I've been measuring IDSS on a batch of JFETS (On semi MMBFJ310) -- they are specified to run from 24 to 60 mA. (IDSS is the drain current with the gate grounded and the drain-source voltage in this case at
10V).Looking at the parameters available for JFETs in SPICE, there doesn't seem to be an explicit IDSS available. Anyone know how to include it? Thanks.
Stephen
JFET's don't specifically have an IDSS equivalent to that of CMOS, since drain current at Vgs=0 is still quite voltage sensitive (saturation region)....
Idrain = BETA·(1+LAMBDA·Vds)·(Vgs-VTO)^2
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
"Jim Thompson" schrieb im Newsbeitrag news: snipped-for-privacy@4ax.com...
Hello Stephen,
You need at least the threshold voltage VTO too.
LAMBDA is used to model the slope of Ids versus Vds in the saturated region. After you have fitted the curves with BETA, VTO and LAMBDA, you will discover that you have to make some compromise with these parameters to get the best fit for gain(conducatnce), Idss(Ids at 0V Vgs) and the threshold voltage.
Best regards, Helmut
I haven't found spice to be very useful when working with typical JFET circuits. That's because critical production JFET parameters vary over such a wide range that either a) one is tricked into thinking he's got a good circuit, thanks to his spot-on spice JFET, or b) the circuit has been well designed not to be badly affected by the JFET's wide range of parameters, in which case spice modeling may not reveal much new information about the circuit.
For example, consider a JFET follower that's biased with a current source in its source, the drain current can be easily predicted without resorting to spice.
I used to use rudimentary JFET's as "kick-starters" for low-current bandgaps. But now that many processes have depletion mode MOSFET's, they're no longer needed.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
Now I wish you could buy them as discretes. With discretes I mean something in SOT23 or SC75 at under $0.05/1000. That would be the ideal thing to get fuel cell converters and stuff kick-started. JFETs are ok but not very steep and a bit wimpy. The BF862 looks nice but NXP, single-sourced and pricey.
-- Regards, Joerg http://www.analogconsultants.com/
Thanks. By assuming lambda =3D 0 (it's that way in all of the J310 models I've seen) I can just scale beta to change IDSS for at least a rough order model.
Stephen
Yep. That'll work.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
ent
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You can buy depletion MOSFETs as discretes. They're single sourced, but from reliable suppliers, and with alternates that can do the same simple job if one becomes hard to get. They're easy on the pocketbook. An obsession with five-cent parts is going to box you into a pretty tight corner.
As for cheap sot-23 JFETs, the bf862 has a rather large die, meant for low-noise. Still, its reel price is only 16 cents. Or use the smaller-die jellybean J108 for 12 to 13 cents in sot-23 packages.
The J112 is even cheaper - MMBFJ112 for 7.7 cents from Mouser. Idss delivers 5mA, min. BTW, most of these jellybean J1xx types are multiply sourced.
Those can't really compete with the BF862 from a Vgs versus Id perspective, take a look at figure 7:
I don't mind the occasional >5c part but then there has to be a darn good reason why I need it. What I do try to avoid is single-source because of all the grief I've seen with such situation. Some companies are worse than others and there are some (such as Analog Devices) whom I trust a whole lot more than others.
-- Regards, Joerg http://www.analogconsultants.com/
Picky, picky - you want a large-die, high-current part for a small-die price and second sources as well. Harrumph! The large-die J105, J106, and J107, etc., parts compete nicely with the bf862 for high-current capability. But they must not be in very much demand, because, sot-23 versions aren't stocked by the distributors I checked. Perhaps the 16-cent bf862 isn't so bad, afetr all?
Fairchild's JFTJ105 uses the sot-223 power-tab package, which shows just how serious those J105 parts are, Idss = 500mA, min (eat your heart out bf862). Mouser stocks 'em, 50 cents qty 2k. And worth every penny!
In closing, I'll just point out you can do a lot with 5mA, and a 3-cent BJT can be used to beef that up if necessary.
Just a note to complete the story, Newark has a nice range of Fairchild's sot-23 mmbfJ1xx JFETs in stock.
Whoa, an...er...JGBT?!
Tim
-- Deep Fryer: A very philosophical monk. Website @
Yep! By any of several means...
,-----+---------- |--' | ,--->|--, |/ High-current self-biased current source | '---| | |\\V | | Rset | '--/\\/\\--+-- '----------------------'
power source ---+--------------, | | | |/V Automatic startup, with disable '--/\\/\\--+---| JFET current sink & PNP switch | |\\ | | |--' '--/\\/\\--- startup ,--->|--, power | | Rset | +--/\\/\\--, '-------| -------+--- gnd | disable -->--'
The disable signal can be activated after the desired power pathway has been established.
It's not just current, it's also how far it'll swing when you only have
500mV total to start with. But the BF862 is single-sourced and NXP, and there I am bit careful right now. Q2/2007 financials were rather dismal as I had expected, Q3 supposedly comes out Oct-30. Let's see. If dismal again I'd think we could see chop-chop time pretty soon. Or in corporate speak there might come a "right-sizing" of staff and product portfolio.
That's a pretty fat JFET. Doesn't look nearly as steep as the BF862 though but I bet one can buyilt a very nice high-IP3 preamp with that.
One trick is to use a start-oscillator that then starts another (bigger) start oscillator which then starts the real inverter. Reminds me of the start procedure of the old Dornier-27 we used for parachuting. Only one guy really mastered that without too much popping and banging, a pilot with serious bush pilot skills.
-- Regards, Joerg http://www.analogconsultants.com/
Yeah, I wonder, too. I know they're upsizing the wafer size in their Hamburg fab (discretes, I think) from 6" to 8" or something like that. And they're hiring. They've called me twice, and it's something I think would be interesting (fab engineering stuff), but I'm wary of a company that's run by a financial consortium that neither knows nor cares about semiconductors, engineering, employees or all the other things that make industry fun. Providing good quality at a good price with qualified and motivated staff on a long-term basis doesn't seem to be a concept that investors grasp.
robert
Yep. A very long time ago right out of university I almost started with them in Hamburg. The former Valvo plant, in the branch that made the NXA CCD imaging chips. Somehow I had a hunch they might even lose that to Japan, which they ultimately did. In hindsight I am glad I chose a smaller US company. I would not have been a very good fit in a larger corporation as I like efficiency and absolutely despise bureaucratic hurdles.
-- Regards, Joerg http://www.analogconsultants.com/
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