Massachusetts Institute of Technology Department of Electrical Engineering and Computer Science 6 002 Circuits and Electronics Spring 2003 Handout S03 034 Lab 2 MOSFET Inverting Amplifiers First Order Circuits Introduction This lab examines the behavior of an inverting MOSFET amplifier It begins by examining the static input output relation of the amplifier and concludes by examining the dynamic behavior of the same amplifier when used as a digital logic inverter You should complete the pre lab exercises in your lab notebook before coming to lab Then carry out the in lab exercises between March 17 and March 21 After completing the in lab exercises have a TA or LA check your work and sign your lab notebook Finally complete the post lab exercises in your lab notebook and turn in your lab notebook on or before Monday April 7 Pre Lab Exercises 2 1 Consider the inverting MOSFET amplifier shown in Figure 1 Using the SCS MOSFET model determine vOUT as a function of vIN for 0 vIN vOUT vT Also sketch and clearly label vOUT as a function of vIN over the same range 2 2 Determine the small signal gain of the MOSFET amplifier shown in Figure 1 assuming that its 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 vIN steps from 0 V to VI at t 0 Second assume 2 VI at t 0 Then determine vOUT t for t 0 given that vIN steps that vOUT R1R R 2 from VI to 0 V at t 0 2 4 For both transients determined in Pre Lab Exercise 2 3 determine the time at which v OUT 2 reaches a given VT where 0 VT R1R R VI 2 VS R vIN vOUT Figure 1 inverting MOSFET amplifier for Pre Lab Exercises 2 1 and 2 2 In Lab Exercises As part of the in lab exercises you will measure the threshold voltage and gate to source capacitance of 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 in Figure 1 To begin construct the amplifier as shown in Figure 3 and connect the signal generator and oscilloscope as shown Next set the signal generator to produce a 1 kHz sine wave with a peak to peak amplitude of 3 V and an offset of 1 5 V Thus the signal generator will produce a biased sine wave between 0 V and 3 V Set the oscilloscope to operate in its X 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 relation displayed on the oscilloscope Finally compare the displayed relation to that sketched in Pre Lab Exercise 2 1 Record the following data First record the value of vIN above which vOUT just begins to fall This is the threshold voltage vT of the MOSFET see the sketch from Pre Lab Exercise 2 1 Second record the values of vIN which correspond to vOUT values of 5 V 4 V 3 V 2 V and 1 V Alternatively you may find it easier and much more accurate to use the signal generator as a programmable vIN source and measure vOUT with a multimeter 2 2 This exercise measures the small signal gain of the amplifier shown in Figure 1 when its output bias voltage is 2 V To begin construct Circuit 1 shown in Figure 4 Adjust the potentiometer until vOUT 2 V as measured by the multimeter Next connect the signal generator and the oscilloscope as shown in Circuit 2 Set the signal generator to vIN R1 R2 C vOUT Figure 2 network for Pre Lab Exercises 2 3 and 2 4 Oscilloscope Channel 1 Signal Generator 5V 1k Oscilloscope Channel 2 Figure 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 Measure the amplitude of both vin and vout which are the sinusoidal components of vIN and vOUT respectively use AC coupling in Channel 1 of the oscilloscope to accurately measure v in The ratio of the amplitudes is the small signal gain Finally adjust the input bias with the potentiometer and observe the variation in vOUT 2 3 This exercise measures the gate to source capacitance CGS of the MOSFET First construct the circuit shown in Figure 5 Set the signal generator to produce a 20 kHz square wave with an amplitude of 5 V peak to peak and an offset of 2 5 V The oscilloscope should display a first order step response Measure the time constant of that step response Second remove the 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 is used as a digital logic inverter Construct the circuit shown in Figure 6 the 100 k resistor in this circuit models the Thevenin resistance of whatever drives the inverter Next connect the oscilloscope and signal generator as shown Set the signal generator to produce a 20 kHz square wave with an amplitude of 5 V peak to peak and an offset of 2 5 V Finally use the oscilloscope to measure the delay from the time at which the signal generator switches high to the time at which the inverter output begins to switch low Also measure the delay from the time at which the signal generator switches low to the time at which the inverter output begins to switch high Since the output of the inverter begins to switch when the MOSFET gate voltage passes by vT the two delays may not be the same see Pre Lab Exercise 2 4 5V 5V 1k 10k Oscilloscope Channel 1 Signal Generator vOUT 10k vIN Circuit 1 1k Oscilloscope Channel 2 vOUT Circuit 2 Figure 4 measuring the small signal gain of the MOSFET amplifier Oscilloscope Channel 1 Oscilloscope Channel 2 Signal Generator 100k Figure 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 Lab Exercise 2 1 explains the input output relation measured during In Lab Exercise 2 1 The model contains four parameters which are required to numerically evaluate the input output relation VS R vT and K From Figure 3 VS 5 V and R 1 k Further vT was measured during In Lab Exercise 2 1 Thus only K is unknown Use the value of vIN recorded for vOUT 1 V to determine K Then use the numerical parameters and the model to graph vOUT as a function of vIN for 1 V vOUT 5 V On this graph also plot the data …
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