I had an ephemt on the bench and some test gear handy, so I played with it a bit. This is cool:
These are "RF" parts, so the data sheets and appnotes say very little about DC behavior.
There must be a use for this, somewhere.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
I had an ephemt on the bench and some test gear handy, so I played with it a bit. This is cool:
These are "RF" parts, so the data sheets and appnotes say very little about DC behavior.
There must be a use for this, somewhere.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com Bottom trace looks bogus. Why doesn't the parasitic diode turn on when the gate is zero volts as in the top view? Cheers, Harry
Huh, I know you guys are always going on about pHemt's, but I don't know them. Can you swap around the drain and source and see the same thing? (Are they symmetrical wrt S and D?)
George H.
It sure looks like this part does not have a substrate diode.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Wat would happen if you used them along with a high speed op amp in one of those "precision rectifier" circuits?
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That's probably not worth the trouble. You can get 0.2 pF tiny schottky diodes that would be better at low level. This phemt is potentially a 1-amp, 3 pF diode, which is otherwise hard to find.
A 1N5817 is close to 200 pF!
There are precision rectifiers that use switches, instead of diodes, and a small phemt might be good there, if you need to go fast. They seem to sort of behave like jfets, but lots more transconductance and much less capacitance.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Den fredag den 18. december 2015 kl. 21.25.11 UTC+1 skrev John Larkin:
it seems they don't have a diode but instead sorta work both ways, I guess shorting G-D means it won't turn on in reverse
-Lasse
And a 10-50 MHz 1/f noise corner instead of 1kHz, and an Early voltage so l ow that some don't even work as source followers. Nice and quiet in the fla tband, though--0.3 nV 1-Hz noise.
Another thing about the low drain impedance--they don't necessarily play ni cely with the other children, because their reverse isolation isn't as good as you'd think from their low C_DG. Just today I was working with a 3 GHz differential photodiode front end with an ATF38143 in it. It's nice and sta ble on its own, with or without a load, but when I connected a Mini Circuit s amp to it, the combination howled at 2 GHz. It took a 9 dB pad to make it stop.
Cheers
Phil
2 GHz howl - the sound made by a 250 nm timber wolf.
-- Grizzly H.
Smaller species howl higher. ;)
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Looks like sort of. I haven't worked out all the permutations, but it does make an interesting device, a diode with gate-controlled polarity.
Phemts have huge transconductances, blinding speed, and really small d-g capacitance. ECL (0.8 volt swing) is generally enough to turn them on and off. And the paddle is the source, not the drain like a mosfet, which is really handy. They are almost always spec'd as RF amps (lots of s-params) but make great switches, once you characterize them yourself.
They must not be really symmetrical, because Cg-d is way less than Cg-s.
Yes, they love to oscillate. I use them as switches, and usually tame them with a gate resistor. But then, I don't care about noise!
Early voltage doesn't matter much either, if they are either on or off.
Mini-circuits sometimes does weird things. Often, actually. I was using one of their SMA-connectorized lowpass filters today. It's labeled as a 120 MHz Bessel. Its rise time is just about 1 ns, and it overshoots a bit.
Beware their MMICs. They sometimes change sources and things go to hell.
what gate current did you mesure?
-- \_(?)_
I didn't, in this little diode experiment. But normally, the gate looks like a diode to the source, sort of like a jfet.
I think there is some sort of beta effect, when the gate starts to conduct, or maybe that's just extreme enhancement. But these things turn on hard when you poke in a few mA of gate current.
It would be interesting to measure the gate current in diode modes; I'll try that.
Thanks again for the (for you) an "as usual" CLEAR (_NOT_ grey on grey) diagram. May others follow.
Maybe we can take up a go fund me collection and buy that certain person some nice white paper and a box of #2 pencils. :)
Kinda.
The "insulated gate emitter" symbol is a simple-minded shorthand for a generic NMOS. It has absolutely no bearing on reality, as the arrow does not denote a junction at all (indeed, it's pointing in /exactly the opposite direction/).
The symbol would be perfectly fine, actually, if the arrow were removed entirely (so it looks like an insulated JFET), and the gate is drawn asymmetrically to one side to suggest the preferred common (source) terminal. There still needs to be some hint at what channel doping is; perhaps a substrate diode to the middle, but not connected anywhere?
The EPC GaN FETs have the same problem. Sadly, no one pays attention to notation and typography anymore; in effect, the world is awash in Comic Sans and equivalents.
Tim
-- Seven Transistor Labs Electrical Engineering Consultation Website: http://seventransistorlabs.com
On Dec 19, 2015, Tom Miller wrote (in article ):
The problem is that the scanner is set for text plus photos, versus for text only. If photos, the scanner will adjust on the assumption of 18% gray, and so white paper becomes gray. If text (and no photo), the scanner will assume that the page is 95% white and 5% black, and so such things as drawings will come out clear (and smudges will vanish).
Joe Gwinn
I posted the connections and the resulting waveforms. Try it yourself if you think I'm making this up.
Yesterday I tried a circuit that actually uses an ephemt as a diode, and it works better than any diodes I can find.
I draw device symbols the way that they make sense to me.
There are at least five commmon ways to draw an n-channel fet, each used by respected sources. I draw mine a lot like AoE, but with the gate connection a little different. Lobby Congress for a law if that bothers you.
How do you draw logic symbols? Do you use the correct IEC617/IEEE91 symbols?
How do you draw resistors? Wiggles or boxes?
How about a 2-terminal bandgap reference? Some barbarians show them as if they were *zener diodes*, which of course they aren't.
Post one of your complex schematics and we'll review it for standards compliance.
I sketch circuits with an ink roller-ball pen or a Sharpie on grid paper, or markers on a whiteboard. I usually photograph them with a handheld camera, not a scanner. Irfanview reduces them from 18 zillion pixels down to something sensible, and crops/tweaks to make nice pics.
I draw real, production-destined schematics with pencil on D-size vellum. My CAD guy does design entry from that.
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