• Max current for a JFET connected as diode

    From neo5bass@gmail.com@21:1/5 to All on Wed Apr 13 13:15:16 2022
    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?

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  • From jlarkin@highlandsniptechnology.com@21:1/5 to neo5bass@gmail.com on Wed Apr 13 17:47:35 2022
    On Wed, 13 Apr 2022 13:15:16 -0700 (PDT), "neo5...@gmail.com" <neo5bass@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

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  • From Rich S@21:1/5 to jla...@highlandsniptechnology.com on Wed Apr 13 17:57:35 2022
    On Thursday, April 14, 2022 at 12:47:47 AM UTC, 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.


    Hello Neo
    It depends on the manufacturer.

    1986 Siliconix FET Databook, pg. 1-3,
    the term is "Gate Forward Current" with symbol
    I_G(f)
    where the "_" means the G(f) part is subscripted.

    On Semiconductor = "Gate Current",
    NXP = "forward gate current"; both use
    I_G (without the 'f')

    Fairchild = "Forward Gate Current": I_GF

    The specs seem to range from 10mA to 50mA (J111 family)

    cheers, Rich S.

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  • From whit3rd@21:1/5 to neo5...@gmail.com on Wed Apr 13 18:23:31 2022
    On Wednesday, April 13, 2022 at 1:15:20 PM UTC-7, neo5...@gmail.com wrote:

    I've been using N channel Jfets connected as diodes for input protection ....it could pass tens to a few hundred mA. Does anyone know what characteristic of a JFET would define the max current

    It's likely the package. Whatever the max current is for the FET, is likely the fusing limit for the bond
    wire connection, and will dominate a gate-current scenario. The solder-bump packages should
    have highest current capability, IMHO.

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  • From neo5bass@gmail.com@21:1/5 to All on Thu Apr 14 05:44:39 2022
    So which bjt's have such low leakage?
    Since Im thinking of it how much current can the collector base junction take without failing?
    - D

    A transistor c-b junction can be a far better diode. Some of the
    smaller transistors leak femtoamps but still behave like PN diodes.

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  • From Rich S@21:1/5 to neo5...@gmail.com on Thu Apr 14 07:23:47 2022
    On Thursday, April 14, 2022 at 12:44:43 PM UTC, 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?
    - D

    A transistor c-b junction can be a far better diode. Some of the
    smaller transistors leak femtoamps but still behave like PN diodes.

    in AoE3, chapter 5, H&H like the PN4117 JFET.
    John's advice has been immortalized , p.294.
    footnote 4. "BFT25".

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  • From jlarkin@highlandsniptechnology.com@21:1/5 to richsulinengineer@gmail.com on Thu Apr 14 07:43:31 2022
    On Thu, 14 Apr 2022 07:23:47 -0700 (PDT), Rich S
    <richsulinengineer@gmail.com> wrote:

    On Thursday, April 14, 2022 at 12:44:43 PM UTC, 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?
    - D

    A transistor c-b junction can be a far better diode. Some of the
    smaller transistors leak femtoamps but still behave like PN diodes.

    in AoE3, chapter 5, H&H like the PN4117 JFET.
    John's advice has been immortalized , p.294.
    footnote 4. "BFT25".

    Even dumb bipolar gumdrops can have pA leakage. PNPs seem to be better
    for some reason. I recall that 2N3906 is pretty good c-b. But it could
    vary by manufacturer.

    I missed that AoE reference. Cool. I'm on p 360 too!





    --

    I yam what I yam - Popeye

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  • From Dave Platt@21:1/5 to neo5...@gmail.com on Thu Apr 14 09:04:08 2022
    In article <3c1fd9b6-b713-4923-977c-dde93f56a582n@googlegroups.com>, neo5...@gmail.com <neo5bass@gmail.com> wrote:

    So which bjt's have such low leakage?

    In "Troubleshooting Analog Circuits", Bob Pease recommended the 2N390
    and 2N3707 for this application. Gold-doped diodes and transistors
    are apparently leakier.

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  • From whit3rd@21:1/5 to neo5...@gmail.com on Thu Apr 14 12:20:31 2022
    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.

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  • From Phil Hobbs@21:1/5 to All on Thu Apr 14 16:25:02 2022
    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

    --
    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

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  • From John Larkin@21:1/5 to All on Thu Apr 14 13:20:10 2022
    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.





    --

    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

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  • From Phil Hobbs@21:1/5 to Phil Hobbs on Thu Apr 14 16:32:50 2022
    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
    --
    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

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  • From whit3rd@21:1/5 to John Larkin on Thu Apr 14 13:50:32 2022
    On Thursday, April 14, 2022 at 1:20:21 PM UTC-7, John Larkin wrote:
    On Thu, 14 Apr 2022 12:20:31 -0700 (PDT), whit3rd <whi...@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.

    True, but there's the 'how much current' question to consider; small geometry isn't
    always a good tradeoff, if it's a protection diode application.

    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.

    The tradeoff with RF transistors includes low Rbb, so keeping base
    current down might not be an issue. Low-noise audio requires high current gain forward, which implies low base doping and high Rbb is acceptable. Those wouldn't be a great choice for protection.

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  • From jlarkin@highlandsniptechnology.com@21:1/5 to pcdhSpamMeSenseless@electrooptical. on Thu Apr 14 14:35:51 2022
    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.



    --

    I yam what I yam - Popeye

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  • From jlarkin@highlandsniptechnology.com@21:1/5 to pcdhSpamMeSenseless@electrooptical. on Thu Apr 14 14:38:40 2022
    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.



    --

    I yam what I yam - Popeye

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  • From Rich S@21:1/5 to All on Thu Apr 14 17:41:49 2022
    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

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  • From Phil Hobbs@21:1/5 to Rich S on Fri Apr 15 09:17:47 2022
    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.

    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

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  • From Phil Hobbs@21:1/5 to jlarkin@highlandsniptechnology.com on Fri Apr 15 09:15:59 2022
    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 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

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  • From Phil Hobbs@21:1/5 to jlarkin@highlandsniptechnology.com on Fri Apr 15 09:19:13 2022
    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.

    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

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  • From neo5bass@gmail.com@21:1/5 to All on Fri Apr 15 06:56:03 2022
    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?

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  • From Phil Hobbs@21:1/5 to John Larkin on Fri Apr 15 10:05:32 2022
    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

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  • From neo5bass@gmail.com@21:1/5 to jla...@highlandsniptechnology.com on Fri Apr 15 07:11:17 2022
    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 - Popeye


    Thanks for including that thread here. I very much appreciate everyone's useful comments on this topic.

    Don

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  • From jlarkin@highlandsniptechnology.com@21:1/5 to pcdhSpamMeSenseless@electrooptical. on Fri Apr 15 07:41:29 2022
    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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From jlarkin@highlandsniptechnology.com@21:1/5 to pcdhSpamMeSenseless@electrooptical. on Fri Apr 15 07:42:27 2022
    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 Hobbs

    Quantity, not quality.



    --

    I yam what I yam - Popeye

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From jlarkin@highlandsniptechnology.com@21:1/5 to neo5bass@gmail.com on Fri Apr 15 07:48:04 2022
    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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From jlarkin@highlandsniptechnology.com@21:1/5 to pcdhSpamMeSenseless@electrooptical. on Fri Apr 15 07:34:02 2022
    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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to jlarkin@highlandsniptechnology.com on Fri Apr 15 10:58:34 2022
    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?


    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 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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to jlarkin@highlandsniptechnology.com on Fri Apr 15 10:52:49 2022
    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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From jlarkin@highlandsniptechnology.com@21:1/5 to pcdhSpamMeSenseless@electrooptical. on Fri Apr 15 08:40:05 2022
    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 Hobbs

    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.



    --

    I yam what I yam - Popeye

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Jan Panteltje@21:1/5 to pcdhSpamMeSenseless@electrooptical. on Fri Apr 15 15:40:03 2022
    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.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to Jan Panteltje on Fri Apr 15 12:34:03 2022
    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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to jlarkin@highlandsniptechnology.com on Fri Apr 15 12:38:07 2022
    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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From John Larkin@21:1/5 to pcdhSpamMeSenseless@electrooptical. on Fri Apr 15 10:08:14 2022
    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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From John Larkin@21:1/5 to pcdhSpamMeSenseless@electrooptical. on Fri Apr 15 10:12:39 2022
    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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to Jan Panteltje on Fri Apr 15 14:06:02 2022
    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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Phil Hobbs@21:1/5 to John Larkin on Fri Apr 15 14:42:25 2022
    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

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  • From whit3rd@21:1/5 to neo5...@gmail.com on Fri Apr 15 13:03:57 2022
    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?).

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  • From Phil Hobbs@21:1/5 to All on Fri Apr 15 16:26:13 2022
    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

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