From: email@example.com (carltons)
Subject: Re: hi-capacity voltage controlled capacitor?
References: <firstname.lastname@example.org> <email@example.com> <3DA72D96.9F9514DE@NOSPAMboeing.com>
User-Agent: NewsWatcher-X 2.2.3b2
NNTP-Posting-Date: Sun, 13 Oct 2002 12:50:53 GMT
Date: Sun, 13 Oct 2002 12:50:53 GMT
In article ,
firstname.lastname@example.org (N. Thornton) wrote:
> Watson 'Atto Parsec' Name wrote in message
> > In article ,
> > email@example.com talked about...
> > [snip]
> > > A row of caps and a row of FETs would do it. Perhaps you have
> > > something more elegant in mind.
> > If the FET has an on resistance of an ohm or so then it could lower the Q
> > of the capacitor drastically, especially if the cap was in a resonant
> > circuit. Capacitor ESR can be important.
> Then don't use FETs with such a high on R :)
> Regards, NT
> > > A ceramic cap with separate ac and dc paths would also work. dc bias
> > > controls capacitance, and the signal is ac coupled to it.
> > > Regards, NT
Please forgive my ignorance as I've not been following this thread. I
gather that there is a problem with ESR in some caps. If this has not
been pointed out before now, there is a reason why the parameter is called
ESR. Current, as in circuit current, does not flow through a cap. Is
this magic or a violation of Kirchoff's laws? No. Charge is exchanged
from one plate to the other of the cap, this current is called
displacement current. Since this is a real world there is a limit. It is
called ESR and is an apparent, mesureable and different parameter from the
circuit resistance outside the component. Rds, as in FET Rds, is real and
is part of the normal thinking of circuits and circuit current. If you
need lower ESR and can't get the right cap, then you will have to parallel
some or redesign for less sensitivity to the ESR. Someone else may have a
better suggestion as I'm not God, but I wouldn't mind hearing it. I'm
still an engineer in training after all. :-)
The best tools an engineer has are his two optical ports on the anterior
portion of the cabesa and the two aural ports on each side of his head.