On 2023-03-03 10:36, John Larkin wrote:> 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 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=0Yup. (I have a couple of reels of BFT25As.) I haven't tried using the reverse beta for anything--interesting idea. One time I was working on a current-feedback loop working in the low nanoamps, with a few BC-connected BFT25As to provide emitter degeneration for the main device. I found that the bandwidth tanked below a few nanoamps, because the BC-connected transistor is actually the world's simplest feedback amplifier, and (despite being a 5-GHz transistor) it was running out of f_T. Using the BE diodes by themselves fixed it. 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 ●March 3, 20232023-03-03
Reply by ●March 3, 20232023-03-03
On Fri, 3 Mar 2023 14:24:26 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 2023-03-03 10:36, John Larkin wrote: >> 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 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 > >Yup. (I have a couple of reels of BFT25As.) I haven't tried using the >reverse beta for anything--interesting idea. >I have a couple of hundred too. There is probably a decent replacement somewhere. Reverse beta is about 4.>One time I was working on a current-feedback loop working in the low >nanoamps, with a few BC-connected BFT25As to provide emitter >degeneration for the main device. > >I found that the bandwidth tanked below a few nanoamps, because the >BC-connected transistor is actually the world's simplest feedback >amplifier, and (despite being a 5-GHz transistor) it was running out of f_T. > >Using the BE diodes by themselves fixed it.Phemts and GaN parts make interesting diodes too. Jfets are terrible diodes, and the PAD parts are jfets.
