, University of the Pacific, Stockton, CA
This paper will present one example laboratory exercise from the course that will illustrate the
use of LabVIEW virtual instruments for comprehensive device analysis, circuit design, circuit
simulation, and prototype testing. It should be noted that students in the course have previously
completed two laboratory sessions concerned with learning the fundamentals of G-language
programming and virtual instrument operation. They have the capability of running LabVIEW
virtual instruments (VIs) and editing them to customize their measurements when so desired.
The laboratory has always included a student feedback questionnaire, and the results of the
student survey of the effectiveness of this laboratory exercise will be reviewed.
NMOSFET Device Analysis
The first part of the laboratory exercise concerns the measurement of the characteristic
parameters for an NMOSFET transistor. In the lecture portion of the course, the students have
studied the theory of operation of the MOSFET transistor and are now familiar with the
characteristic equations that govern its current-voltage relationship. They are also familiar with
the data sheet for the transistor that is specified in the laboratory, and will be able to compare
their subsequent measurement of its characteristic parameters with values they have obtained
from that data sheet.
The students are first instructed to review the description window and the wiring diagram (figure
1) for the NMOSFET analysis VI that they will use in this part of the exercise. The objective
here is to have them become familiar with how the VI determines the characteristic parameters
Figure 1. Wiring Diagram for the NMOSFET Analysis Virtual Instrument
Proceedings of the 2004 American Society for Engineering Education Pacific Southwest Section Conference
April 1-2, 2004, University of the Pacific, Stockton, CA
of the transistor from selected voltage measurements in a test circuit. Next, they are instructed to
build a parameter test circuit for their transistor on a protoboard, following directions in the
description window, and according to the schematic on the front panel of the VI. They are then
instructed to connect three differential channels of the data acquisition board in their workstation
to appropriate points on their protoboard circuit in order to measure the required test voltages.
When they activate the RUN button on the virtual instrument, it does the rest of the measurement
and analysis work for them.
Figure 2 shows the front panel for the NMOSFET analysis VI. Notice that this VI is
programmed to directly record test circuit voltages and solve the relevant equations for the
characteristic parameters (Kn – tranconductance, VTO – gate threshold voltage, and λ -- channel
Figure 2. NMOSFET Analysis VI Front Panel Showing Characteristic Parameter Measurements
Proceedings of the 2004 American Society for Engineering Education Pacific Southwest Section Conference
April 1-2, 2004, University of the Pacific, Stockton, CA
length modulation) of the transistor [2]. It also displays the I-V characteristic of the transistor for
the particular gate-source voltage (VGS) of the test circuit. The students are required to use
graphical cursors to identify the “pinchoff voltage” from the I-V characteristic and then execute a
final measurement and analysis run with the VI to precisely determine the characteristic
parameters of the transistor. The students can then compare their measurements with the
specifications they have previously determined from the transistor data sheet.
In this way, the virtual instrument simplifies the test circuit measurements, conveniently
implements the calculation of characteristic parameters from the characteristic equations, and
allows the students to appreciate the actual I-V characteristic of their particular NMOSFET
transistor. Most students report being excited to see how the actual transistor behaves in
comparison to the theory that they have learned, and how characteristic parameters determined
from the data sheet compare with those actually measured for their particular transistor.
Four-resistor Bias Circuit Design
With accurate measurements of the NMOSFET characteristic parameters in hand, the students
can now approach the design of a four-resistor dc biasing circuit to cause the transistor to operate
at a specified dc Q-point. In the classroom lecture, the design procedure was developed and
illustrated. Now the students can implement that procedure using a convenient VI. Again, they
are instructed to review both the documentation window of the VI for information about its use
Figure 3. Wiring Diagram for the Four-resistor Bias Circuit Design VI
Proceedings of the 2004 American Society for Engineering Education Pacific Southwest Section Conference
April 1-2, 2004, University of the Pacific, Stockton, CA
and the wiring diagram (figure 3) to appreciate the implementation of the design procedure.
They are then instructed to enter characteristic parameter data for their transistor and the desired
Q-point for the design on the front panel of this VI (figure 4). The Gate Margin parameter in the
Figure 4. Front Panel of the Four-resistor Bias Circuit Design VI
VI is actually the voltage across the RS resistor in the bias circuit. It is presented, in this case, as
a variable design parameter that can be used to adjust the resistors in the design to more closely
approximate available 5% tolerance values [2]. This is easy to do using the LabVIEW
continuous run feature, which allows a VI to be run repeatedly while a student manually adjusts
a front panel parameter [1].
In this application, the virtual instrument simplifies the circuit design process and shortens the
time students have to spend calculating resistor values in the lab. This time saving feature has
been greeted with nearly universal enthusiasm according to the student evaluations.
Load Line Analysis and Verification of the Bias Circuit Design
Once the circuit design has been completed, the student is instructed to verify the design Q-point
using a graphical load line approach. The VI shown in figure 5 (wiring diagram) and figure 6
Proceedings of the 2004 American Society for Engineering Education Pacific Southwest Section Conference
April 1-2, 2004, University of the Pacific, Stockton, CA
Figure 5. Wiring Diagram for the Load Line Analysis VI
Figure 6. Front Panel for the Load Line Analysis VI Showing Graphical Q-point Verification
Proceedings of the 2004 American Society for Engineering Education Pacific Southwest Section Conference
April 1-2, 2004, University of the Pacific, Stockton, CA
(front panel) allows the student to enter chosen 5% resistor values for the circuit and to
graphically verify the Q-point using the intersection of the simultaneously plotted load line and
transistor I-V characteristic equations. It is then possible for the student to make slight
adjustments to the chosen resistor values and quickly see the effect on the circuit Q-point. This
kind of design adjustment can be done much more rapidly and effectively with LabVIEW than
with circuit simulations using PSpice [3]. Again, student evaluations of this tool have been very
positive in terms of how it helps them to appreciate the effect of resistor variations on the circuit
performance.
Protoboard Circuit Construction and Q-point Measurement
The final step in this laboratory exercise is for the students to construct their circuit design on a
protoboard and use a LabVIEW measurement VI to determine the circuit Q-point. Figures 7 and
8 show the wiring diagram and front panel, respectively, for the VI used to measure the Q-point
voltage and current. The drain-source voltage and voltage across RD are simultaneously
measured, and lowpass filtering is incorporated to reduce noise.
Figure 7. Wiring Diagram for the Q-point Measurement VI
Once again, the VI simplifies the measurement and saves the student valuable laboratory time.
Students have evaluated this VI as moderately helpful to their verification of circuit operation.
Proceedings of the 2004 American Society for Engineering Education Pacific Southwest Section Conference
April 1-2, 2004, University of the Pacific, Stockton, CA
Figure 8. Front Panel of the Q-point Measurement VI Showing Measured Values of Q-point
Voltage and Current from a Protoboard Circuit
Summary and Conclusions
This paper has presented an application of National Instruments LabVIEW software to a
laboratory exercise on MOSFET transistor parameter analysis and dc biasing in an introductory
electronics course. Cost effective special purpose virtual instruments have been developed,
which aid beginning students in analyzing actual transistor characteristics, designing and
verifying a four-resistor dc bias circuit, and confirming the Q-point of the design by direct
measurements using a protoboard circuit. It should be recognized that this approach offers the
beginning student an opportunity to make sophisticated electronic measurements without having
previous experience with laboratory instrumentation. Student evaluations conducted over a
period of five years in the course have verified that this virtual instrument approach is very
effective in helping beginning students to understand the link between transistor theory and
application.
Proceedings of the 2004 American Society for Engineering Education Pacific Southwest Section Conference
April 1-2, 2004, University of the Pacific, Stockton, CA
References
[1] Bishop, R.H., Learning with LabVIEW Student Edition. Prentice-Hall, NJ, 2002
[2] Jaeger, R.C., Microelectronic Circuit Design. McGraw-Hill, NY, 1997.
[3] Herniter, M.E., Schematic Capture with Cadence PSpice, (2nd Ed.). Prentice-Hall, NJ, 2003
Author Information
John T. Oldenburg, Ph.D., is a Professor of Electrical & Electronic Engineering at California State University,
Sacramento.
Proceedings of the 2004 American Society for Engineering Education Pacific Southwest Section Conference
April 1-2, 2004, University of the Pacific, Stockton, CA