I want to use an FET as the first stage in an op amp darlington follower. The ones I have on hand require a threshold signal voltage to begin conducting. Hence, I am loosing swing at the output. Can anyone recommend an FET that turns on from 0V upward, or as close as possible?
Stu Garet
Some way ..somehow there's has to be a ~3V difference between the gate and the source for a mosfet to begin to conduct. Example...Vs is ~-3V and Vg= 0V..... or Vs=0V and Vg=3V Otherwise...it's not a mosfet.. D from BC
Damn new shit coming out all the time! :P So I checked it out... First glance ...the PITA alarms went off..ding ding ding.. I'm guessing it's an expensive min quantity PITA part.. I'll be surprised if the OP will be jumping for joy...
However..it's been an education...so hopefully I'll remember the IC for some quirky design someday.. "Yeah..I think it was some 0 Vgs chip that ran off 5V..." D from BC
Supertex makes depletion-mode mosfets, which can be very handy. They make nice current limiters.
John
Best regards, Spehro Pefhany
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
The OP wrote "turns on from 0V and upward".. I interpreted that as an enhancement mode mosfet. D from BC
well its not an enhancement mode mosfet anyway, but you Can get depletion mode mosfets, theyr just not as common, at least not in power types anyway, theyve been available in small signal devices for some time, usualy combination of enhancement/depletion, particularly the dual gate devices I like to use.
some low voltage logic level power mosfets are getting with lower and lower vgth.
the vgth spread of course will still be an issue.
the jfet type obviously cant be enhancement mode at all. so jfet could be used to compensate for the folowing stages voltage drop, assuming you have supply headroom.
Colin =^.^=
bitch, bitch, bitch ;-)
Small quantities in stock at Mouser, they show reasonable lead times for additional quantity. Prices don't look too bad to me-- between
1.50 and 2.00 in qty 100 for the quad array. Not 3 cents, of course.Often the special the part has 'de'parted by the time you need it.
Best regards, Spehro Pefhany
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Stu, we're confused, what do you mean by "op amp darlington follower"
-- an op amp voltage follower doesn't need or involve a Darlington, which is a bipolar transistor configuration. As for FET input stages, generally we use JFETs. These are depletion type devices, which can be operated at their full drain current, called Idss, where Vgs = zero volts (it's enhancement-mode MOSFETs that need a volt or two to get going). So your question is unclear on that account as well.
Finally, you can get pretty good JFET opamps, which in all but a few special cases are fine. Some special cases would be where you need noise levels below 2.5nV/rt-Hz, where you need to work from low supply voltages, or where the $8 to $15 cost of low-noise JFET opamps is considered to be too high. Tell us more about your project.
Like ye olde SD5400. Hard to obtain these days and for most of my designs now prohibitively expensive. And it's always been so good to me, it is (was?) a true analog engineer's delight.
-- Regards, Joerg http://www.analogconsultants.com
$8-$15? Ouch! Which one would that be? At that cost I'd go discrete.
-- Regards, Joerg http://www.analogconsultants.com
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