Depletion-Mode FETs

How about a few J105 like surface mounted JFETs in parallel?

Fairchild JFTJ105 ...

Going to have to use JFETs. The first thing I learned upon graduating college is that MOSFETs exist neither in depletion mode nor in 4 terminal varieties. Much like unicorns, these are entirely fictional entities that can be found in theory but not catalog. And are really just used by the sorts of people who write engineering textbooks so that they can laugh as they watch your dreams of how to build your circuit shatter on the rocky shores of availability. *glares over at Win*

OK, glare at me if you will. I'm sure you know MOSFETs in ICs make extensive use of the effect of substrate/body potentials. We say little about this, but enough to rightly-so draw your interest.

Our MOSFET introduction (AoE pages 117-118) explicitly highlights this 4th element or connection. In our defense, we must mention it, because of the important role it plays for example in IC mosfet switch designs (AoE pg 145). I included Motorola's 2n4351 as an example of a MOSFET with 4th connection (AoE pg 224), which may need further examination.

It's true that few discrete MOSFETs make the body connection available, but some important ones do, like the popular SD210 series. And there are a few other examples.

In this discussion, we acknowledge that the popular DMOS / VMOS power mosfets don't have a 4th body connection, and furthermore they have a "parasitic" substrate diode connected to the drain. But, hey, they're another (dominant) beast entirely! One would *never* use a huge part like that in a low-current charge and leakage-sensitive application.

Or one of those slick PV optocouplers with a zener in parallel. Some of them output like 9 volts or something open-circuit.

John

How about a 3-V enhancement FET with a lithium battery in series with the gate?

Cheers,

Phil Hobbs

BS! I know several processes for which depletion mode FETs exist... like Fairchild, for example... I've used them as start-up mirrors in several RFID tags.

...Jim Thompson

Discretes available off the shelf from Supertex. They are often used in process current transmitters.

Best regards, Spehro Pefhany

To that I say:

OK, and come to think of it so have I. Just illustrates why one should never say never! :>) When using a power mosfet for analog switching, one usually struggles with finding a small enough FET, to keep down the capacitances. I have found that small say 1A high-voltage parts, which used to be manufactured, have been discontinued, so that the smallest part in a series is no longer available. As this phenomenon was playing itself out a few years ago I made lifetime buys of some of the small FETs in question, to have them for high-speed switches and linear amplifiers.

In article , Phil Hobbs wrote: [....]

A normally closed relay?

In article , Winfield Hill wrote: [...]

I have. If you need an analog switch with nearly zero on resistance, the ability to withstand a couple of 100V and moderately fast switching, a selected power MOSFET can be just the ticket.

Aromat/Nais offers a normally-on SSR aqv414 - 400V withstand, with 22R impedance.

As with most isolating or PVI optocouplers, body size of 'smd' version is large.

RL

It's a shame that all the SSRs (or all the ones I've tried) have a schmitt-like driver in the opto path. If the control were linear, all sorts of interesting things would be possible.

John

Hmm, and they're even distributed and in stock. Looks like I learn something new every day. Having now met this day's quota I can go back to bed.

-- Rob

Well, how about fragile, unreliable, noisy, low transconductance, high power dissipation, high on resistance, needs filament power, not available in surface mount.

John

alright, a person after my heart!

Here's one:

There are others in Russia, the Czech republic, Slovakia, Ukraine and China, but it's obviously a niche market.

Best regards, Spehro Pefhany

Well it looks like the Clare (CPC3703C), SOT-89, 4.0 ohm, DMOS FET driven by a Toshiba (TLP190B) photovoltaic coupler form a nice 0.25A normally closed, floating relay in SMD packages. Add a 3.3 meg resistor from gate to source to insure normally closed operation. Supertex has a line of DMOS and IRF have photovoltaic couplers that could be used. Thanks for all the input and the info regarding the J105 J-FET. . Cheers, Harry