Massachusetts Institute of Technology Department of Electrical Engineering and Computer Science 6 002 Electronic Circuits Fall 2004 Lab 2 MOSFET Inverting Ampli ers First Order Circuits Handout F04 36 Introduction This lab examines the behavior of an inverting MOSFET ampli er It begins by examining the static input output relation of the ampli er and concludes by examining the dynamic behavior of the same ampli er 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 October 25 and October 29 After completing the in lab exercises have a TA or LA check your work and sign your lab notebook Before asking to get checked o make sure you meet all the requirements in the checko list at the end of the In Lab Exercises page 4 Finally complete the post lab exercises in your lab notebook and turn in your lab notebook on or before Wednesday November 3 Bring in your favorite CD for In Lab Exercise 2 3 it is meant to be a fun experiment and its results will not be needed for the post lab exercises If you have a portable CD player or laptop bring that to use for In Lab Exercise 2 3 as the stockroom has only a limited number of CD players and speakers available Pre Lab Exercises 2 1 Consider the inverting MOSFET ampli er shown in Figure 1 Using the SCS MOSFET model write an expression for vOUT as a function of vIN for 0 vIN vOUT VTM Also sketch and clearly label the form of vOUT as a function of vIN over the same range VS R M vIN vOUT Figure 1 inverting MOSFET ampli er for Pre Lab Exercises 2 1 and 2 2 2 2 Write an expression for the small signal gain of the MOSFET ampli er 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 write an expression for vOUT t for t 0 given that vIN steps from 0 V to VI at t 0 R1 vIN R2 C vOUT Figure 2 network for Pre Lab Exercises 2 3 and 2 4 2 4 For the transient determined in Pre Lab Exercise 2 3 determine the time at which vOUT 2 reaches a given VT where 0 VT R1R R VI 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 for the MOSFET labeled as M in every in lab exercise described below Remember that the MOSFET should say 2N7000 on it 2 1 This exercise measures the static input output relation of the MOSFET ampli er shown in Figure 1 To begin construct the ampli er 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 o set 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 To set the oscilloscope in X Y mode turn the SEC DIV knob all the way to the left You should now see the input output relation displayed on the oscilloscope Make a sketch in your notebook of the input output relation and compare the displayed relation to that sketched in Pre Lab Exercise 2 1 Oscilloscope Channel 1 Signal Generator 5V 1k M Oscilloscope Channel 2 Figure 3 measuring the static input output relation of the MOSFET ampli er shown in Figure 1 Also record the following data First record the value of vIN above which vOUT just begins to fall This is the threshold voltage VTM 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 nd 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 ampli er shown in Figure 1 when its output operating point 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 zero and re adjust the potentiometer so that vOUT 2 V Then set the signal generator to 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 vin The ratio of the amplitudes is the small signal gain Retain this circuit for the next exercise 5V 5V 1k 10k M vOUT Circuit 1 Signal Generator Oscilloscope Channel 1 10k vIN 1k M Oscilloscope Channel 2 vOUT Circuit 2 Figure 4 measuring the small signal gain of the MOSFET ampli er 2 3 The experiments in this exercise will use Circuit 2 constructed in In Lab Exercise 2 2 to explore the limits of saturation operation of the ampli er by observing clipping of an output waveform and by listening to distortion in music output Start by adjusting the input bias with the potentiometer and observing the variation in vOUT Now increase the peak to peak amplitude of the sine wave input from the signal generator to 300 mV Observing the output on Channel 2 of the oscilloscope increase the input bias voltage until you see clipping on the bottom part of the output Use DC coupling in Channel 1 of the oscilloscope to make a note of the upper excursion limit of the voltage vIN Similarly decrease the input bias voltage until you see clipping on the top part of the output and make a note of the lower excursion limit of the voltage vIN These upper and lower limits of vIN approximate the input operating limits of the ampli er The multimeters can also be used to nd the upper and lower excursion limits of the DC input voltage Next replace the signal generator with the CD player use the headphones output Set the CD player volume such that the peak to peak amplitude of the music signal vin is approximately 300mV Connect the vOUT signal to an amplifying speaker leave the oscilloscope connection in place and adjust the speaker volume to listen to the music …
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