BJTs as ultra low leakage protection diodes

OK; just did some "reasonable" measurements with the Everlight super bright LED EL-334-15/T2C2-1TVB:

LEDs have no spec for leakage; just set up a fixture in low lighting conditions and test to what is reasonable like i did (4V or less in reverse); see response posted a bit earlier.

Hah! That shows red LEDs to be far superior to white ones (where i got

You are worse than i am...at least i "took the hint" and have stopped posting stuff from investment newsletters...

White LEDs are made of GaN--not the world's greatest semiconductor.

Cheers

Phil Hobbs

• "Equivalent schematics" AFAIK have never shown protection diodes, and in many cases will imply possible operation in regions that just cannot be achieved with a real device. Almost all analog and digital devices have a reverse polarity protection diode "built in" under the bonding pad and many have the same at the outputs; this appears to be standard practice and is ASS-u-ME-ed known and therefore not alluded to by most people in the industry. In a number of cases, an additional diode may be added to the supply rail and/or the doping of the pad-diode may be adjusted for a (say) zener breakdown of 5-8V. Cannot say about that as i have not tried to measure this presumption

- and again industry is fairly quiet..

*** Robert: They do look at Op27 datasheet it also has shown equivalent schematics. I can show you many BJT opamp that have shown protection diodes and also can show you many ultra low bias current opamps without protection.

If it is assumed to be known there should be some references to show that can prove this. I couldn't have find one. I'm not expecting you to show me a reference but I expect people that accuse me of "going beyond competence" just because I posted something that I assume is true to provide references to back their claims. I have provided my references:

2 datasheets op27 and opa627(I can post many more).

Regards, Ebrahim

Sorry; that is all i had for testing.

Getting back to the basic question, breakdown voltages are only guaranteed at a minimum. Suppose you get a device that has exceedingly good breakdown?

I did not understand what is your mean. will you explain more please ?

Regards, Ebrahim

"Ebrahim"

** You are a totally stupid pile of autistic shit, a waste of space a waster of good people's valuable time.

Fuck off and DROP DEAD !!!

ter

Ahem. Well, in 32 posts, has anyone answered Ebrahim's question? Which was, does a C-B junction make a good low-leakage high-voltage diode, and my answer is yes. Likely as low as 10pA, but I'd want to check specific parts.

It's the same question one asks before using a transistor at extremely low collector currents. OK, not too many people use BJT transistors at 10pA, for one thing they're very slow, but it's surprising how well they work down in that territory.

Often when doing this trick, one wants very low capacitance, and I've been disappointed with the high capacitance of the usual candidates, e.g., 5pF. This is what steers people back to the old PAD-1 beauties, spec'd at 0.5pF and 0.3pA

When one is in this pA territory, PCB leakage can be a big problem, and teflon posts and in-air wiring are often used.

***Hello Winfield :) : Thank you very much for your technical and non-political answer. So it makes sense to look for such a candidate among high frequency transistors that should have low capacitance.

Thank you again. Best wishes for you, Ebrahim

The part of this that makes zero sense is the 100V part. Almost all op amps, in fact AFAIK every single one ever sold, has PN junctions between its inputs and its supply pins.(*) That's why the Absolute Maximum Ratings section of datasheets specifies that you mustn't take their inputs more than 0.3V outside the supplies.

Not everything on an IC maps well onto a schematic--e.g. schematics don't show the parasitic SCR inherent in junction-isolated CMOS processes. It's there, though, as you'll find if you try dumping 20 mA into an input.

If you want the same sort of function provided by the input protection diodes, without a big leakage problem, you can use something like this:

0 VDD | _ A | *---Ri------*--------Rf-----------* | | | GGG _ | A | | | | |\\ | 0-------------RRRRR---*---RRRRR---|+\\ | | \\ | | >---*-*---0 *---|- / | | | / | | |/ | | | *---Ri------*---Rf------* | GGG

(Noninverting shown because inverting is reasonably obvious, and negative polarity left as an exercise for the reader.) You have to scale the resistances to fit your problem. The idea is that the first diode has nearly no voltage across it, and hence no leakage or capacitive current. This isn't a complete solution, because it costs you SNR eventually, but it's a lot better than nothing.

The diode needs almost no breakdown voltage at all--certainly not 100V. Something like a BFT25 B-C junction (about 0.3 pF) will work great. (WDNNS PAD-1!)

Cheers

Phil Hobbs

(*) Some chips, e.g. old CMOS->TTL converters use Zeners instead, because their inputs are intended to be overdriven by volts. Also, some rail-to-rail input op amps use charge pumps to allow them to run their input stages on more than VDD-VSS.

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net

Yes, I did.

Which was, does a C-B junction make a good

ftp://jjlarkin.lmi.net/BFT25.JPG

The c-b capacitance of the BFT25 is around 0.5 pF.

The forward and reverse curves should converge around zero volts; I think I may have had 20 or 30 fA of offset in my test rig.

I've used 2N4402's below 1 pA... not sure how much below.

The PAD parts are tiny jfets, so have high channel resistances, a few K typically. So they're not very hard clamps.

Did you see my fA tester box?

ftp://jjlarkin.lmi.net/99S260A.JPG ftp://jjlarkin.lmi.net/99A260A3.JPG ftp://jjlarkin.lmi.net/99A260A1.JPG

ftp://jjlarkin.lmi.net/PAD5.JPG

John

*** Hello Phil: I think I explained 100V in an earlier post in this thread I quote it again: "100V is needed for another use(protecting a high voltage discrete amplifier), not protecting FET opamp. I was hoping that I could find one low leakage diode for both uses. though for the first application(HV Discrete amp) I can live with some leakage"

I have also a High voltage discrete op amp and it has nothing to do with opa627. it is another part of a system. input voltage of up to 100V is allowed in there. It has a bootstrapped FET input stage. I may want to ask a question about it later so maybe I post the schematics of it but first I have to learn ASCII schematics ;) .

About reason of why input voltage shouldn't be more than 0.3 outside the supplies, well I did not know that and your reason seems reasonable. I will investigate about that. Thanks.

*** That's an interesting idea, you actually bootstrapped the first diode. Are you sure that any diode can do this? I mean I think I have done some measurements with 1N4007 and even in ~0 volt, its leakage was high. I don't remember the exact numbers, though I must find my papers.

Thanks Phil Hobbs Regards, Ebrahim

Hello John, Thanks a lot :). Your posts are very useful and give me a lot of inspirations.

Best wishes for you, Regards, Ebrahim

Not a crisis. I used this trick ten years or so ago in the Footprints system, which reduced the cost of a low-resolution thermal imager by two orders of magnitude while maintaining competitive performance. It took advantage of the photosensitivity of the LEDs as well, to supply a few picoamps of bias current under processor control. Ironically, if it had been only 0.5-1 order of magnitude cheaper instead of two, it would have sold much better. Some people won't let you save them money. :(

(I've mentioned it here a few times before--for anyone who isn't tired of hearing about it, the gory details are at

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net

?

waster

//jjlarkin.lmi.net/99A260A1.JPG

Very nice, John. Excellent!