Max current for a JFET connected as diode...

[John Larkin]

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.