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

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

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

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

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.

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

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

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

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

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

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

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?).

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

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.

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

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.

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

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.