Max current for a JFET connected as diode...

N

neo5...@gmail.com

Guest
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?
 
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?
Click to expand...

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
 
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.
Click to expand...

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.
 
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.
 
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.
Click to expand...
 
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.
Click to expand...

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

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
 
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.
 
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?
Click to expand...

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.
 
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.
Click to expand...

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
 
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.
Click to expand...

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
 
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.
Click to expand...

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
 
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.
Click to expand...

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.
Click to expand...

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.
 
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
Click to expand...

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
 
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
Click to expand...

I miss JT\'s harassment. He was good at it.



--

I yam what I yam - Popeye
 
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.
Click to expand...

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
 
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
Click to expand...

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
 
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.
Click to expand...

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
 
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.
Click to expand...

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?
 
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.
Click to expand...

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
 
You must log in or register to reply here.

Welcome to EDABoard.com

Sponsor

Back
Top