miso@sushi.com wrote:> 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
BJTs as ultra low leakage protection diodes
Started by ●June 12, 2009
Reply by ●June 14, 20092009-06-14
Reply by ●June 14, 20092009-06-14
"Ebrahim"> > I did not understand what is your mean. will you explain more please ?** 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 !!!
Reply by ●June 14, 20092009-06-14
On Jun 14, 12:32=A0am, "Phil Allison" <phi...@tpg.com.au> wrote:> "Ebrahim" > >> I did not understand what is your mean. will you explain more please ? > > ** You are a totally stupid pile of autistic shit, a waste of space a was=ter> of good people's valuable time. > > =A0 =A0Fuck off and DROP DEAD =A0!!!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.
Reply by ●June 14, 20092009-06-14
"Winfield Hill" "Phil Allison" "Ebrahim"> >> I did not understand what is your mean. will you explain more please ? > > ** 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 !!!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. ** Since when is C-B junction leakage at high voltages anything like that low ?? You are bulshitting wildly - as per fucking usual. It's the same question one asks before using a transistor at extremely low collector currents. ** What absolute bollocks. The two matters are totally independent issues. Winfield has long be a posturing, fucking PITA, pseudo-academic pommy prig. Now, his rampant congenital autism has rendered him totally useless. I know he will not comprehend a single word of this. The mentally diseased never do. .... Phil
Reply by ●June 14, 20092009-06-14
Winfield Hill wrote:> 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
Reply by ●June 14, 20092009-06-14
Phil Allison wrote:> "Winfield Hill" > "Phil Allison" > "Ebrahim" > >>>I did not understand what is your mean. will you explain more please ? >> >>** 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 !!! > > > 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. > > > ** Since when is C-B junction leakage at high voltages anything like that > low ?? > > You are bulshitting wildly - as per fucking usual. > > > It's the same question one asks before using a transistor at > extremely low collector currents. > > > ** What absolute bollocks. > > The two matters are totally independent issues. > > Winfield has long be a posturing, fucking PITA, pseudo-academic pommy prig. > > Now, his rampant congenital autism has rendered him totally useless. > > I know he will not comprehend a single word of this. > > The mentally diseased never do. > > > > .... Phil > >You're correct Phil, the mentally diseased normally don't understand each other! Touch�!
Reply by ●June 14, 20092009-06-14
Ebrahim wrote:> Winfield Hill wrote: >> 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, > EbrahimThe 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
Reply by ●June 14, 20092009-06-14
On Sun, 14 Jun 2009 03:46:42 -0700 (PDT), Winfield Hill <hill@rowland.org> wrote:>On Jun 14, 12:32�am, "Phil Allison" <phi...@tpg.com.au> wrote: >> "Ebrahim" >> >>> I did not understand what is your mean. will you explain more please ? >> >> ** 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 �!!! > > Ahem. Well, in 32 posts, has anyone answered Ebrahim's > question?Yes, I did. 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.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.> > 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 0.3pAThe PAD parts are tiny jfets, so have high channel resistances, a few K typically. So they're not very hard clamps.> > When one is in this pA territory, PCB leakage can be a big > problem, and teflon posts and in-air wiring are often used.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
Reply by ●June 14, 20092009-06-14
Phil Hobbs wrote:> 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 20mA > into an input. *** 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.> 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!) >*** 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.> (*) 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.Thanks Phil Hobbs Regards, Ebrahim
Reply by ●June 14, 20092009-06-14
John Larkin wrote:>> Ahem. Well, in 32 posts, has anyone answered Ebrahim's >> question? > > Yes, I did. > > 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. > > 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. > >> 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 > > The PAD parts are tiny jfets, so have high channel resistances, a few > K typically. So they're not very hard clamps. > > >> When one is in this pA territory, PCB leakage can be a big >> problem, and teflon posts and in-air wiring are often used. > > 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 >Hello John, Thanks a lot :). Your posts are very useful and give me a lot of inspirations. Best wishes for you, Regards, Ebrahim
