On Fri, 22 Jun 2018 09:27:05 -0700 (PDT), whit3rd <whit3rd@gmail.com> wrote:>On Friday, June 22, 2018 at 9:05:06 AM UTC-7, John Larkin wrote: >> On Fri, 22 Jun 2018 09:02:14 +0200, Piotr Wyderski >> <peter.pan@neverland.mil> wrote: >> >> >I need a low current, low voltage and extremely low leakage diode. > >> Jfets make terrible diodes. Series resistance and capacitance are both >> high. And jfets are expensive. >> >> The c-b junction of a small transistor is a much better diode. BFT25 >> c-b is around 0.35 pF and has leakage that's hard to measure, below 10 >> fA. > >It's not a good part, though, for 20 mA; if you want the c-b diode, with >emitter open, you want something that allows 20 mA base current.BFT25 must be a tiny chip; it's only good for 30 mW dissipation. Something hunkier would be appropriate for 20 mA, but getting pA leakage is easy. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
JFET used as a diode
Started by ●June 22, 2018
Reply by ●June 22, 20182018-06-22
Reply by ●June 22, 20182018-06-22
John Larkin wrote:> Jfets make terrible diodes. Series resistance and capacitance are both > high.Capacitance is not important here, but resistance might be, thanks for pointing this out.> And jfets are expensive.MMBFJ202 costs 7 cents at quantity 10, 4 cents for a full 3000 reel. Would say it's dirt cheap. I've seen them used in this role on several Tektronics schematics. Why didn't they use a bipolar instead?> The c-b junction of a small transistor is a much better diode. BFT25 > c-b is around 0.35 pF and has leakage that's hard to measure, below 10 > fA.So again, how about its reliability? The base region isn't very famous for its rugged physical structure. No long-term degradation? Best regards, Piotr
Reply by ●June 22, 20182018-06-22
On Fri, 22 Jun 2018 20:19:41 +0200, Piotr Wyderski <peter.pan@neverland.mil> wrote:>John Larkin wrote: > >> Jfets make terrible diodes. Series resistance and capacitance are both >> high. > >Capacitance is not important here, but resistance might be, thanks for >pointing this out. > >> And jfets are expensive. > >MMBFJ202 costs 7 cents at quantity 10, 4 cents for a full 3000 reel. >Would say it's dirt cheap. > >I've seen them used in this role on several Tektronics schematics. >Why didn't they use a bipolar instead? > >> The c-b junction of a small transistor is a much better diode. BFT25 >> c-b is around 0.35 pF and has leakage that's hard to measure, below 10 >> fA. > >So again, how about its reliability? The base region isn't very >famous for its rugged physical structure. No long-term degradation? > > Best regards, PiotrIf you don't need fA leakage, go for something bigger than the BFT25. You can get SOT23 transistors that are good for amps of collector current, which implies a lot of base current. The jfet is OK if you like the forward conduction curve, but you should maybe test it. It will have a biggish ohmic component with a large positive TC. There are low leakage p-n diodes, like CMPD6001. They are typically way better than their specs. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●June 22, 20182018-06-22
On 06/22/2018 01:00 PM, George Herold wrote:> On Friday, June 22, 2018 at 11:37:48 AM UTC-4, Piotr Wyderski wrote: >> George Herold wrote: >> >>> What sort of leakage current do you need? >> >> <=100nA for T<=80C >> >>> And how much V_rev. ? >> >> 5V >> >> Best regards, Piotr > > Speed? The bc or be junction of a 2n3904 would probably work. > B. Pease "Bounding an clamping techniques" has a nice discussion of diodes... > I can't find a good copy on the web. :^( > > George H. >There's this one, but it's a bit blurry. https://electrooptical.net/www/sed/Bob-Pease-on-bounding-and-clamping-techniques.pdf 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
Reply by ●June 22, 20182018-06-22
George Herold wrote:> Speed?None, milliseconds would be fine.> B. Pease "Bounding an clamping techniques" has a nice discussion of diodes... > I can't find a good copy on the web. :^(Thanks for the reference, George. Best regards, Piotr
Reply by ●June 22, 20182018-06-22
On 22.6.18 17:14, Phil Hobbs wrote:> On 06/22/2018 04:58 AM, whit3rd wrote: >> On Friday, June 22, 2018 at 12:02:20 AM UTC-7, Piotr Wyderski wrote: >>> I need a low current, low voltage and extremely low leakage diode. >>> The gate junction of MMBFJ202 seems extremely well-suited >> >> Yep, should work fine. The JFETs are tested for leakage current, >> one would hope lots of other diodes are just as good in >> reverse blocking (but would require sorting). Tested >> transistors, with B-C shorted, might do well - and were >> discussed here not long ago. > > Not B-C shorted--it's the base-collector junction that has the low > leakage. You just leave the emitter open. BFT25As are great for that, > but of course won't take 20 mA. >Since when there has been base and collector in an JFET? -- -TV
Reply by ●June 22, 20182018-06-22
On 22.6.18 19:04, John Larkin wrote:> On Fri, 22 Jun 2018 09:02:14 +0200, Piotr Wyderski > <peter.pan@neverland.mil> wrote: > >> I need a low current, low voltage and extremely low leakage diode. >> The gate junction of MMBFJ202 seems extremely well-suited for the job. >> The datasheet says its absolute maximum gate current is 50mA. >> Can I operate it reliably at 20mA for years? I mean, would there >> be any relevant degradation mechanism or is this junction an ordinary >> diode and I shouldn't worry about it? >> >> Best regards, Piotr > > Jfets make terrible diodes. Series resistance and capacitance are both > high. And jfets are expensive. > > The c-b junction of a small transistor is a much better diode. BFT25 > c-b is around 0.35 pF and has leakage that's hard to measure, below 10 > fA. > > https://www.dropbox.com/s/ft0tsikhdi90rgq/BFT25.JPG?raw=1 > > Most gumdrop bipolars will leak well below 1 pA.When other options do not work, the PAD-1 seems to be still around. A pair of them performed beautifully in a fA electrometer. -- -TV
Reply by ●June 22, 20182018-06-22
On 06/22/2018 03:33 PM, Tauno Voipio wrote:> On 22.6.18 17:14, Phil Hobbs wrote: >> On 06/22/2018 04:58 AM, whit3rd wrote: >>> On Friday, June 22, 2018 at 12:02:20 AM UTC-7, Piotr Wyderski wrote: >>>> I need a low current, low voltage and extremely low leakage diode. >>>> The gate junction of MMBFJ202 seems extremely well-suited >>> >>> Yep, should work fine. The JFETs are tested for leakage current, >>> one would hope lots of other diodes are just as good in >>> reverse blocking (but would require sorting). Tested >>> transistors, with B-C shorted, might do well - and were >>> discussed here not long ago. >> >> Not B-C shorted--it's the base-collector junction that has the low >> leakage. You just leave the emitter open. BFT25As are great for >> that, but of course won't take 20 mA. >> > > Since when there has been base and collector in an JFET? >The referent was "tested transistors", i.e. BJTs, not JFETs. If you short the gate of a JFET to drain or source, it stops acting like a diode. 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
Reply by ●June 22, 20182018-06-22
On Friday, June 22, 2018 at 2:48:45 PM UTC-4, Phil Hobbs wrote:> On 06/22/2018 01:00 PM, George Herold wrote: > > On Friday, June 22, 2018 at 11:37:48 AM UTC-4, Piotr Wyderski wrote: > >> George Herold wrote: > >> > >>> What sort of leakage current do you need? > >> > >> <=100nA for T<=80C > >> > >>> And how much V_rev. ? > >> > >> 5V > >> > >> Best regards, Piotr > > > > Speed? The bc or be junction of a 2n3904 would probably work. > > B. Pease "Bounding an clamping techniques" has a nice discussion of diodes... > > I can't find a good copy on the web. :^( > > > > George H. > > > There's this one, but it's a bit blurry. > > https://electrooptical.net/www/sed/Bob-Pease-on-bounding-and-clamping-techniques.pdf >Right I've got the same blurry copy on my computer. I've got an older paper copy some where. George H.> 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
Reply by ●June 22, 20182018-06-22
On Fri, 22 Jun 2018 22:35:59 +0300, Tauno Voipio <tauno.voipio@notused.fi.invalid> wrote:>On 22.6.18 19:04, John Larkin wrote: >> On Fri, 22 Jun 2018 09:02:14 +0200, Piotr Wyderski >> <peter.pan@neverland.mil> wrote: >> >>> I need a low current, low voltage and extremely low leakage diode. >>> The gate junction of MMBFJ202 seems extremely well-suited for the job. >>> The datasheet says its absolute maximum gate current is 50mA. >>> Can I operate it reliably at 20mA for years? I mean, would there >>> be any relevant degradation mechanism or is this junction an ordinary >>> diode and I shouldn't worry about it? >>> >>> Best regards, Piotr >> >> Jfets make terrible diodes. Series resistance and capacitance are both >> high. And jfets are expensive. >> >> The c-b junction of a small transistor is a much better diode. BFT25 >> c-b is around 0.35 pF and has leakage that's hard to measure, below 10 >> fA. >> >> https://www.dropbox.com/s/ft0tsikhdi90rgq/BFT25.JPG?raw=1 >> >> Most gumdrop bipolars will leak well below 1 pA. > > >When other options do not work, the PAD-1 seems to be still around. >A pair of them performed beautifully in a fA electrometer.Yes, but high Rs, and expensive. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
