[sdiy] MOSFET leakage.

[Ian Fritz]

Hello all --

Following up on the VCO using a MOSFET switch (BS170) that I recently 
described, I decided to have a try at directly measuring the MOSFET leakage 
current.  Wasn't there some discussion here recently on how to go about 
this?  Here's one way:  :-)

To make the measurement I used a low leakage opamp (OPA602, I_bias < 4pA), 
with a 10nF cap (feedback configuration) followed by a gain-of-five 
amplifier.

Here's an old lab-rat trick for measuring a signal with a background 
present: First connect the MOSFET from the opamp to the (+) power supply 
rail and measure the ramp rate of the voltage output.  Then connect the 
MOSFET from the opamp to the (-) rail and do the same.  The MOSFET leakage 
current changes sign with the reversed connection, but leakage from other 
sources such as the opamp bias current do not.  So the device leakage is 
obtained by subtracting the two results and dividing by two.  The other 
leakage contributions are obtained by averaging the two results.

The leakage current is then the capacitance times the ramp rate, which for 
the BS170 turned out to be 1.8 pA.  (The opamp bias current came out at 0.4 
pA.)  The corresponding channel resistance is 10^13 Ohms.  Yes, you *can* 
measure these levels at home!

The measured leakage level would correspond to a cycle time of nearly two 
hours for my VCO, assuming that the integrator's offsets were trimmed 
out.  I don't think I'll be bothering with that.  :-)

I spent some time yesterday looking for better devices to use for the 
switch.  Speed isn't much of an issue, but lower capacitance would be 
nice.  I found interesting thru hole devices from Supertex (VN2106 and 
VN0104) and many interesting smd parts from Fairchild and others (BSS138 
and FDV301N).  Anyone have experience with these or any other 
suggestions?  Thanks.


   Ian