EE 321L Analog Electronics Laboratory Fall 2010Lab 12MOSFET variable gain amplifier and amplitude modulationPre-Lab1. Decide which NMOS transistor to use for the experiment.2. Sketch the experiment setup in the first section (step 1). Tentatively estimate thechannel resistance based on measurements from your previous lab.3. Sketch the variable gain amplifier in the second section. Guess at the size of the feedbackresistor based on the estimated channel resistance from before.4. Sketch the demodulation circuit in the fourth section and write the expression for thecapacitance.In this lab you will use the NMOS as a varia ble gain resistor to amplitude modulate a carrierwith a signal. Using a rectifier and low-pass filter you will then demodulate the original signalfrom the carrier.Measuring the channel resistance1. Pick a NMOS tr ansistor on the MOSFET IC, but not the one whose source is connectedto VSS! Bias VDD= +8 V and VSS= −5 V using volta ge dividers from ±15 V supplies.Connect the source to ground and the gate to a variable 0 − 6 V supply through a100 kΩ r esistor. The drain is connected to VDDthrough a resistor, RD.2. For several values of the g ate voltage (in the 0-6 V range), measure the drain currentand vDSfor small values of vDS(you will need to adjust RDto make vDSsmall).3. Use these measurements to compute and plot the channel resistance as a function ofgate voltage. Decide, for use later, on a mid-range gate voltage for which the resistancevaries roughly linearly with g ate voltage.Building the variable gain amplifierIn this section yo u will build a variable gain amplifier using an op-amp, in which the gain iscontrolled by the voltage on the NMOS ga te.4. Using the same setup as in the previous section use the NMOS as a variable gainresistor in a non-inverting amplifier, connecting the source to ground and the drain tothe inverting input on an op-amp (Use the LF411 with the ±15 V supplies). Pick afeedback resistor which produces a gain in the range of 10 to 100 for the mid-rangegate voltage.1EE 321L Analog Electronics Laboratory Fall 20105. Apply a small-amplitude sinusoidal carrier o f moderately high frequency to the non-inverting input of the amplifier. How high can you go in frequency before you loosegain?6. Pick a carrier frequency well below the maximum frequency and show how you cancontrol the output amplitude of the amplifier by varying the gate voltage.Amplitude modulationIn this section you will modulate a sinusoidal signal (the carrier) with a lower-frequencyinput signal.7. Attach a second function generator to the gate, through a 100 kΩ resistor, and use itto supply a signal of much lower frequency with an offset equal to the mid-range gatevoltage from earlier. Show how the output signal amplitude is modulated by the gatesignal.8. Attach a second 100 kΩ between drain and gat e. Does t his reduce the distortion?Building the demodulation circuitIn this section you will use a simple rectifier to demodulate the low-frequency input signalfrom the carrier.9. Attach the op-amp output through a diode to a 1 kΩ resistor to create a half-waverectifier with positive voltage swing only. Plot an example of the modulated rectifiedcarrier (it may be easier to trigger on t he signal input).10. Use a capacitor to create a low-pass filter with a time-constant much longer than thecarrier and much shorter than the signal.11. Plot the output signal together with the input signal. Are they
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