From: Kevin McMurtrie
Subject: Re: Strange going-on with a floating MOSFET gate
User-Agent: MT-NewsWatcher/3.2 (PPC Mac OS X)
Date: Sun, 05 Jan 2003 04:03:49 GMT
NNTP-Posting-Date: Sat, 04 Jan 2003 20:03:49 PST
In article ,
Winfield Hill wrote:
>Reporting on some strange business with a floating MOSFET gate...
> But then something very strange happened. At reading #157 the drain
> current jumped a few uA. Thereafter it increased by a few uA at each
> 15-second recording for a steady rate of 5uA/minute. This means that
> the gate voltage was increasing by about 1mV per minute. I thought,
> aha! given the FET's -2mV/C tempco that's like a temperature decrease
> of about 0.5C per minute. But oops! this funny business went on for
> the rest of the experiment, for a total change of +45mV in 80 minutes.
> I know the lab temperature didn't go down by 22C, to below freezing!
> OK, fine, you say. The steady change of +45mV total resulted from a
> leakage current _into_ the FET's gate of about I = C dV/dt = + 0.5fA.
> While there'd be no problem with observing this low leakage current,
> we know this explanation cannot be correct, because all the other
> FET voltages were _lower_ rather than higher than the gate voltage.
>. ,-----+---- current -----,
>. | | recorder |
>. D volt +
>. float --- G meter supply
>. 2.04V S 695mV -
>. | | |
> The gate voltage should have dropped, and the drain current should
> also have dropped. Instead the current rose from 540uA to almost
> 900uA during the two hours. !!!
> An additional very strange thing happened, twice. More later.
> - Win
Did you shield the gate?
Somewhere I have a gizmo that amplifies the voltage on a bare wire to
drive two LEDs, using a JFET op-amp. Walking around the room or even
stirring the air causes voltage changes.
Air filters will create a big charge too. They're designed so that the
friction of the air against the filter fibers creates static
electricity. That's how those fluffy air filters are >99% effective
even though you could probably shake grains of sand through them.