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Old 22nd Apr 2021, 1:52 am   #15
G0HZU_JMR
Dekatron
 
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
Default Re: Tektronix P6201 FET Probe repair

Of course what I should have done was just use Genesys to process the s-parameter data from the VNA to display the input shunt capacitance at the source and also at the drain and then look for a difference when the drain and source are swapped! I've now done this and the plot is below.

As you can see in the plot the 'normal' pinout has the lowest drain capacitance by just over 0.1pF. However, as expected, it is the other way around at the source. Here, the shunt capacitance is slightly higher at the source with the normal pinouts. It also shows that the difference is just over 0.1pF.

The two differences aren't quite the same (they don't cancel) but this may be because of differing dc voltage at the two pins. There may be a slight scaling effect because the drain runs 5V higher than the source in these tests and this may change the internal physics a bit. I'm just guessing though. These results look really good to me and they seem to indicate the subtle difference in internal capacitance could be caused by the differences in the die layout in Andy's datasheet diagram a few posts back.

Just a reminder, these plots are of data taken with the 2N4416A in common/grounded gate at 5Vds and 1mA Id. The plots compare the effect of swapping the drain and source pins around to see if the device is fully symmetric.
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Regards, Jeremy G0HZU
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