Ebrahim wrote:> miso@sushi.com wrote: > >> What exactly are you trying to protect? IC, fet, ?? > > Hello, > I'm Trying to protect input of Ultra low bias current FET OpAmps. > > Regards, > EbrahimThen a neon will not do as it takes around 60V to "break down" after ionizes; 90V beforehand.
BJTs as ultra low leakage protection diodes
Started by ●June 12, 2009
Reply by ●June 13, 20092009-06-13
Reply by ●June 13, 20092009-06-13
Robert Baer wrote:> In a way i have "measured" leakage current in LEDs; low microamps for > remaining visibility which i think is astounding, and <1nA in the 1V > (forward) region with corresponding reverse values. > Certainly not in the region you want.Wow, uA and nA :-o ! then they can't be used as an ultra low leakage. Even humble 1N4148 has lower leakage ;) .
Reply by ●June 13, 20092009-06-13
"Ebrahim" <Engineer.Ebi@gmail.com> wrote in message news:h0vdqr$314$1@news.eternal-september.org...> miso@sushi.com wrote: > >> What exactly are you trying to protect? IC, fet, ?? > > Hello, > I'm Trying to protect input of Ultra low bias current FET OpAmps. > > Regards, > Ebrahimnot cheap but http://www.fairchildsemi.com/ds/FJ/FJH1100.pdf
Reply by ●June 13, 20092009-06-13
Robert Baer wrote:> Ebrahim wrote: >> Robert Baer wrote: >>> Ebrahim wrote: >>>> Hello, I want is there anyone that has used a BJT instead of ultra >>>> low leakage diode(<10pA) in a serious design. The reason that I do >>>> not use JFET is that I need high breakdown(100V) voltage. So I also >>>> need Bootstrapping I have down some measurements with different >>>> JFETs and BJTs and I didn't find quite big differences. >>> Do not think BJTs in general are that good; try an LED... >> >> >> >> Hello Robert :) >> Thanks for replying to my post. The main problem with LEDs is their >> low breakdown voltage. I have not measured leakage current of an LED >> but it makes sense because GaAs Reverse saturation current is about 10 >> times less than Si. Have you used LED as an ultra low leakage diode ? >> have you measured leakage currents of LEDs? >> >> Regards, >> Ebrahim > In a way i have "measured" leakage current in LEDs; low microamps for > remaining visibility which i think is astounding, and <1nA in the 1V > (forward) region with corresponding reverse values. > Certainly not in the region you want.Nanoamps? Wow. Is that spec'd anywhere? If so then they'd be really useful as limiters. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
Reply by ●June 13, 20092009-06-13
On Sat, 13 Jun 2009 10:05:11 +0430, Ebrahim <Engineer.Ebi@gmail.com> wrote:>miso@sushi.com wrote: > >> What exactly are you trying to protect? IC, fet, ?? > >Hello, >I'm Trying to protect input of Ultra low bias current FET OpAmps. > >Regards, >EbrahimThen limit the current before you try to clamp it. You're not going to have a lot of luck finding a half-amp, 100V diode that leaks picoamps. Why 100 volts? The opamp can't stand that. Use a resistor or, if you can't stand the Johnson noise, a pair of depletion fets or some such. SSRs are interesting as low-resistance current limiters. A high-value resistor bypassed with a small cap is interesting here too. If the fet has internal ESD diodes, that may be all you need. John
Reply by ●June 13, 20092009-06-13
John Larkin wrote:> On Sat, 13 Jun 2009 10:05:11 +0430, Ebrahim <Engineer.Ebi@gmail.com> > wrote:> Then limit the current before you try to clamp it. You're not going to > have a lot of luck finding a half-amp, 100V diode that leaks picoamps. > Why 100 volts? The opamp can't stand that.*** John: 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 but for protecting the FET opamp I really need ultra low leakage and about 500mA load current.> Use a resistor or, if you can't stand the Johnson noise, a pair of > depletion fets or some such. SSRs are interesting as low-resistance > current limiters.*** The FET Opamp that I try to protect is opa627 ( http://focus.ti.com/docs/prod/folders/print/opa627.html) resistor is out of question because of Johnson noise as you mentioned(because voltage could rise up to 100V in some rare occasions and it needs a really big resistor) and for the same reason depletion FETs can't be used. SSR(Solid State Relays ?) How?> A high-value resistor bypassed with a small cap is interesting here > too. If the fet has internal ESD diodes, that may be all you need.*** No, Ultra low bias current opamps usually do not have internal protection diodes. Cheers, Ebrahim
Reply by ●June 13, 20092009-06-13
Ebrahim wrote:> Robert Baer wrote: >> Ebrahim wrote: >> Do not think BJTs in general are that good; try an LED... > > Hello Robert :) > Thanks for replying to my post. I have not measured leakage current of > an LED but it makes sense because GaAs Reverse saturation current is > about 10 times less than Si. Have you used LED as an ultra low leakage > diode ? have you measured leakage currents of LEDs? > > Regards, > EbrahimIt's far lower than that. I've measured ordinary red display LEDs whose leakage was below 100 fA from -5V to +0.5V bias. 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
Reply by ●June 13, 20092009-06-13
"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 !!!
Reply by ●June 13, 20092009-06-13
Ebrahim wrote:> John Larkin wrote: >> On Sat, 13 Jun 2009 10:05:11 +0430, Ebrahim <Engineer.Ebi@gmail.com> >> wrote: > > >> Then limit the current before you try to clamp it. You're not going to >> have a lot of luck finding a half-amp, 100V diode that leaks picoamps. >> Why 100 volts? The opamp can't stand that. > *** > John: > 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 but for protecting the FET > opamp I really need ultra low leakage and about 500mA load current. > > >> Use a resistor or, if you can't stand the Johnson noise, a pair of >> depletion fets or some such. SSRs are interesting as low-resistance >> current limiters. > *** > The FET Opamp that I try to protect is opa627 ( > http://focus.ti.com/docs/prod/folders/print/opa627.html) > resistor is out of question because of Johnson noise as you > mentioned(because voltage could rise up to 100V in some rare occasions > and it needs a really big resistor) and for the same reason depletion > FETs can't be used. SSR(Solid State Relays ?) How? > > >> A high-value resistor bypassed with a small cap is interesting here >> too. If the fet has internal ESD diodes, that may be all you need. > *** > No, Ultra low bias current opamps usually do not have internal > protection diodes.What op amp have you ever seen that didn't, and how do you know? Such a device might be quite interesting in applications, but I don't know of one. Even femtoamp leakage devices (e.g. the venerable LMC660) have protection diodes. You can use them for resetting integrators, by briefly inverting the power rails (with suitable current limits). Win Hill posted a circuit for that in this very group a few years back. There are folks here who post stuff that's way beyond their competence...you've been here long enough to know who some of them are. Joining that group isn't necessarily the best way to achieve respect. Just saying. 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
Reply by ●June 13, 20092009-06-13
Phil Hobbs wrote:> Ebrahim wrote:> What op amp have you ever seen that didn't, and how do you know? Such a > device might be quite interesting in applications, but I don't know of one.> Even femtoamp leakage devices (e.g. the venerable LMC660) have > protection diodes. You can use them for resetting integrators, by > briefly inverting the power rails (with suitable current limits). Win > Hill posted a circuit for that in this very group a few years back. > *** Hello Phil Look at data sheet of OPA627, It has shown equivalent schematic. I do not see any protection diodes in inputs.> There are folks here who post stuff that's way beyond their > competence...you've been here long enough to know who some of them are. > Joining that group isn't necessarily the best way to achieve respect. > Just saying.I really do not understand what you are talking about. I'm not here to join some internal groups, I'm not here to woo members. I'm not aware of factions of this newsgroup and honestly I do not care about them if they exist. I'm here to discuss technical issues. I never claim something that I'm not. I ask questions and if I know something about a topic I post my opinion. If someone provided documents and proved that I'm wrong about something, then I learn from him and thank him, provided that he present his reasons politely and respectfully. I do not remember I've claimed anything here. so your post really surprised me. regards, Ebrahim
