Jan Panteltje wrote:> 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.The Q that you'd need for the aliased waveform to be that clean would be fairly impressive. 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
jlarkin@highlandsniptechnology.com wrote:> 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.Ah, right, didn't pay enough attention.> >> >>> >>> 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> >>> I 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.A transformer might be a win--you save a volt when the on-time is short enough, and don't lose anything much when it isn't. 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 12:38:07 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>jlarkin@highlandsniptechnology.com wrote: >> 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. > >Ah, right, didn't pay enough attention.It was a breadboard with dangling wires, not very tight. -- If a man will begin with certainties, he shall end with doubts, but if he will be content to begin with doubts he shall end in certainties. Francis Bacon
Reply by ●April 15, 20222022-04-15
On Fri, 15 Apr 2022 09:19:13 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>jlarkin@highlandsniptechnology.com wrote: >> On Thu, 14 Apr 2022 16:25:02 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> 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. >>> >>> Cheers >>> >>> Phil Hobbs >> >> Any guesses about SiGe transistors as diodes? I should get some and >> try them. One figure of merit is capacitance * reverse current. >> >> >> > >Haven't tried it. I should hang some on my nice shiny HP 4145B and find >out. I use BFU520As to replace BFT25As for actual three-terminal jobs.Is that SiGe? I'd like to get a good SiGe to play with. That BFU does look nice. Lots of voltage, which we need sometimes. -- If a man will begin with certainties, he shall end with doubts, but if he will be content to begin with doubts he shall end in certainties. Francis Bacon
Reply by ●April 15, 20222022-04-15
Jan Panteltje wrote:> 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. > > >Yeah, I just didn't read it right. Moving on, nothing to see here, folks,.... ;) 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
John Larkin wrote:> On Fri, 15 Apr 2022 09:19:13 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> jlarkin@highlandsniptechnology.com wrote: >>> On Thu, 14 Apr 2022 16:25:02 -0400, Phil Hobbs >>> <pcdhSpamMeSenseless@electrooptical.net> 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. >>>> >>>> Cheers >>>> >>>> Phil Hobbs >>> >>> Any guesses about SiGe transistors as diodes? I should get some and >>> try them. One figure of merit is capacitance * reverse current. >>> >>> >>> >> >> Haven't tried it. I should hang some on my nice shiny HP 4145B and find >> out. I use BFU520As to replace BFT25As for actual three-terminal jobs. > > Is that SiGe? I'd like to get a good SiGe to play with. > > That BFU does look nice. Lots of voltage, which we need sometimes. >No, it's follow-on to the late lamented BFG series. The BFG403 was even nicer than the BFG25A, which was nicer than the BFT25A, which was nicer than the OG MRF9331 that Win and Paul enthused about in AOE II. (I have a reel of BFG403s too.) The SiGe you want is either BFP640 or BFP650. The BFP780 was recently discontinued as well. Get your transistors while they're hot, folks! Actual measured parameters: beta ~ 250, V_A ~ 300 V at AC, essentially infinite at DC due to thermal changes of V_BE. You will definitely want to put a BLM15BA050SN1D bead in the base though! 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 Friday, April 15, 2022 at 6:56:07 AM UTC-7, neo5...@gmail.com wrote:> > 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...> 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?The 'max rating' is in reference to device deviations from its specifications, and zener ratings assume you care, critically, about the threshold voltage, not leakage. Most of the use-a-transistor-as-diode discussion is about typical actual leakage, but NOT in a test scenario where breakdowns and wacky bias conditions occur, so some of the observations won't be complete in the sense of including age and 'abuse' conditions. If it were practical, I'd consider that you want an input resistor, and a clamp-to-ground, i.e. antiparallel diodes, into an inverting node of an op amp; since the inverting node is always pseudo-ground, those clamp diodes have zero applied bias, and leakage is a near non-issue (unless they're photovoltaic?).
Reply by ●April 15, 20222022-04-15
whit3rd wrote:> On Friday, April 15, 2022 at 6:56:07 AM UTC-7, neo5...@gmail.com wrote: > >>> 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... > >> 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? > > The 'max rating' is in reference to device deviations from its specifications, and zener ratings assume > you care, critically, about the threshold voltage, not leakage. > > Most of the use-a-transistor-as-diode discussion is about typical actual leakage, but NOT in > a test scenario where breakdowns and wacky bias conditions occur, so some of the observations > won't be complete in the sense of including age and 'abuse' conditions. > > If it were practical, I'd consider that you want an input resistor, and a clamp-to-ground, i.e. antiparallel > diodes, into an inverting node of an op amp; since the inverting node is always pseudo-ground, > those clamp diodes have zero applied bias, and leakage is a near non-issue (unless they're > photovoltaic?). >If it's coming from outside the box, you _do_ want that on the inputs. In high performance applications you often have to stand on one leg a bit to get there. For instance, the protection diode can be an RF Schottky plus a lightly-biased zener, to get both low capacitance and good clamping. Other times, I bootstrap the protection circuitry such that a hard fault gets caught but the capacitance and leakage mostly go away. A simpler example is protecting a noninverting buffer using a series pairs of Schottkies from the input to both supplies (4 diodes in all), biased with a resistor to the output. The diodes connected to the input have nearly zero volts across them, and hence nearly zero leakage, but transients get dumped to the supplies harmlessly. (One might need a couple of series resistors to make this really robust.) 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 ●March 2, 20232023-03-02
On Wednesday, April 13, 2022 at 8:47:47 PM UTC-4, jla...@highlandsniptechnology.com wrote:> On Wed, 13 Apr 2022 13:15:16 -0700 (PDT), "neo5...@gmail.com" > <neo5...@gmail.com> wrote: > > >Hi; > >I've been using N channel Jfets connected as diodes for input protection for a long time. Everything seems to just work. Recently an application came up where the low leakage of a diode connected FET would be useful but when forward biased it could pass tens to a few hundred mA. Does anyone know what characteristic of a JFET would define the max current through the Diode connected device where the Anode is the shorted Drain-Source and the Cathode is the Gate? Would it be the Jfets Id Max? > Jfets make pretty terrible diodes. The PAD1 series of picoamp leakage > diodes are actually jfets inside, just expensive. Jfet diodes have a > lot of equivalent series resistance. > > https://www.mouser.com/ProductDetail/InterFET/PAD1?qs=OxRSArmBDfzNntVAJAN1dw%3D%3D > > Idmax could well blow out the gate. Try it. > > A transistor c-b junction can be a far better diode. Some of the > smaller transistors leak femtoamps but still behave like PN diodes. > > > > > -- > > I yam what I yam - PopeyeI have used the 2n4117 or mmbt4117 trick in the past and learned that they are only "diod-y" up to about 1-2mA forward current. After that they look more like resistors.
Reply by ●March 3, 20232023-03-03
On Thu, 2 Mar 2023 13:35:58 -0800 (PST), "neo5...@gmail.com" <neo5bass@gmail.com> wrote:>On Wednesday, April 13, 2022 at 8:47:47?PM UTC-4, jla...@highlandsniptechnology.com wrote: >> On Wed, 13 Apr 2022 13:15:16 -0700 (PDT), "neo5...@gmail.com" >> <neo5...@gmail.com> wrote: >> >> >Hi; >> >I've been using N channel Jfets connected as diodes for input protection for a long time. Everything seems to just work. Recently an application came up where the low leakage of a diode connected FET would be useful but when forward biased it could pass tens to a few hundred mA. Does anyone know what characteristic of a JFET would define the max current through the Diode connected device where the Anode is the shorted Drain-Source and the Cathode is the Gate? Would it be the Jfets Id Max? >> Jfets make pretty terrible diodes. The PAD1 series of picoamp leakage >> diodes are actually jfets inside, just expensive. Jfet diodes have a >> lot of equivalent series resistance. >> >> https://www.mouser.com/ProductDetail/InterFET/PAD1?qs=OxRSArmBDfzNntVAJAN1dw%3D%3D >> >> Idmax could well blow out the gate. Try it. >> >> A transistor c-b junction can be a far better diode. Some of the >> smaller transistors leak femtoamps but still behave like PN diodes. >> >> >> >> >> -- >> >> I yam what I yam - Popeye > >I have used the 2n4117 or mmbt4117 trick in the past and learned that they are only "diod-y" up to about 1-2mA forward current. After that they look more like resistors.All diodes get ohmic at some current, and often have a zero-TC point around the transition. That can be useful. You get low leakage from small junctions, and small junctions get ohmic at lower currents than big ones. That's one reason that small-signal RF transistors tend to have low leakage. If you connect collector to base and use that config as a diode, you get better, beta-enhanced, forward conduction and not extremely worse leakage, but low breakdown voltage. Connecting b to e is interesting too; that gives reverse-beta gain and more voltage. https://www.dropbox.com/sh/cuoy2p93jkmyxtz/AAANclqWKW4e5xHfRn3LlPUDa?dl=0
