Massachusetts Institute of TechnologyDepartment of Electrical Engineering and Computer Science6.002 – Circuits and ElectronicsSpring 2003Handout S03-034 - Lab #2: MOSFET Inverting Amplifiers & First-OrderCircuitsIntroductionThis lab examines the behavior of an inverting MOSFET amplifier. It begins by examining thestatic input-output relation of the amplifier, and concludes by examining the dynamic behavior ofthe same amplifier when used as a digital logic inverter. You should complete the pre-lab exercisesin your lab notebook before coming to lab. Then, carry out the in-lab exercises between March 17and March 21. After completing the in-lab exercises, have a TA or LA check your work and signyour lab notebook. Finally, complete the post-lab exercises in your lab notebook, and turn in yourlab notebook on or before Monday April 7.Pre-Lab Exercises(2-1) Consider the inverting MOSFET amplifier shown in Figure 1. Using the SCS MOSFETmodel, determine vOUTas a function of vINfor 0 ≤ vIN≤ vOUT+ vT. Also, sketch andclearly label vOUTas a function of vINover the same range.(2-2) Determine the small-signal gain of the MOSFET amplifier shown in Figure 1 assuming thatits MOSFET is biased into saturated operation.(2-3) Consider the network shown in Figure 2. First, assume that vOUT= 0 at t = 0. Then,determine vOUT(t) for t ≥ 0 given that vINsteps from 0 V to VIat t = 0. Second, assumethat vOUT=R2R1+R2VIat t = 0. Then, determine vOUT(t) for t ≥ 0 given that vINstepsfrom VIto 0 V at t = 0.(2-4) For both transients determined in Pre-Lab Exercise 2-3, determine the time at which vOUTreaches a given VTwhere 0 < VT<R2R1+R2VI.+-RVS+-vOUTvINFigure 1: inverting MOSFET amplifier for Pre-Lab Exercises 2-1 and 2-2.In-Lab ExercisesAs part of the in-lab exercises, you will measure the threshold voltage and gate-to-source capacitanceof a MOSFET. These parameters will be used to interpret the results of other in-lab exercises.Therefore, use the same MOSFET in every in-lab exercise described below.(2-1) This exercise measures the static input-output relation of the MOSFET amplifier shown inFigure 1. To begin, construct the amplifier as shown in Figure 3, and connect the signalgenerator and oscilloscope as shown. Next, set the signal generator to produce a 1-kHz sinewave with a peak-to-peak amplitude of 3 V and an offset of 1.5 V. Thus, the signal generatorwill produce a biased sine wave between 0 V and 3 V. Set the oscilloscope to operate in itsX-Y mode with an X-axis (Channel #1) sensitivity of 500 mV per division and a Y-axis(Channel #2) sensitivity of 1 V per division. You should now see the input-output relationdisplayed on the oscilloscope. Finally, compare the displayed relation to that sketched inPre-Lab Exercise 2-1.Record the following data. First, record the value of vINabove which vOUTjust begins tofall. This is the threshold voltage vTof the MOSFET; see the sketch from Pre-Lab Exercise2-1. Second, record the values of vINwhich correspond to vOUTvalues of 5 V, 4 V, 3 V, 2 Vand 1 V. Alternatively, you may find it easier and much more accurate to use the signalgenerator as a programmable vINsource and measure vOUTwith a multimeter.(2-2) This exercise measures the small-signal gain of the amplifier shown in Figure 1 when itsoutput bias voltage is 2 V. To begin, construct Circuit #1 shown in Figure 4. Adjustthe potentiometer until vOUT= 2 V as measured by the multimeter. Next, connect thesignal generator and the oscilloscope as shown in Circuit #2. Set the signal generator to+-+-vOUTCR2R1vINFigure 2: network for Pre-Lab Exercises 2-3 and 2-4.+-5V SignalGeneratorOscilloscopeChannel #1OscilloscopeChannel #21kΩFigure 3: measuring the static input-output relation of the MOSFET amplifier shown in Figure 1.produce an unbiased 1-kHz sine wave with a peak-to-peak amplitude of 100 mV. Measurethe amplitude of both vinand vout, which are the sinusoidal components of vINand vOUT,respectively; use AC coupling in Channel #1 of the oscilloscope to accurately measure vin.The ratio of the amplitudes is the small-signal gain. Finally, adjust the input bias with thepotentiometer, and observe the variation in vOUT.(2-3) This exercise measures the gate-to-source capacitance CGSof the MOSFET. First, constructthe circuit shown in Figure 5. Set the signal generator to produce a 20-kHz square wave withan amplitude of 5 V peak-to-peak and an offset of 2.5 V. The oscilloscope should display afirst-order step response. Measure the time constant of that step response. Second, removethe MOSFET from the circuit, and measure the time constant again.(2-4) This exercise measures the delay of the MOSFET amplifier shown in Figure 1 when it isused as a digital logic inverter. Construct the circuit shown in Figure 6; the 100-kΩ resistorin this circuit models the Thevenin resistance of whatever drives the inverter. Next, connectthe oscilloscope and signal generator as shown. Set the signal generator to produce a 20-kHzsquare wave with an amplitude of 5 V peak-to-peak and an offset of 2.5 V. Finally, use theoscilloscope to measure the delay from the time at which the signal generator switches highto the time at which the inverter output begins to switch low. Also, measure the delay fromthe time at which the signal generator switches low to the time at which the inverter outputbegins to switch high. Since the output of the inverter begins to switch when the MOSFETgate voltage passes by vT, the two delays may not be the same; see Pre-Lab Exercise 2-4.+-5V SignalGeneratorOscilloscopeChannel #1OscilloscopeChannel #21kΩ+-vIN10kΩ+-vOUT5V+-vOUT1kΩ10kΩCircuit #1Circuit #2Figure 4: measuring the small-signal gain of the MOSFET amplifier.+- SignalGeneratorOscilloscopeChannel #1OscilloscopeChannel #2100kΩFigure 5: measuring the gate-to-source capacitance of the MOSFET amplifier.Post-Lab Exercises(2-1) This exercise examines how well the MOSFET amplifier model developed during Pre-LabExercise 2-1 explains the input-output relation measured during In-Lab Exercise 2-1. Themodel contains four parameters which are required to numerically evaluate the input-outputrelation: VS, R, vTand K. From Figure 3, VS= 5 V and R = 1 kΩ. Further, vTwasmeasured during In-Lab Exercise 2-1. Thus, only K is unknown. Use the value of vINrecorded for vOUT= 1 V to determine K. Then, use the numerical parameters and themodel to graph vOUTas a function of vINfor 1 V ≤ vOUT≤ 5 V. On this graph, also plotthe data measured during In-Lab Exercise
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