EGRE 207 – Electrical Circuits II
EGRE 306 – Introduction to Microelectronics
Spring 2021
Lab 3 –MOSFET biasing
Part I – Introduction to MOSFET
Using Multisim, you will first measure the I-V characteristics of an n-channel MOSFET so that you can predict the transistor’s behavior. Then, you will take data to use the transistor as a voltage-controlled resistor.
Set-Up
Using Multisim, place a 2N7000 NMOS transistor and connect the Source pin to ground. Then, connect two separate DC power sources to the Gate and Drain pins. Finally, connect a cu
ent probe to measure the drain cu
ent.
Part I – Simulation of Threshold Voltage
To measure the threshold voltage, you will apply a large VDS voltage (10V) and slowly increase the gate voltage until you obtain a cu
ent. Remember that if VGS < Vt, then the transistor will be in the cutoff region and “no” cu
ent will flow.
Continue to increase VGS and measure and plot the ID vs. VGS graph for your transistor. Your resulting graph should resemble the figure below. Do not exceed 200mA.
Use a cu
ent of 1.0mA to determine the threshold voltage (Vt).
Part II – MOSFET as a Voltage Controlled Resistor (VCR)
When the MOSFET is in the triode region (small vDS), it can be used as a Voltage Controlled Resistor. By varying the gate voltage while in this region, the resistance of the channel will change.
Now, you will take data to help you cali
ate your voltage-controlled resistor.
1. Set vDS to a small value (~50mV).
2. Slowly change VGS and monitor the drain cu
ent for each gate voltage.
3. After you have completed this step for at least 5 different gate voltages, change vDS to ~100mV and repeat.
For each VGS value, you now have 2 data points. You can determine the resistance of each VGS value by using the following example as a guide:
vDS = 0.05V ID = 0.3mA at VGS=3V
ID = 0.4mA at VGS=4V
vDS = 0.1V ID = 0.5mA at VGS=3V
ID = 1.1mA at VGS=4V
For VGS = 3V The drain cu
ent increased 0.2mA when the drain voltage
increased by 0.05V. The channel resistance at VGS = 3V is:
0.05V / 0.2mA = 250
For VGS = 4V The drain cu
ent increased 0.7mA when the drain voltage
increased by 0.05V. The channel resistance at VGS = 4V is:
0.05V / 0.7mA = 71.4
Using this example as a guide, determine the resistance of your transistor for 5 different gate voltages.
4. Recall :
Using your results, determine the MOSFET transconductance value (kn) for your transistor (calculate it for each of your ten data points).
5. Determine the co
ect bias conditions (VGS) so that your transistor has a resistance of 110 Ohms.
Part I Data. XXXXXXXXXXVt = ____________
Part II Data XXXXXXXXXXkn = ____________
Design: For rDS = 110 Ohms: VGS = ____________
Lab 3 – MOSFET biasing
Part II - DC Biasing of MOSFET
Using Multisim, you will first estimate the parameters of a 2N7000 n-channel MOSFET so that you can properly bias the transistor. Then, you will investigate alternate configurations of the bias circuit.
Part I – Estimation of Threshold voltage and MOSFET Transconductance (kn)
1. Connect two separate power supplies to the Drain and Gate of your MOSFET. Apply a large VDS voltage (5V).
2. Connect the Source to ground and use the ammeter to measure the drain cu
ent (between the VDD source and the drain of the transistor).
3. Adjust the Gate voltage until the cu
ent is approximately equal to 20mA. Remember that if VGS < Vt, then the transistor will be in the cutoff region and “no” cu
ent will flow.
4. Record the gate voltage (VGS1).
5. Increase VDD until the drain cu
ent is approximately equal to 80mA (~4 times the original ID)
6. Record the gate voltage (VGS2).
Using the data from steps 3, 4, 5, 6 and
ID = 0.5 kn (VGS1 – Vt)2 4ID = 0.5 kn (VGS2 – Vt)2
you can estimate the Threshold Voltage (use exact values for ID, 4ID, VGS1 and VGS2).
Then, using your data and your estimate of Vt, you can find kn.
VGS1 = ______________ ID1 = ______________
VGS2 = ______________ ID2 = ______________
Vt = ________________ kn = ________________
Q1: For all of these measurements, what region of operation is the transistor operating in?
Discuss with your partner how you know this.
Part II – DC Biasing Circuit for MOSFET
1. Build the circuit on the right, using VDD = 15V,
RG2 = 150k, and RS = 0 (short circuit).
2. Find (calculate) RG1 and RD so that there will be a drain
cu
ent of ~20mA and VDS ~VDD/2
(The VDD voltage is split evenly between
the resistor and the transistor).
3. Attach RG1 and RD into your circuit, and
measure VD and ID.
Analysis Hints:
a. Assume the transistor is operating in the saturation region.
. Recall that the cu
ent entering the gate is zero.
RG1 = _____________ RD = _____________
VG (actual) = _____________ VD (actual) = _____________ID (actual) = _____________
Q2: What are pros/cons of this biasing circuit (RS = 0).
Part III – Another DC Biasing Circuit for MOSFET
1. Build the circuit on the right, using your
original transistor, VDD = 15V,
RG2 = 150k, and RD = 470.
2. Find (calculate) RG1 and RS so that VD ~VDD/2 and VDS ~VDD/3.
3. Attach RG1 and RS into your circuit, and
measure VD, VS, and ID.
Analysis Hints:
a. Assume the transistor is operating in the saturation region.
. Recall that the cu
ent entering the gate is zero.
RG1 = _____________ RS = _____________ VG (actual) = _____________
VD (actual) = _____________ VS (actual) = _____________ID (actual) = _____________
Q3: What are pros/cons of this biasing circuit?
Lab 3 Part I and Lab 3 Part II will be combined into a single report
Assume that the audience for this report is an EGRE 306 student who didn’t complete the lab, but wants to know exactly what you did. He/she has been to class, so they are familiar with the concepts and the equipment that you use.
The following information from Part II should be included to the lab report:
**Only have one of each section listed below, containing all of your work for each part (Part I and Part II) of the lab**
1. Cover Page (As seen on previous templates, include: name, class, title, section, date, partner (if you have one), honor pledge - signed, etc.)
2. Introduction to the lab: Explain what the purpose of this lab was and a
ief description of the purpose for each part of the lab you completed (max of 2 paragraphs).
3. Background and Theory: This includes relevant background information which explains why your experiment worked the way that it did.
If taking reference from other sources, be sure to include citations.
3. Procedure: This is an explanation of what you did, experimentally, in the lab. Do NOT include results in this section. Include schematics of circuits made.
4. Experimental Results: This is where you present the results of your experiments (data tables, screenshots, graphs, etc.). In addition, explain your results.
5. Conclusion: This is where you write a summary of the experiment, including answers to all of the questions that were included in the lab handout.
To Be Included:
items that you should include in the appropriate sections are:
1. Schematic diagram of your Part I circuit (using Multisim). Include voltmete
ammeter
where you measured voltage and cu
ent.
· Description of HOW you determined Vt for your transistor.
· Step-by-step explanation of how you determined the resistance values in Part III. A table of your data may be helpful.
· From the Part II data, create a plot of rDS (channel resistance) versus Vov for your transistor, where Vov = VGS - Vt. Discuss the relationship between rDS and Vov.
2. Explanation (include experimental procedure, data table, and calculations) of how you
determined the threshold voltage and MOSFET transconductance parameters for your transistor. (This will be a lengthy answer)
3. Schematic diagram of your Part II circuit (using Multisim). Include voltmete
ammeter
where you measured voltage and cu
ent.
4. Part II calculations, showing how you determined RD and RG1 that satisfy the given
criteria. Be sure to EXPLAIN your calculations, not just a list of equations.
5. Data Table with all of your measured values from step II.3 and II.4.
6. Schematic diagram of your Part III circuit (using Multisim). Include
voltmete
ammeter where you measured voltage and cu
ent.
7. Part III calculations, showing how you determined RS and RG1 that satisfy the given
criteria. Be sure to EXPLAIN your calculations, not just a list of equations.
8. Data Table with all of your measured values from step III.3 and III.4.
9. Answers/Discussion for Q1 – Q3.