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ECEN 317
Stephen Schultz
Winter 2009
N-Channel MOSFET Characteristics
OBJECTIVES
1. To experimentally verify n-channel MOSFET characteristics.
2. To become familiar with MOSFET operation.
3. Build a discrete MOSFET amplifier
EQUIPMENT
1.
2.
3.
4.
5.
6.
Dual dc power supply
MOSFET lab chip--AMI 14157-002
Digital multimeter (DMM)
Oscilloscope
Function generator
AMI MOSFET chip (See the associate sheet.)
CAUTIONS
This and subsequent lab experiments use a CMOS chip fabricated by AMI Semiconductors
(AMIS) for educational purposes. The devices on these chips can be damaged when exposed to
electrostatic discharge or high-voltages (above 5 V). Please take the following precautions:
 Use only one chip per group.
 Avoid walking around the room with the chip in your hand.
 Avoid pulling the chip in and out of the breadboard repeatedly.
 Turn the dc voltages to 0 V before connecting to the chip.
A schematic of the devices and circuits fabricated on the lab ship is attached to this lab
experiment. The chip pin numbers are shown along with the circuit schematics; a pin diagram
is also shown. Note the following:
1. ALL VSS and VDD pins should be connected appropriately to ensure proper operation of
any device on the chip.
2. An approximately 300  ESD protection resistor is present between each pin and its
circuit or device. This may cause measurement errors for drain currents over 200 A.
THEORY
While discrete MOSFET power stages are used regularly, the discrete MOSFET amplifier stage
is rarely used for general amplifying circuits. The transconductance of small MOSFET stages
are much lower than the transconductance of the BJT stage, thus BJT stages are used in most
amplifying applications. Although this lab may not relate to practical circuits, some underlying
principles of MOSFET IC amplifier design are demonstrated in this lab.
The midband voltage gain of the stage of Figure 1 can be written as
AMB   gm rds // RD
where gm is the transconductance and rds is the drain-to-source impedance of the MOSFET.
Unfortunately, the transconductance of the device varies with dc drain current as
C W
g m  4 ox
ID
2 L
If rds >> RD, the midband voltage gain can be written as
AMB   g m RD
This gain can be maximized by increasing gm and RD. If ID is increased to raise gm, the dc
voltage drop across RD increases and may force the device into its triode region. If RD is
increased to raise the gain, the dc drain current must be lowered to keep the device in the active
region, resulting in lower values of gm.
It is left to the student to show why increasing the value of RD while decreasing ID by an equal
percentage leads to larger increases in the voltage gain than increasing the current while
lowering the value of RD.
+5 V (VDD)
36
R1
RD
35
0.1  F
Vout
32
50 
R2
34
26
Vin
Figure 1
PRELIMINARY
No Preliminary Work in this laboratory because you need the MOSFET parameters that will be
measured in the laboratory.
PROCEDURE
+5 V (VDD)
36
35
VDC
+

32
34
26
Figure 2
Part 1 - Measurement of MOSFET parameters
1. Connect the MOSFET circuit. The simple MOSFET is circuit 1 on the AMI chip. There is
a separate sheet describing the AMI MOSFET chip on the web page.
a. Place the lab chip on the breadboard and make the appropriate power (5 V) and
ground connections. Use the +5V output of the power supply.
b. Measure VDD on one of the chip pins with the DMM.
c. Before connecting the NMOS circuit of Figure 2, set the second DC power supply to
0 V. Now connect the circuit shown.
2. Measure the threshold voltage VT
a. Manually sweep VGS from 0 to 2.6 V in 0.2-V increments and measure the
corresponding values of drain current, ID.
b. Record these values in a table.
c. Determine the value for VT. (Vary VGS until ID=0.)
+5 V (VDD)
36
RD
35
Vout
VDC
34
32
+

26
Figure 3
3. Calculating
a.
b.
c.
d.
 Cox W
.
2 L
Add a drain resistor to get the circuit shown in Figure 3. Choose a value for RD such
that VDS is near VGS-VT=1 V. Use the value for current measured in the previous
part. (This should be around 8 k. Use a close value from your kit.)
Adjust VGS until VGS-VT=VDS.
 Cox W
Calculate
.
2 L
 Cox W
VGS  VT 2 (since VDS=VDSP)
ID 
2 L
Using the largest current value from the table calculated in part 2 calculate  using
the following equation.
 Cox W
VGS  VT 2 1 +  5  VGS + VT
ID 
2 L
 

Part 2 – Common Source Amplifier
1. Calculate the midband gain of the common source MOSFET amplifier shown in Figure 1
with RD=2.2 k and VDSQ=3.5V. Use the MOSFET parameters measured in Part 1.
2. Construct the circuit shown in Figure 1 with RD=2.2 k, and R1 and R2 such that
a. R1 || R2 ≥ 100 k and
b. VDSQ = 3.5 V
There is a separate sheet describing the AMI MOSFET chip on the web page
3. Measure the voltage gain at f = 10 kHz (AMB) after adjusting the peak-to-peak value of
output voltage to 1 Vpp.
4. Calculate gm assuming that rds >> RD.
5. Measure the lower 3-dB frequency of the voltage gain.
6. Measure the upper 3-dB frequency of the voltage gain.
7. Increase the input voltage amplitude until the output signal start to distort.
8. Include a table similar to Table 1.
Table 1
Parameter
gm
Calculated
Measured
Error
AMB
fL
fH
--
--
Kn
--
--

--
--
R1
R2
Be sure to compare the measured values to the calculated values.