ELECTRICAL AND COMPUTER ENGINEERING
THE CITADEL
ELEC 313 Lab 7
MOSFET Amplifier Configurations
OBJECTIVE:
to measure the gain and output resistance of two MOSFET amplifier configurations
EQUIPMENT:
MOSFET -- 2N7000
resistors -- 100 kΩ , 33 kΩ , 1 kΩ (x2), 470 Ω , 100 
capacitors -- 0.1 μF (x2)
triple-output DC power supply -- Hewlett Packard E3631A
function generator -- Hewlett Packard 33120A
multi-meters -- Hewlett Packard 34401A, Fluke 8010A
oscilloscope -- Agilent 54622D
PRE-LAB:
1.
In the circuit of Figure 1, determine the values of R1 and RS that make VG = 4 V and ID = 4 mA
when VDD =18 V . Assume a gate threshold voltage of VTN = 2 V and Kn = 75 mA/V2 .
Round each resistor value to the nearest value in the Equipment list (above).
Figure 1. Common-source amplifier circuit.
2.
Copy Figure 1 into your notebook.
Using the true (standard) resistor values, VDD = 18 V, VTN = 2 V, Kn = 75 mA/V2 ,
calculate the voltages at all three MOSFET terminals (VG, VD, VS) and the drain current (ID).
Label these values (where they will be measured) on your schematic.
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PROCEDURE:
Common-Source Amplifier
1.
Ensure that you have the resistors, capacitors, and MOSFET necessary to build the circuit of Figure 1.
a. Make sure (measure) that no resistor is more than 10% above/below its nominal value.
b. Make sure (measure) that both capacitors are within 20% of their nominal values.
c. Make sure that “2N7000” is written on the transistor.
2.
Build the circuit of Figure 1. Use the +25-V/1-A supply on the HP E3631A to set VDD = 18 V .
3.
Measure and record the DC voltages at all terminals of the transistor (VG, VD, VS).
Measure the drain current (ID).
4.
Set the function generator to output a sine wave at 20 kHz, 200 mVPP, 0 VDC offset. Connect it to Vin .
5.
Connect Channel 1 of the oscilloscope to Vin and Channel 2 to Vout .
6.
Adjust the function generator to set Vin to 200 mVpp as measured on the oscilloscope.
(“Quick Meas”  “Source: 1”  “Select: Peak-Peak”  “Measure Pk-Pk”)
7.
Record the peak-to-peak output voltage Vout from Channel 2 of the oscilloscope.
Note if the waveform Vout is inverted with respect to the waveform Vin .
8.
Repeat steps 6 and 7 for Vin = 300, 400, 500, 600, 700, 800, 900, and 1000 mVpp .
Make a table of values for Vout vs. Vin .
9.
Connect a 1-k load resistor (RL) from Vout to ground and reset Vin to 200 mVpp .
Record Vout,load with the load resistor connected.
Source-Follower Amplifier
10. Construct the circuit of Figure 2 by removing the 1-k drain resistor
and moving the output capacitor from the drain to the source of the transistor.
Figure 2. Source-follower circuit.
11. Measure and record the DC voltages at all terminals of the transistor (VG, VD, VS).
Measure the drain current (ID).
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12. Adjust the function generator to set Vin to 200 mVpp as measured on the oscilloscope.
13. Record the peak-to-peak output voltage Vout from Channel 2 of the oscilloscope.
Note if the waveform Vout is inverted with respect to the waveform Vin .
14. Repeat steps 12 and 13 for Vin = 300, 400, 500, 600, 700, 800, 900, and 1000 mVpp .
Make a table of values for Vout vs. Vin .
15. Connect a 1-k load resistor (RL) from Vout to ground and reset Vin to 200 mVpp .
Record Vout,1k with the 1-k resistor connected.
16. Replace the 1-k load resistor with a 100- load resistor.
Record Vout,100 with the 100- resistor connected.
DATA ANALYSIS:
Common Source Amplifier
1.
Compute percent error for VG , VD , VS , ID (vs. calculations from the pre-lab).
2.
Compute the no-load AC voltage gain of the common-source amplifier Av for each (Vin, Vout) pair.
From this collection of values, estimate Av for the common-source configuration.
3.
Based on the no-load and loaded output voltages for Vin = 200 mVPP ,
calculate the output resistance Rout of the circuit from
V o u t,lo a d  V o u t,n o -lo a d
RL
R L  R out
Source Follower Amplifier
4.
Compute the no-load AC voltage gain of the source-follow amplifier Av for each (Vin, Vout) pair.
From this collection of values, estimate Av for the source-follower configuration.
5.
Based on the no-load and (100-) loaded output voltages for Vin = 200 mVPP ,
calculate the output resistance Rout of the circuit.
QUESTIONS:
1.
What is a common-source amplifier? How/why is it useful?
2.
What is a source-follower amplifier? How/why is it useful?
3.
For the common-source amp, do your measured VG, VD, VS, ID values match theory? Why or why not?
4.
What is the purpose of the two capacitors (just after Vin, just before Vout)?
5.
How is the output-resistance equation (Data Analysis, step #3) derived?
6.
For the source-follower, does the output voltage drop significantly (from the no-load case)
for RL = 1 k? Why or why not?
LAB REPORT:
Complete your lab report in the format requested by your instructor.
See Lab #1 for a complete list of requirements.
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