Recently, Somebody Somewhere wrote these words
>
>
> Declan Moriarty wrote:
>
> >/rambling preamble
> >
> >My reading on transconductance goes back to Ia/Vg Curves, chasing
> >linearity on valves (as advanced a science as surfing tidal waves)
> >and such 'prehistoric' notions.
> >
> >My current requirement is in switching circuitry, and particularly,
> >replacing fets in switching circuitry.
> >
> I have designed switching circuits using FETs for many years and I
> couldn't tell you what the transconductance value was for any FET I
> used. In short I couldn't explain the relevance of gfs to switching
> applications because I don't believe there is any relevance. I think
> you are barking up the wrong tree. I wouldn't pay too much attention
> to gate threshold either. If I am using it for a switch I want to know
> at what vgs the device is guaranteed to be saturated. Then, will the
> FET handle the current and voltage. Finally what is Rd-on? My circuit
> current squared times Rd-on will give me my actual dissipation.
> Pretty simple.
>
> Caveat: Most power FETs have a Vgs for saturation (not threshold) of 4
> to 5 volts. This is too high for interfacing with logic. There are
> FETs known as logic-level FETs with built-in charge pumps that
> saturate around 2 volts. If this is what you need, check out Harris
> Semiconductor.
>
This question arose because in my particular niche, speed is everything.
I found myself trying to find a logic level fet in the stock of a
supplier who didn't discriminate between logic level fets, and less
sensitive ones, and wondered if could the gfs be extrapolated into a
reckoning of the saturation voltage. That's worth knowing if I could do
it.
Secondly, I want to educate the technical support guys there who I know
fairly well on a personal basis. When you are talking down to
'engineers' you have to be fairly sure of your facts.
Alois gave a separate exposition on the subject, and it is a purely
linear characteristic.
--
With best Regards,
Declan Moriarty.
--
Author: Declan Moriarty
INET: [EMAIL PROTECTED]
Fat City Hosting, San Diego, California -- http://www.fatcity.com
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