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UNIVERSITY OF NORTH CAROLINA AT CHARLOTTE Department of Electrical and Computer Engineering Experiment No. 10 - MOSFET Amplifier Input/Output Impedances Overview: The purpose of this experiment is to familiarize the student with the input and output impedances of a single-stage MOSFET amplifier. In previous experiments the DC biasing and AC amplification in MOSFET amplifiers has been studied. This experiment will build upon these factors in a MOSFET amplifier and will expand into the impedances looking into the input and output of such an amplifier. The amplifier case in study will be the common-source configuration. Here the techniques of properly biasing a MOSFET amplifier and setting up AC amplification will be used as well as new methods of experimentally measuring the input/output impedances. By hand calculations, the impedances are usually found by setting up either a test voltage source and finding the resulting input current into a point of interest, or by setting up a test current and finding the resultant voltage at the point of interest. The following is a display of the first method, setting up a test voltage vx and finding the resulting input current ix. R1 Rd Cd M1 ix Cg 8 1uF Vdd 6 RL 1uF 7 Vx R2 Rs Cs 1uF 0 Figure 1. Inserting a Test Voltage to Find Input Impedance Here the capacitors are considered significantly large enough so that they appear as an AC ground and the DC supply is considered as an AC ground. This same approach can be applied to the output. It is important when solving for the ouput impedance that the point where the input source was connected to the original amplifier circuit be set as an AC ground point. The detailed technique for finding the input and output impedances can be found in any analog electronics textbook. 15V Pre-Lab – MOSFET Impedances 1. For the circuit shown in Figure 2, find values for R1, R2, RD and RS such that the MOSFET is biased with an IDS of approximately 400μA, a VGS of 2.3V, a VD of 9V, and a VS of 4V. Verify that the MOSFET is in saturation. Assume: μoCox(W/L) = 650μA/V2 , VT = 1.19V , RL = 10kΩ, and that the capacitors are open circuit to DC. Use only standard resistor values less than 150kΩ. Since all transistors have a variance in their characteristics, when building this circuit in the lab. session, it may be necessary to make adjustments of the bias voltages to provide for the best signal swing. This can usually be accomplished by adjusting the value of RS. R1 Rd Cd M1 Cg 1uF Vs 8 1uF + 6 15V RL + 7 R2 vout Rs vin Cs 1uF - 0 Figure 2. MOSFET Common-Source Amplifier 2 Vdd - 2. For the circuit of Figure 2, use the calculated values for R1, R2, RD and RS, assume the load resistance RL is 10kΩ, and that capacitors and DC power supplies represent a short circuit to AC. Find the voltage gain AVG-D = vout/vin, the input impedance ri, and the output impedance ro. Also, assume: μoCox(W/L) = 650μA/V2 ; VT = 1.19V ; and VA = 100V. *Note: The values given for the μoCox(W/L) and VT were found using measured data on the CD4007UBE chip [1]. (INSTRUCTOR’S SIGNATURE_____________________________DATE 3 ) Lab Session – MOSFET Impedances 1. Observe the schematic shown in Figure 3 below. R1 Rd Cd M1 Cg 1uF Vs 8 1uF + 6 Vdd 15V RL + 7 R2 vout Rs vin Cs 1uF - - 0 Figure 3. MOSFET Common-Source Configuration 2. Before connecting the circuit shown in Figure 3, prepare the power supply for VDD to ensure a DC voltage of 15V and adjust the function generator for VS to a voltage level of approximately 100mVp-p at a frequency of 1kHz. 3. Now connect the circuit as shown using your values for the resistors R1, R2, RD, and RS calculated in the Prelab section. Use a load resistance RL of 10kΩ. Measure and record the values of VD, VG, VS. 4. Measure the voltage vout using an oscilloscope and calculate the voltage gain AVG-D = vout/vin. If the output waveform is distorted, adjust the input voltage, and, if necessary, the bias voltages by changing RS to remove the distortion. 5. Vary the frequency of the function generator in order to obtain the necessary data to plot a frequency response curve for the voltage gain. 6. Measure the input impedance ri. To do this, use the setup shown in the circuit diagram in Figure 4. Use the original input signal from the function generator (100mVp-p @ 1kHz). 4 CS Amp Cg Rx ro 1k Vx + 1uF + Av i ri Vin - Vout - Figure 4. Input Impedance Measurement Setup 7. Measure the value of vin. Set up an equation relating vx and vin. Use that equation to solve for the value of the AC input impedance zi. 8. Vary the frequency of the function generator in order to obtain the necessary data to plot a frequency response curve for AC input impedance zi. 9. Measure the output impedance ro. To do this, use the setup shown in the circuit diagram in Figure 5. Use the same input signal from the function generator (100mVp-p @ 1kHz). Cg CS Amp ro Cd 1uF ri 1uF Avi Rx + 100k Vx Vout - Figure 5. Output Impedance Measurement Setup 10. Measure the value of vout. Set up an equation relating vout and vx. Use that equation to solve for the value of the AC output impedance zo. Make sure that RL is not connected for this measurement! 11. Vary the frequency of the function generator in order to obtain the necessary data to plot a frequency response curve for the output impedance zo. 5 Lab Session – MOSFET Impedances (Data Sheet) INSTRUCTOR'S INITIALS DATE: 6 Post Lab – MOSFET Impedances 1. Using the measured bias voltages VD, VG, and VS, calculate the bias current IDS. Compare these values with those calculated in the Prelab. Also compare the measured voltage gain, input impedance, and output impedance at 1kHz with that obtained in the Prelab. 2. Plot a frequency response curve for the voltage gain, input impedance, and the output impedance from the measured data collected in Steps 5, 8, and 11. Use Excel or a similar spreadsheet software to plot the curves. Note: When plotting the frequency response curves make sure the x axis is log (i.e. 1 10 100 1000, etc.) in Excel. 3. Comment on the behavior of the voltage gain, input impedance, and the output impedance as they relate to frequency. Reference: [1] The values given for the μoCox(W/L) and VT found using the measured data on the CD4007 chip were provided courtesy of Daniel Evans, PhD candidate under Dr. David M. Binkley, Associate Professor at the University of North Carolina at Charlotte and Clark Hopper M.S, R.A. and Harold Hearne M.S., R. A. both also at the University of North Carolina at Charlotte. 7