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Measure Static COSS of a RF LDMOS Power Fet.

Question asked by paulh on Sep 20, 2007
Latest reply on Sep 20, 2007 by Dr_joel
Perhaps some of you might be able to share some insight on a good
method to measure the static Output Capacitance of a vhf ldmos rf power fet. I am using a hp8753 VNA

Looking in the data sheet I see for the static COSS measurement they
have

VGS=0V, VDS=12.5V F=1Mhz  COSS typical = 24Pf

The device can work up to 500Mhz but I will be using it at 200Mhz.
My idea is to try measure the actual static output capacitance at my
intended frequency of operation in an attempt to reduce the number of
passes on the design for the output matching network.

What I was thinking of doing is, Put the device in a small test fixture
apply VCC, ground the gate, and measure the drain capacitance
with a series 1000pf DC/Blocking cap so as not to have 12.5vdc feeding back to the vna port.

Would this be a valid test or am I way off base here ?.

I am also curious as to why alot of the manufacturers of LDMOS
rf power fets supply Coss measured at 1Mhz as opposed to the typical
frequency that these devices are used at. Looking at many manufacturers
data sheets it appears that the 1Mhx measurement is a industry standard.

Thanks

Paul.  

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