John Larkin wrote:> On Thu, 14 Apr 2022 12:20:31 -0700 (PDT), whit3rd <whit3rd@gmail.com> > wrote: > >> On Thursday, April 14, 2022 at 5:44:43 AM UTC-7, neo5...@gmail.com wrote: >>> So which bjt's have such low leakage? >>> Since Im thinking of it how much current can the collector base junction take without failing? >> >> It's not an easy question, low-leakage isn't a high test priority for BJTs. >> At a guess, you'd want low-gain audio (low-frequency) parts, which have relatively >> high base doping. > > Small geometry RF transistors seem to have fA reverse currents when > used as diodes. I'd expect audio parts to be much bigger. > > https://www.dropbox.com/sh/cuoy2p93jkmyxtz/AAANclqWKW4e5xHfRn3LlPUDa?dl=0 > > Note that my diode connection exploits reverse beta to keep the base > current down, so it should be good for rated max Ic. That will limit > max reverse voltage.It's possible for it to oscillate, though! The normal diode-connected transistor runs out of beta at low currents, too. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Max current for a JFET connected as diode
Started by ●April 13, 2022
Reply by ●April 15, 20222022-04-15
Reply by ●April 15, 20222022-04-15
On Thursday, April 14, 2022 at 5:38:51 PM UTC-4, jla...@highlandsniptechnology.com wrote:> On Thu, 14 Apr 2022 16:32:50 -0400, Phil Hobbs > <pcdhSpamM...@electrooptical.net> wrote: > > >Phil Hobbs wrote: > >> whit3rd wrote: > >>> On Thursday, April 14, 2022 at 5:44:43 AM UTC-7, neo5...@gmail.com wrote: > >>>> So which bjt's have such low leakage? > >>>> Since Im thinking of it how much current can the collector base > >>>> junction take without failing? > >>> > >>> It's not an easy question, low-leakage isn't a high test priority for > >>> BJTs. > >>> At a guess, you'd want low-gain audio (low-frequency) parts, which > >>> have relatively > >>> high base doping. > >>> > >>> How about a zener diode? Should be good for high currents, certainly > >>> specified for 'em. > >>> > >> > >> The best ones I know about are BFT25As (RIP, sniff sniff), which are > >> small-geometry 5-GHz NPNs. > > > >See, e.g. this thread from a dozen years ago: > > > >"Types of diode-connected BJTs" > > > ><https://groups.google.com/g/sci.electronics.design/c/wonzon1fZGI/m/f31wyYfP4aUJ> > > > >Cheers > > > >Phil Hobbs > I miss JT's harassment. He was good at it. > -- > > I yam what I yam - PopeyeThanks for including that thread here. I very much appreciate everyone's useful comments on this topic. Don
Reply by ●April 15, 20222022-04-15
On Fri, 15 Apr 2022 10:05:32 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>John Larkin wrote: >> On Thu, 14 Apr 2022 12:20:31 -0700 (PDT), whit3rd <whit3rd@gmail.com> >> wrote: >> >>> On Thursday, April 14, 2022 at 5:44:43 AM UTC-7, neo5...@gmail.com wrote: >>>> So which bjt's have such low leakage? >>>> Since Im thinking of it how much current can the collector base junction take without failing? >>> >>> It's not an easy question, low-leakage isn't a high test priority for BJTs. >>> At a guess, you'd want low-gain audio (low-frequency) parts, which have relatively >>> high base doping. >> >> Small geometry RF transistors seem to have fA reverse currents when >> used as diodes. I'd expect audio parts to be much bigger. >> >> https://www.dropbox.com/sh/cuoy2p93jkmyxtz/AAANclqWKW4e5xHfRn3LlPUDa?dl=0 >> >> Note that my diode connection exploits reverse beta to keep the base >> current down, so it should be good for rated max Ic. That will limit >> max reverse voltage. > >It's possible for it to oscillate, though!Idiot data sheets seldom specify s-params in the reverse beta mode! (repeat usual rf rant) -- I yam what I yam - Popeye
Reply by ●April 15, 20222022-04-15
On Fri, 15 Apr 2022 09:17:47 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>Rich S wrote: >>>>> The best ones I know about are BFT25As (RIP, sniff sniff), which are >>>>> small-geometry 5-GHz NPNs. >> >> >> Re BFT25 (without the A, has higher Noise Figure, lower f_T) >> about 1700, left in stock at DigiKey >> https://www.digikey.com/en/products/base-product/nxp-usa-inc/568/BFT25/35932 >> >> Though I assume if one chose another of similar >> construction & specs, they should see similar results (?) >> if not in the ballpark... >> = RS > >Oh, I have three reels of BFT25As in stock. Doesn't help much with >customer designs.Know of a good sub? SAV541 makes a couple of interesting diodes... which *can* oscillate https://www.dropbox.com/s/bi2mmj2edn8gul5/SAV541_diode_2.jpg?raw=1 Dearly departed ATF-50189 was a 1-amp, 20 volt, low Vf, 3 pF diode https://www.dropbox.com/s/4ql98sfpm6oarnd/Ephemt_Diode.JPG?raw=1 -- I yam what I yam - Popeye
Reply by ●April 15, 20222022-04-15
On Fri, 15 Apr 2022 09:15:59 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>jlarkin@highlandsniptechnology.com wrote: >> On Thu, 14 Apr 2022 16:32:50 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> Phil Hobbs wrote: >>>> whit3rd wrote: >>>>> On Thursday, April 14, 2022 at 5:44:43 AM UTC-7, neo5...@gmail.com wrote: >>>>>> So which bjt's have such low leakage? >>>>>> Since Im thinking of it how much current can the collector base >>>>>> junction take without failing? >>>>> >>>>> It's not an easy question, low-leakage isn't a high test priority for >>>>> BJTs. >>>>> At a guess, you'd want low-gain audio (low-frequency) parts, which >>>>> have relatively >>>>> high base doping. >>>>> >>>>> How about a zener diode?�� Should be good for high currents, certainly >>>>> specified for 'em. >>>>> >>>> >>>> The best ones I know about are BFT25As (RIP, sniff sniff), which are >>>> small-geometry 5-GHz NPNs. >>> >>> See, e.g. this thread from a dozen years ago: >>> >>> "Types of diode-connected BJTs" >>> >>> <https://groups.google.com/g/sci.electronics.design/c/wonzon1fZGI/m/f31wyYfP4aUJ> >>> >>> Cheers >>> >>> Phil Hobbs >> >> I miss JT's harassment. He was good at it. > >But when Fields got involved, it went to a whole new level. ;) > >Cheers > >Phil HobbsQuantity, not quality. -- I yam what I yam - Popeye
Reply by ●April 15, 20222022-04-15
On Fri, 15 Apr 2022 06:56:03 -0700 (PDT), "neo5...@gmail.com" <neo5bass@gmail.com> wrote:>On Thursday, April 14, 2022 at 3:20:35 PM UTC-4, whit3rd wrote: >> On Thursday, April 14, 2022 at 5:44:43 AM UTC-7, neo5...@gmail.com wrote: >> > So which bjt's have such low leakage? >> > Since Im thinking of it how much current can the collector base junction take without failing? >> It's not an easy question, low-leakage isn't a high test priority for BJTs. >> At a guess, you'd want low-gain audio (low-frequency) parts, which have relatively >> high base doping. >> >> How about a zener diode? Should be good for high currents, certainly specified for 'em. > >A zener is too leaky for this application alone. It is a Zener circuit but where where I'm adding the extra forward voltage drop of a low leakage diode in series so the zeners leakage current isnt causing error on the signal line below the zener clamp voltage. Jfet clamps are very good at staying out of the way in terms of their leakage. I've used them for many microphone or other audio signal line protection but never had to pay much attention to the current through them when conducting. - Just that it does when called for. This application is a little different. This is to protect a "universal" voltage input where the range of signal can be 0 to 10 mV or a large as 0 to 10V. But also prevent damage when some dope connects the +24V power supply rail to it. > >AofE does have the closest thing I've read that discusses some of the question but doesn't really speak to the question of the limit of current through the clamp. Logically is would be below the max rating for the device but how far below?See AoE3 page 361. The idea is to use depletion fets as current limiters. The LND150s add a couple of k equivalent resistance, but there are hunkier parts, like DN2530. Ixys make a self-protecting SSR that can be fun as a current limiter. -- I yam what I yam - Popeye
Reply by ●April 15, 20222022-04-15
jlarkin@highlandsniptechnology.com wrote:> On Fri, 15 Apr 2022 10:05:32 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> John Larkin wrote: >>> On Thu, 14 Apr 2022 12:20:31 -0700 (PDT), whit3rd <whit3rd@gmail.com> >>> wrote: >>> >>>> On Thursday, April 14, 2022 at 5:44:43 AM UTC-7, neo5...@gmail.com wrote: >>>>> So which bjt's have such low leakage? >>>>> Since Im thinking of it how much current can the collector base junction take without failing? >>>> >>>> It's not an easy question, low-leakage isn't a high test priority for BJTs. >>>> At a guess, you'd want low-gain audio (low-frequency) parts, which have relatively >>>> high base doping. >>> >>> Small geometry RF transistors seem to have fA reverse currents when >>> used as diodes. I'd expect audio parts to be much bigger. >>> >>> https://www.dropbox.com/sh/cuoy2p93jkmyxtz/AAANclqWKW4e5xHfRn3LlPUDa?dl=0 >>> >>> Note that my diode connection exploits reverse beta to keep the base >>> current down, so it should be good for rated max Ic. That will limit >>> max reverse voltage. >> >> It's possible for it to oscillate, though! > > Idiot data sheets seldom specify s-params in the reverse beta mode! > > (repeat usual rf rant)Even with reduced beta, f_T is often higher in reverse mode, I've heard. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●April 15, 20222022-04-15
jlarkin@highlandsniptechnology.com wrote:> On Fri, 15 Apr 2022 09:17:47 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> Rich S wrote: >>>>>> The best ones I know about are BFT25As (RIP, sniff sniff), which are >>>>>> small-geometry 5-GHz NPNs. >>> >>> >>> Re BFT25 (without the A, has higher Noise Figure, lower f_T) >>> about 1700, left in stock at DigiKey >>> https://www.digikey.com/en/products/base-product/nxp-usa-inc/568/BFT25/35932 >>> >>> Though I assume if one chose another of similar >>> construction & specs, they should see similar results (?) >>> if not in the ballpark... >>> = RS >> >> Oh, I have three reels of BFT25As in stock. Doesn't help much with >> customer designs. > > Know of a good sub? > > SAV541 makes a couple of interesting diodes... which *can* oscillate > > https://www.dropbox.com/s/bi2mmj2edn8gul5/SAV541_diode_2.jpg?raw=1Getting a 12-GHz f_max transistor to oscillate at 2 kHz requires a real gift. ;) Thermal, I gather?> > Dearly departed ATF-50189 was a 1-amp, 20 volt, low Vf, 3 pF diode > > https://www.dropbox.com/s/4ql98sfpm6oarnd/Ephemt_Diode.JPG?raw=1There's a TI app note on using GaN FETs as ideal diodes. (GaN FETs are also 2-D electron gas devices, like pHEMTs.) <https://www.ti.com/lit/an/snoa932/snoa932.pdf> Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●April 15, 20222022-04-15
On Fri, 15 Apr 2022 10:58:34 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>jlarkin@highlandsniptechnology.com wrote: >> On Fri, 15 Apr 2022 09:17:47 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> Rich S wrote: >>>>>>> The best ones I know about are BFT25As (RIP, sniff sniff), which are >>>>>>> small-geometry 5-GHz NPNs. >>>> >>>> >>>> Re BFT25 (without the A, has higher Noise Figure, lower f_T) >>>> about 1700, left in stock at DigiKey >>>> https://www.digikey.com/en/products/base-product/nxp-usa-inc/568/BFT25/35932 >>>> >>>> Though I assume if one chose another of similar >>>> construction & specs, they should see similar results (?) >>>> if not in the ballpark... >>>> = RS >>> >>> Oh, I have three reels of BFT25As in stock. Doesn't help much with >>> customer designs. >> >> Know of a good sub? >> >> SAV541 makes a couple of interesting diodes... which *can* oscillate >> >> https://www.dropbox.com/s/bi2mmj2edn8gul5/SAV541_diode_2.jpg?raw=1 > >Getting a 12-GHz f_max transistor to oscillate at 2 kHz requires a real >gift. ;) > >Thermal, I gather?2K was the sinewave generator. The oscillations are the fuzz on the negative swing.> >> >> Dearly departed ATF-50189 was a 1-amp, 20 volt, low Vf, 3 pF diode >> >> https://www.dropbox.com/s/4ql98sfpm6oarnd/Ephemt_Diode.JPG?raw=1 > >There's a TI app note on using GaN FETs as ideal diodes. (GaN FETs are >also 2-D electron gas devices, like pHEMTs.) > ><https://www.ti.com/lit/an/snoa932/snoa932.pdf> > >Cheers > >Phil HobbsI was recently playing with ideas about an ideal full-bridge rectifier. There is an LTC chip to do that with four mosfets, but I need more voltage than that chip can handle. EPC ganfets as diodes might be a compromise on Von, without elaborate gate driver stuff. -- I yam what I yam - Popeye
Reply by ●April 15, 20222022-04-15
On a sunny day (Fri, 15 Apr 2022 10:58:34 -0400) it happened Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote in <0a4e27a5-782a-b21b-600e-262eef831edf@electrooptical.net>:>jlarkin@highlandsniptechnology.com wrote: >> On Fri, 15 Apr 2022 09:17:47 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> Rich S wrote: >>>>>>> The best ones I know about are BFT25As (RIP, sniff sniff), which are >>>>>>> small-geometry 5-GHz NPNs. >>>> >>>> >>>> Re BFT25 (without the A, has higher Noise Figure, lower f_T) >>>> about 1700, left in stock at DigiKey >>>> https://www.digikey.com/en/products/base-product/nxp-usa-inc/568/BFT25/35932 >>>> >>>> Though I assume if one chose another of similar >>>> construction & specs, they should see similar results (?) >>>> if not in the ballpark... >>>> = RS >>> >>> Oh, I have three reels of BFT25As in stock. Doesn't help much with >>> customer designs. >> >> Know of a good sub? >> >> SAV541 makes a couple of interesting diodes... which *can* oscillate >> >> https://www.dropbox.com/s/bi2mmj2edn8gul5/SAV541_diode_2.jpg?raw=1 > >Getting a 12-GHz f_max transistor to oscillate at 2 kHz requires a real >gift. ;) > >Thermal, I gather?Looks like it is oscillating in the GHz range sort of self-quencing. bottom wave is full of RF? Wire length -> wave length etc, maybe resistor is inductive too.
