From: Robert Baer
X-Mailer: Mozilla 4.75 [en] (Win98; U)
Subject: Re: mitigating charge injection in switched capacitor systems...
Date: Sat, 14 Dec 2002 09:26:13 GMT
NNTP-Posting-Date: Sat, 14 Dec 2002 01:26:13 PST
Organization: EarthLink Inc. -- http://www.EarthLink.net
Ken Smith wrote:
> In article <_moK9.2019$X56.firstname.lastname@example.org>,
> Kevin Aylward wrote:
> >Ken Smith wrote:
> >> Yes. what happens is: a charge is capacitively coupled in from the
> >> gate to the source when the gate moves. The channel then becomes a
> >> very high resistance and traps this charge as the source of the FET.
> >> If you are making your own gate driver for the fet, you can control
> >> the shape of the switching wave form to reduce this effect quite a
> >> bit.
> >Vary dubious if the load is also a capacitor. It wont matter what the
> >ramp time of the control voltage is. Its a capacitor divider.
> No, until the FET turns off it is an RC circuit with the resistance being
> the channel resistance of the FET. If the gate moves slowly, most of the
> charge goes to the driven electode and not onto the load capacitor.
> email@example.com forging knowledge
Well, yes, but if one wants the FET to act as a "reasonable" switch,
then the rise/fall times must be relatively fast.
A "dirty" trick is to use a capacitor (or maybe another fet) driven
with an opposite polarity pulse, to cancel the capacitive divider