Laboratory No. 5- MOSFETs And BJTs DC Characteristics

ECSE 331
Laboratory No. 5
MOSFETs And BJTs DC
Characteristics
Objective:
Investigate the i-v characteristics of a MOSFET and BJT transistor. Design a resistor biasing network
to establish the DC operating point of a transistor.
Equipment Required:
1. NI Elvis-II+ test instrument
2. PC with ELVIS-II+ software installed
3. Heat Gun
4. Aerosol Freeze Spray to cool diode
5. Hand-held thermal imager
6. Components:
a. BS170 NMOS transistor
b. 2N2222A npn transistor
c. 10 kΩ potentiometer
d. resistors: 100 Ω, 10 kΩ
Description of the NI Elvis-II+ Test Instrument:
The National Instruments Educational Laboratory Virtual Instrumentation Suite (NI ELVIS-II+
) is a
hands-on design and prototyping platform that integrates the 12 most commonly used instruments –
including oscilloscope, digital multi-meter, function generator, bode analyzer, and more. It connects to
a PC through a USB connection, providing quick and easy acquisition and display of measurements.
The National Instruments Educational Laboratory Virtual Instrumentation Suite consists of two main
components:
1. The bench-top workstation (NI ELVIS-II+
), which provides instrumentation hardware and
associated connectors, knobs, and LEDs as shown in Fig. 1(a). A prototyping
board (breadboard) sits on top of the workstation, plugged into the NI ELVIS-II+ platform,
and offers hardware workspace for building circuits and interfacing experiments.
2. NI ELVIS-II+ software, which includes soft front panel (SFP) instruments. Fig. 1(b) illustrates
the PC screen view of the oscilloscope interface.
The Elvis-II+ prototype board consists of 5 separate areas: four small prototype areas on the
peripheral of the board, and a much larger central prototype area. The boards on the peripheral are
used to interface to the internal data acquisition board of the Elvis-II+ system. There are marking
along the perimeter of the board indicating the connection. The student was introduced to this test
bench back in Laboratory 1. The student should refer back to this lab for a detailed description of the 
G. Roberts/Y. Li, Feb. 28, 2017 2
NI ELVIS-II+ test system.
Practical Information for the Student:
In performing this experiment, you will construct several building blocks of modern transistor circuits.
On doing so, keep the following points in mind:
1. In the figures, more positive voltages are always shown closer to the top of the page, so that
current flows in the circuit from top to bottom.
2. Keep all connecting leads as short as possible and pushed down to the surface of the circuit
board. You want to avoid a “rat’s nest” of wiring whereby unwanted coupling capacitances and
series inductances are created. These parasitics can prevent your circuit from working
correctly.
3. Use different color wires in your circuit. The standard convention is red for positive supply
voltages, blue for negative supply voltages and black for ground. Use colors that are different
than these for signals. If you use a single color for everything, your circuit should still work.
However, the debug process will be much more difficult if something goes wrong. It is
important for the student to understand that when asked to design a circuit it is not limited to
just the arrangement and component selection for the circuit but also to the fact that the
design may not work the first time it is assembled. In general, all design will require at least
two iterations before one can declare success!
4. In understanding circuits, as well as designing them, keep in mind that that there are two
interleaved problems: the dc design, which establishes the operating points, and the ac
(a)
(b)
Fig. 1: (a) Elvis-II+ instrumentation hardware with prototyping board, and (b) menu
for various virtual instrument.
3 G. Roberts/Y. Li, Feb. 28, 2017
design, which is responsible for how the circuit responds to signals. The student must design
both problems at the same time; if you make a change to one, be sure that you haven’t
changed the other.
Write-Up Requirements:
A good laboratory report should contain a brief description of what the experiment was about,
including circuit diagrams, and what you did, your data, your results, and anything else called for in
the assignment, such as questions inserted in the laboratory. Answers to these questions require
observations that need to be made at the time you do the experiment.
The laboratory report should be written using the IEEE paper style consisting of a double-column
single-space format, and must adhere to the following:
1. Title page - Title of the assignment/project, authors' name, and course name.
2. Abstract - Abstract of the assignment/project report.
3. Introduction
4. Main body of the assignment/project report including figures.
6. Conclusions
7. References
8. Appendices
Procedure:
I. MOSFET iD-vDS Characteristics Using a Curve Tracer
In a similar manner to that which was performed for the diode in Laboratory 4, here the student
will extend the use of the curve tracer to one that can sweep the MOSFET drain-source voltage
vDS while measuring its drain current iD for different gate voltages.
The student will recall that the curve tracer apparatus consists of the function generator
generating a triangular voltage waveform and a current probe constructed from a differential
amplifier with a 100-W sense resistor. The A and B channels of the oscilloscope will then enable
one to display the drain current as a function of the drain-source voltage. The function generator
and the oscilloscope can be found in the Elvis-II+ test system.
Construct the circuit shown in Fig. 2 using the 741 op-amp and the BS170 NMOS transistor. The
BS170 is a single packaged device. The pin-outs for this IC are shown in Fig. 3. The student
should already be familiar with the pinouts of the 741 from previous experiments.
Using the NMOS transistor, connect the drain terminal to the function generator, connect the
wiper of a 10-kW potentiometer to the gate terminal operating with 5 V across its main terminals,
and connect the source terminal to ground as shown in Fig. 2. Set the function generator feature
to generate a saw-tooth waveform over a 0 V to +10 V signal range. Connect the VX node to
channel A of the oscilloscope and the VY to channel B of the oscilloscope. As the voltage VY is
100-times larger than the current, either scale the input by a factor of one-hundred, or simply
keep track of the y-axis units and adjust any plot accordingly. Set the oscilloscope to plot channel
A versus B, so that an iD-vDS characteristic will appear on your screen. Beginning with the gate 
G. Roberts/Y. Li, Feb. 28, 2017 4
voltage set to 0 V, observe that the drain current iD is zero while the drain-source voltage vDS
sweeps from 0 to 10 V. Next, adjust the screw of the potentiometer so that the drain current iD
begins to flow. According to the manufacturers data sheet the threshold voltage is approximately
2 V. Measure the gate voltage at which this point and record it. This voltage is the threshold
voltage Vt of the MOSFET. Next, increase the gate voltage in increments of 500 mV until the gate
voltage reaches 5 V, and capture the iD-vDS characteristics. In your report, combine all the curve
traces into a single plot and observe the overall behavior. What is the effective Early voltage of
your device?
In this next test, set the gate voltage to 3 V by adjusting the potentiometer and the drain voltage
to 5 V. This will require that the saw tooth signal source be replaced with a DC variable voltage
source. Once complete, measure the gate voltage and drain current, and record these results.
Next, adjust the gate voltage by approximately 100 mV. Again, measure the gate voltage and
drain current, and record these results. Compute the transconductance gm of your device using
the following formula:
Draw the equivalent small-signal model of your MOSFET for low-frequency operation.
II. MOSFET Temperature Effects
In this part of the laboratory, you will measure the temperature effects of the MOSFET that you
used in part A of this experiment. This will require access to a hot-air blowgun, a can of freeze
spray (spray-on liquid Nitrogen) and a hand-held thermal imager. These are available in the
microelectronics laboratory in Rm 4090 of the Trottier building.
For this step, return to the curve tracer set-up involving the BS170 NMOS transistor shown in Fig.
2. Remember to reconnect the saw tooth waveform signal to the drain of the MOSFET and set to
a 0 – 10 V sweep. Display the iD-vDS characteristic for the MOSFET using your oscilloscope for a
gate voltage of 3 V. Use the hand-held thermal imager and determine the temperature of the
MOSFET. Record your iD-vDS characteristic at this temperature. Now, using the can of freeze
spray, blow the freeze-spray coolant directly onto the MOSFET, while at the same time exercising
its i-v characteristic. Measure the temperature of the MOSFET using the hand-held thermal
imager and record this information along with the iD-vDS characteristics. Now, repeat this
procedure but this time use the hot-air blowgun, blow hot air onto the MOSFET. Avoid heating up
the op-amp circuit. This will raise the temperature of the MOSFET. Measure the temperature of
the MOSFET and record this information along with the iD-vDS characteristics. Compare the three
plots taken at the three different temperatures. How do they compare? How do you expect the
iD-vDS characteristics to change with temperature? Provide an explanation with supporting theory.
Can you suggest a method in which to reduce the changes in MOSFET behavior as a function of
temperature?
III. BJT iC-vCE Characteristics Using a Curve Tracer
In this part of your laboratory you will investigate the iC-vCE characteristics of the 2N2222A npn
transistor. Using your curve-tracer circuit, place the BJT into the test port as shown in Fig 4. The
pin-outs for this device is shown in Fig. 5. Set the base voltage using the 10-kW potentiometer so
that it is 0 V. Set the saw tooth waveform output from the function generator to produce a signal
that varies between 0 V to 10 V. Observe the iC-vCE characteristic on the oscilloscope in the same
gm = ΔiD
ΔvGS
5 G. Roberts/Y. Li, Feb. 28, 2017
manner described in part I of this experiment. You should see a flat line represent zero collect
current. Next, increase the base voltage by steps of about 50 mV to no more than 750 mV –
approximately two turns of the screw adjustment control of the potentiometer. Be aware that the
BJT is extremely sensitive to this voltage, so take care with this step. Capture the iC-vCE
characteristics corresponding to each of these base voltage settings. In your report, combine
about ten traces into a single plot and observe the overall behavior. What is the effective Early
voltage of your device?
In this next test, set the base voltage to 650 mV by adjusting the potentiometer and the collector
voltage to 5 V. This will require that the saw tooth signal source be replaced with a DC
programmable source. Once complete, measure the base voltage and collector current, and
record these results. Next, adjust the base voltage to 700 mV . Again, measure the base voltage
and collector current, and record these results. Compute the transconductance gm of your device
using the following formula:
Draw the equivalent small-signal model of your BJT for low-frequency operation.
IV. BJT Temperature Effects
For this step, return to the curve tracer set-up involving the BJT shown in Fig. 4. Remember to
reconnect the saw tooth waveform signal to the collector of the BJT and set to a 0 – 10 V sweep.
Display the iC-vCE characteristic for the MOSFET using your oscilloscope for a base voltage of 700
mV. Use the hand-held thermal imager and determine the temperature of the BJT. Record your
iC-vCE characteristic at this temperature. Now, using the can of freeze spray, blow the freeze-spray
coolant directly onto the BJT, while at the same time exercising its i-v characteristic. Measure the
temperature of the BJT and record this information along with the iC-vCE characteristics. Now,
repeat this procedure but this time use the hot-air blowgun, blow hot air onto the BJT. Avoid
heating up the op-amp circuit. This will raise the temperature of the BJT. Measure the
temperature of the BJT and record this information along with the iC-vCE characteristics. Compare
the three plots taken at the three different temperatures. How do they compare? How do you
expect the iC-vCE characteristics to change with temperature? Provide an explanation with
supporting theory.
This concludes this lab.
gm = Δi
C
ΔvBE
G. Roberts/Y. Li, Feb. 28, 2017 6
Fig. 2: Curve-trace set-up for measuring the MOSFET i-v characteristics.
Fig. 3: Pin out for the BS170 NMOS transistor.
(Pin 1: Drain, Pin 2: Gate, Pin 3: Source).
7 G. Roberts/Y. Li, Feb. 28, 2017
Fig. 4: Curve-trace set-up for measuring the BJT i-v characteristics.
Fig. 5: Pin out for the BJT