The Cyber-Spy.Com Usenet Archive Feeds Directly
From The Open And Publicly Available Newsgroup
This Group And Thousands Of Others Are Available
On Most IS NNTP News Servers On Port 119.
Cyber-Spy.Com Is NOT Responsible For Any Topic,
Opinions Or Content Posted To This Or Any Other
Newsgroup. This Web Archive Of The Newsgroup And
Posts Are For Informational Purposes Only.
Reply-To: "fred bartoli"
From: "fred bartoli"
Subject: Re: Strange going-on with a floating MOSFET gate
Date: Mon, 6 Jan 2003 12:59:16 +0100
X-Newsreader: Microsoft Outlook Express 5.00.2314.1300
Organization: Guest of ProXad - France
NNTP-Posting-Date: 06 Jan 2003 12:58:52 MET
Winfield Hill a écrit dans le message :
> Norman Yarvin wrote...
> > Winfield Hill wrote:
> >> Tony wrote...
> >>> Winfield wrote:
> >>>> But what I didn't expect was what happened immediately when
> >>>> removing the clip lead to the gate. Without fail the current
> >>>> would instantly drop by about 2x, indicating a roughly -75mV
> >>>> change in gate voltage. ...
> >>> If you offered your hand (and croc clip) back to the
> >>> gate (without actually connecting to it) did you get
> >>> the corresponding +75mV change on the gate voltage?
> >> Generally the new readings were stable, movements of my hand
> >> nearby and other activity had little effect, that's why I went
> >> ahead and let the meter log 2 hours of data. The FET's tiny
> >> antenna wire and all that. But I haven't tried your idea.
> > Remember that it's not potential that's conserved, it's charge.
> > Removing the clip lead removed nearby charges, thus changing
> > the potential (voltage).
> > Whether this is the correct explanation depends on the relative
> > sizes of the different capacitances involved ... as measured at
> > the first instant that current could no longer flow between the
> > alligator clip and the transistor, as the clip was pulled away.
> Yes I have thought of such a scenario; it has a certain appeal.
> But a primary problem is that everything is presumed to be in
> equilibrium in the instant before the breaking contact, thus no
> current is flowing (ignoring ac pickup), and therefore (at least
> in the dc case) the argument for charge transfer at the moment
> of severed contact fails.
think that when removing the gate connection you change the field lines
distribution, i.e. you change the effective external capacitance seen by the
One can distinguish 3 steps :
1) gate connected => one field distribution
2) *just after* gate disconnect with the gate connection in the same place
as before => same field distribution
3) gate connection removed => some field lines that before were closing on
the gate connection have now to close on the gate lead. This implies
additionnal capacitance, which can be significant.
this capacitance is splitted between the drain and source paths so the
effect depends on the voltages of both wires.
The amplitude of this effect of course depends on the leads shapes, nearby
or wide apart. You can test this by :
1) changing the relative position of the leads.
2) see what happens when disconnecting the gate and keeping the wire near
the gate lead.
3) operate with a lower gate voltage (lower drain current), a symetrical
leads arrangement and a drain voltage twice as the gate voltage. This of
course should lead to null Vgs change. With a higer drain voltage you should
see positive gate deltaV.
> - Win
Go Back To The Cyber-Spy.Com
Usenet Web Archive Index Of
The sci.electronics.design Newsgroup