Microwave Design and Measurements
Project #5 – Transistor Modeling & Characterization
PRE-LAB: (Due Monday, June 30th)
1. Review DC and RF operation of a GaAs MESFET
2. Be prepared to measure DC values for device transconductance, Idss and Vpo.
3. Review on-line data sheet for NEC NE721S01 device:
http://www.cel.com/pdf/datasheets/ne721s01.pdf
4. Create a linear equivalent circuit model for a common-source MESFET on AWR or
ADS. Determine values of each element to fit the published S-parameters given in the
NEC data sheet for a bias condition of Vds=3V, Ids=10mA. Plot your equivalent
model S11 and S22 parameters over the 2-20 GHz frequency band and compare with
the published values. Send your equivalent circuit with final values and the S11 and
S22 comparison plots to Dr. Lehmann by midnight, Monday, June 30th.
LAB PROCEDURE:
1. CAUTION: When handling any static-sensitive device, such as a microwave transistor,
make sure you are properly grounded. ESD wrist straps are provided for each bench and
must be used when handling and assembling the transistor into the test fixture and when
making DC and RF measurements.
2. Measure and record the following device characteristics:
a) Idss
b) Vpo
c) Gm @ Vd=3v, Ids=Idss
d) Gm @ Vd=3v, Ids=50%Idss
e) Gm @ Vd=3v, Ids=10mA
f) Document device number, gatelength, and gatewidth from NEC data sheet
3. Calibrate the VNA for 2-port S-parameter measurements over the frequency range of 100MHz
to 8GHz (501 points). Check your calibration by disconnecting the cables, re-connect and
measure the “thru” S-parameters of the cables.
4. Determine reference plane extensions:
a) For both S11 and S22 measurements the length of the 50Ω transmission line between the
VNA cable and the device-under-test (DUT) must be accurately known.
b) Measure the electrical length of the “50Ω microstrip thru” line provided to you in lab. The
total electrical length represents the length of the 50Ω transmission line in the transistor
fixture plus two SMA connectors. Therefore, half of the “thru” length is on the input side
of the transistor fixture; half is on the output side of the transistor.
c) Record the total “thru” length in degrees and seconds. Record what you will use for the
reference plane extension on the input and the output when measuring the transistor in its
test fixture.
Input Reference Output Reference
Plane Extension Plane Extension
Through holes (vias
to ground; 2 places)
DUT
Microstrip Device Test Fixture
5. Mount your microwave transistor in the test fixture provided. Carefully connect the gate tab to
the RF input port and drain tab to the RF output port using conductive epoxy or solder. The case
of the RF package must make good electrical contact to ground through both ground tabs. Use
solder or conductive epoxy to make the ground connections to the back side of the Duroid.
Biasing of the transistor is accomplished by connecting the appropriate voltages to the input
bias tee connections on the rear panel of the VNA.
IMPORTANT NOTES:
Check the power supply current limit to ensure it is set at 200mA or less.
When biasing the FET apply the negative gate voltage FIRST! After setting the gate
voltage to approximately -1 V, then apply the drain voltage. Adjust the gate voltage for
desired drain current. Failure to bias the transistor in the proper sequence may lead to
catastrophic results!
Use a volt meter to ensure your gate and drain voltages are what you desire at the device
terminals.
6. Using the appropriate reference plane extensions measured in step 4, measure the microwave
transistor 2-port S-parameters at:
a) Vd=3v, Id=50%Idss
b) Vd=3v, Id=10mA
Save all 2-port S-parameters for later use.
Compare measured S-parameter data to data sheet values (plot S11 and S22 on
same Smith Chart). Comment on differences.
Determine equivalent circuit model parameters based upon measured data. Note
any major differences between model values with measured data vs. published data.
DUE DATE: Device equivalent circuit schematics and plotted S11 and S22 (simulated &
data sheet values) are due Monday, June 30th, by midnight (late inputs will not be accepted;
you will receive no credit for this portion of the project if email is received late.)