[sdiy] MOSFET leakage.

[René Schmitz]

Hello Ian and 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:  :-)

Weren't we discussing cap leakage?

> 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.

Did you follow any of the advice that was given in that earlier thread?
(Dead-bug, Air and Teflon insulation or the like...)

> 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!

Thats really not bad! Very good! Even more so if you consider that a 
MOSFET can switch more current than a JFET at the high end. So you can 
just raise the cap proportionally and you would still get a very large 
sweep range. Also this shows that one has to interpret the values in the 
  datasheets really carefully. Here the current is 500 times lower than 
what you would expect from the datasheet.

I have experimented with BS108s as discharge switches in my VCO3, but 
didn't like the effects of the channel capacitance.
I had to put a small resistor in series with it, since the discharge was 
faster than the integrators OPs slew rate. That tamed the overshoot a 
bit but it was still somewhat dirty.

> 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.

With a larger cap the gate to channel capacitance would have less 
impact. Also don't forget that one could neutralize the capacitance, by 
putting a second one in parallel, which gets a reset signal of opposite 
direction.

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159