Revision: March 09, 2015

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1 Lab Assignment 7: MOSFET Circuits Produced in cooperation with Revision: March 09, Overview In this lab assignment, we will experimentally determine the characteristic curves for both n and p channel MOSFETs. We will use these curves to estimate MOSFET model parameters. Finally, we will design and construct several simple circuits incorporating MOSFETS. Before beginning this lab, you should be able to: Sketch circuit symbols for n- and p- channel MOSFETs; labeling gate, source and drain terminals (Module 7.1) Sketch typical characteristic curves for n-channel MOSFETs, labeling gate-tosource voltage, drain-to-source voltage, and drain current (Module 7.1) Identify triode and saturation regions on n-channel MOSFET characteristic curves (Module 7.1) Determine MOSFET drain current when the gate-to-source voltage is less than the threshold voltage (module 71) Calculate MOSFET output resistance and transconductance (Module 7.2) State governing equations for MOSFET characteristics in both triode and saturation regions. After completing this lab, you should be able to: Experimentally determine the characteristic curves for both n and p channel MOSFETs Use experimental characteristic curves to determine MOSFET model parameters Use MOSFETs to construct the following simple circuits: (a) Variable resistor (b) Current mirror (c) Buffer amplifier This lab exercise requires: EE 352 Analog Parts Kit Breadboard Function Generator, oscilloscope, DC power supplies Symbol Key: Demonstrate circuit operation to teaching assistant; teaching assistant should initial lab notebook and grade sheet, indicating that circuit operation is acceptable. Analysis; include principle results of analysis in laboratory report. Numerical simulation (using PSPICE or MATLAB as indicated); include results of Matlab numerical analysis and/or simulation in laboratory report. Record data in your lab notebook. Contains material Digilent, Inc. 7 pages

2 Lab Assignment 7: MOSFET Circuits Page 2 of 7 I. MOSFET Characteristics Pre-lab: None Using the TEK 571, obtain a printout and sketch in your lab notebook the family of curves relating to both the ZVN2110A NMOS and ZVP2110A PMOS discrete enhancement type transistors. In addition, using the diode function of the TEK 571 obtain printouts of the i D -v GS characteristics for both MOSFETs. Hint: For the NMOS, connect G & D to the anode connection and S to the cathode. Note: Connections for these MOSFETs are provided in Appendix A of this lab assignment. Using the printouts obtained above, determine V t, K, V A, λ, and g m for both n and p channel enhancement-type MOSFETs. Equations governing these parameters are available in Sedra and Smith. Post-Lab Exercises: Using the MOSFET parameters determined above, use PSPICE to simulate i D vs. v DS characteristics for both the n- and p-channel MOSFETs. Compare your simulated and measured characteristic curves. II. Triode Region Operation Pre-lab: None In the triode region, the i D -v DS characteristics for an ideal MOSFET are approximately linear for a given value of v GS. This allows the device drain-to-source characteristics to operate as a linear resistor whose value is controlled by v GS. Our goal in this portion of this lab assignment is to use a discrete ZVN 2110A MOSFET to create a variable resistance within a voltage divider. The basic circuit is shown in Figure 1. The variable resistance will allow us to set the output voltage, v o, by adjusting the MOSFET gate voltage, v i. The objective for this assignment is to design the circuit shown in Figure 1 (e.g. choose a value for R) to provide a vo which ranges from 10% to 50% of v DD, depending on the input voltage v i. One possible design approach is as follows: (a) Determine values of r DS as a function of v GS. (or v i in Figure 1). This may require you to regenerate the characteristic curves you determined in Part I for the MOSFET, in order to emphasize the triode region. (b) For the available range of r DS determined in part (a), determine a value for R to provide the desired output voltage range (10% to 50% of the reference voltage, v DD ). Assume that the circuit of Figure 1 corresponds to a voltage divider where the MOSFET provides a variable resistance r DS which varies with the applied voltage v GS.

3 Lab Assignment 7: MOSFET Circuits Page 3 of 7 (c) Estimate the expected output voltage, v o, as a function of the input voltage, v I, for your value of resistance, R. (d) Construct and test the circuit and compare your results with the expected results of part (c). Does your circuit meet the design requirements? (Note: To operate in the triode region, v DD will need to be rather small, typically around 0.1 to 0.2 volts for the ZVN2110A NMOS in your parts kit. If you wish, you can test your circuit using an input voltage v i equal to a time-varying signal of small amplitude created by your function generator; note that v i will need a DC offset higher than the threshold voltage V t for your device, which you should have determined in Part I.) Demonstrate your circuit to the TA and have them initial your lab notebook. v DD R v o v i Figure 1. Variable resistance voltage divider. III. MOSFET Buffer Amplifier A buffer amplifier is useful in cases where it is not necessary to increase voltage across the amplifier, but an increase in power is desired. Buffer amplifiers in general provide little or no voltage gain but have high input resistance relative to the source resistance and low output resistance relative to the load. One possible implementation of a buffer amplifier, is the single stage MOSFET amplifier shown in Figure 2. The operation of this circuit can be qualitatively described as follows: When the input voltage V in is less that the MOSFET s threshold voltage, the MOSFET is cut off and no current flows to the load. As V in increases above the threshold voltage, the MOSFET conducts and power is applied to the load. The output voltage V out increases as V in increases, so the circuit acts as a noninverting amplifier. It is important to note that the power supplied to the load is provided by the power supply V DD, not the input V in. It is also important to note that the circuit provides no power to the load if V in is negative; for linear operation, therefore, the circuit will generally be biased at a positive value of V in. In this lab, we will characterize the large-signal operation of the buffer amplifier shown in Figure 2. We will use this characterization data to determine the input signal necessary to provide a specified set of operating conditions to the load.

4 Lab Assignment 7: MOSFET Circuits Page 4 of 7 Figure 2. MOSFET-based buffer amplifier Pre-lab: (a) The voltage applied to the load resistor in the circuit shown in Figure 1 can range from zero volts to something less than V DD. If you assume that V out ranges from zero to V DD volts, calculate a range of power which can be delivered to the load (in terms of V DD and R L ). (b) Determine the power delivered to the circuit of Figure 2, in terms of V in and R G. (a) Connect the buffer amplifier to a DC power supply as shown in Figure 3. Use the IRF510 MOSFET included in your parts kit as the MOSFET in the buffer amplifier. Pin descriptions for the IRF510 are provided in Appendix A of this lab assignment. You may wish to use a decade resistor box for the load resistor, depending upon your expectations from the pre-lab for power delivery to the load (the resistors in your analog parts kit are generally rated for about 0.25W of power). (b) Determine a transfer characteristic for the buffer amplifier/motor combination: To do this: Apply 10V DC at V DD as shown. Vary the input voltage, V in, applied to the buffer amplifier and measure the voltage applied to the load, V Load. Use a range of V in which allows V Load to span as much as possible of the 0-10V range allowed by V DD. Tabulate your results and plot V in vs. V Load in your lab notebook. (c) From your transfer characteristic of part (b), determine and plot the input power applied to the buffer amplifier vs. the output power provided to the load. What is the gain in power provided by the amplifier, when the voltage applied to the load is approximately 5V? Demonstrate your circuit to the TA and have them initial your lab notebook and the lab checklist.

5 Lab Assignment 7: MOSFET Circuits Page 5 of 7 Figure 3. Circuit connections to determine system transfer characteristic. IV. Current Mirror It is often desirable in IC design to have a constant current source. In general, in these devices, a constant DC current is generated at one location in the circuit and is replicated at other locations in the circuit. One application for such devices is biasing amplifier stages. Consider the circuit of Figure 4. If the current i D2 is fixed by setting v DD and R and appropriate biasing of the MOSFET Q 2, the current i D1 is fixed by the relative characteristics of the MOSFETs Q 1 and Q 2. If Q 1 and Q 2 are identical, then i D1 = i D2. Figure 4. Constant current source.

6 Lab Assignment 7: MOSFET Circuits Page 6 of 7 Pre-lab: Carefully review Example 4.3 and work Exercise 4.12 in Sedra and Smith. Design and construct a current mirror with a reference current of 1mA. (Note: the total current from your current mirror is to be twice the reference current.) The reference current can be set with a variable resistor and a DC voltage supply. Your MOSFETs in the circuit will need to be well matched; use the CMOS Dual Complementary Pair Plus Inverter, CD4007, to implement the matched MOSFETs. Demonstrate your circuit to the TA and have them initial your lab notebook and the lab checklist. Lab Report: In your lab report, provide a summary of the results of this lab assignment. You should include, at a minimum, all items indicated on the lab checklist. Append the lab checklist sheet with teaching assistant initials indicating completed lab demos to your report.

7 Lab Assignment 7: MOSFET Circuits Page 7 of 7 Appendix A Transistor and IC Pin Descriptions Figure A1. Pin descriptions fo ZVN & ZVP 2110A MOSFETs Figure A2. Pin description for IRF 510 MOSFET. Figure A2. CMOS Dual Complementary Pair Plus Inverter CD4007. Additional details can be found in the file hcf4007.pdf on the class web site.