The dual-gate MOSFETS are going away

[Radio Wrangler]

Arthur Clarke wrote "Silently, and without any fuss the stars were going out"

I've just been trawllng through all the manufacturers websites and there seems to recently have been a spate of discontinuance notices for these parts. I was looking for an alternative to the BF904 I've used in quantities, but all the low voltage ones have gone. I spotted one infineon part but it was merely NRND.

Some of the Japanese 12v types are the only ones lingering. but for how long?

The mass market has moved on to other things and the parts it used to require aren't needed in economic quantities anymore.

David

[turretslug]

A shame, and another sign of the times- at one time it seemed that every time you opened a hobbyist magazine, there was another project involving 40673s or BF960s. I daresay that there must be millions of TV tunerheads, sat-boxes and elderly mobile phones lying around with them in but, unlike 6AK5s or 6CW4s, they're less easy to harvest "on the fly".

[kellys_eye]

Are they not still available as SMD devices?

[turretslug]

BF904 is (or was!) a surface mount type. I hate to say this, but perhaps the give-away hi-fi tuners currently cluttering the world will end up being robbed of their FM front-end MOSFETS, with heated and entrenched camps over whether Hitachi or Toshiba devices offer better frequency range and which date-codes offer lowest noise....

[MrBungle]

Not quite there in the transition frequency department, but I've stockpiled a number of 40673's after a nice surprise in a box of knackered looking components from ebay. Got a life time supply of them and 2n4416's.

You can still get the 40673's from Spectrum comms for a not too respectable £2.50 each new though. I suspect this stock may not last.

[Karen O]

Is it possible to approximate a dual gate mosfet by using two ordinary fets in cascode arrangement?

[Al (astral highway)]

Such a shame! Wonderful inventions, I've used in an UHF mixer and a current project features one in a star role!

[emeritus]

I remember using types 3N140 and 3N141 in the early 1970s for electronically switching different bandwidth IF filters. I must still have some used examples in my odds and ends box. I remember the warnings about possible damage from static electricity, but we never seemed to suffer any problems by not strapping all the leads together in storage. Possibly because the lab was neither carpeted nor heated enough for static to be a problem.

[Philips210]

Originally Posted by Karen O

Is it possible to approximate a dual gate mosfet by using two ordinary fets in cascode arrangement?

Hi.

Yes, I was thinking along the same lines. I believe that a dual gate MOSFET can be simulated by using a JFET and a bipolar transistor in a cascode arrangement.

Regards
Symon.

[Radio Wrangler]

It is possible, but single gate small RF MOSFETs went away some time ago, killed of by the success of the dual gate version.

We use them at work. New full spec devices from trustworthy sources in large quantities. Except were having to make a final large buy. This won't last forever and we'd like to continue on with the products which use them. The BF904 is SMT, it's designed to run from 5v and it has an interesting little on-chip current mirror to simplify and stabilise biasing.

NXP has dropped it. looking deeper all their dual gate MOSFETs have gone at once. Spreading the net wider, Infineon (nee Siemens) have dropped all dual gate MOSFETs too except one I found was marked NRND.

Looking around there are some of the high voltage types from Japan soldiering on, but not for long I expect before they too join the stampede.

Pity. 25dB of gain with easy AGC control, high Z to interface directly with tuned circuits, simple bias and just pennies each.... but gorn away.

David

[MrBungle]

Indeed. It makes sense from a commercial perspective though if you're selling silicon. It's cheaper doing digital processing so everyone is focused on getting signals into that domain right away. That means you hit an IC up front and there's no room for dual gate FETs there unless they're on the die.

Incidentally this is what lead to me doing software. I saw the end coming of all the fun stuff. Last minute specialisation on VLSI design and out the door designing financial software. Fooey wish I'd been born 30 years earlier.

[Synchrodyne]

It’s a bit scary to realize that a technology that one learned about when it was new and state-of-the-art (and which one still thinks of that way) has passed through obsolescence to being obsolete and is now on the way to being unobtainium. 1969 I think was the year when quite a bit of new equipment featured dual-gate mosfets, although there had been some releases in 1968 (The Rogers Ravensbourne 2 FET FM tuner, released very early in 1968, comes to mind in this context.) So that is not far off 50 years ago. Heavens to Murgatroyd! The protected-gate versions I think were commercially available for 1969 equipment production, so prior use of the unprotected type may have been only by the brave, as it were.

Dual-gate mosfet front ends quickly became de rigueur in FM hi-fi tuner front ends, and were also used in consumer-level and some intermediate-level professional HF receivers. From about 1966 some FM tuner makers had used jfet cascodes as RF amplifiers. These gradually gave way to dual-gate mosfets, but sometimes it took a few years, suggesting that the performance gain in this case with mosfets over jfets was only incremental, and did not justify an immediate change. Thus you had the situation in the early 1970s where makers such as Heathkit and McIntosh were using dual-gate mosfets in the (unusually high quality) AM front ends of some of their FM-AM equipment, but jfets in the FM front ends.

Cascode jfets were also found in some HF equipment, such as the RF amplifiers of the GEC RC410-R and Marconi Hydrus. The latter also used a cascode jfet 1st mixer. The Eddystone EC964 used an interesting mix of dual-gate mosfet and cascode jfet stages (including a triple for the 2nd mixer).

So if reversion is indicated, there are certainly prior examples of cascode jfet use in FM and HF equipment in situations where dual-gate mosfets were subsequently “normal”. In the VHF TV tuner case, cascode jfet RF amplifiers were proposed in the late 1960s, but not taken up as they had insufficient agc range, whereas dual-gate mosfets could at least match the VHF TV RF valves, cascodes, guided-grid triodes, etc., in all key parameters, and so were used from about 1969.

The paradox is though that a while back I saw somewhere that dual-gate mosfets were sometimes used as a repair substitutes for cascode TIS88 jfets in some FM front ends – B&O equipment being involved I think.

Hybrid jfet/bipolar cascodes appear to have been used from about 1970; there is some mention in this thread: http://www.vintage-radio.net/forum/s...718#post622718. There was also some early use of mixed-fet cascodes, such as jfet/single-gate mosfet and the reverse. As RW has already said, single-gate mosfets faded out quite early on. RCA applied its protected-gate technology only to the dual-gate type, suggesting that where only one gate was required, they could be tied together, with the extant (unprotected) single-gate devices reserved for applications where extremely high input impedances were required, higher than allowed by devices with protection diodes.

One could say that dual-gate mosfets were the legitimate solid-state small-signal successors to the double-triode cascode RF and triode-pentode VHF frequency changer valves, both of which, in dedicated form, date back to 1951, and were used in new equipment until the early 1970s. So I think that the mosfets had a longer period of use in new equipment. To some extent dual-gate mosfets also succeeded dedicated VHF pentodes (in all cutoff forms), although ICs probably did more of the work there. The first VHF pentode that became a mass-production consumer equipment item, right after WWII, was the sharp cutoff 6AG5, although remote cutoff and semi-remote cutoff VHF pentodes came later. So I suppose that the dual-gate mosfet had a decent run when measured against its direct predecessors of the valve era. And it had versatility – it could be viewed, to a first approximation anyway, as being analogous to a cascode double triode, to a tetrode or pentode, and to a dual-control tetrode or pentode. And if not exactly analogous to, it was used in place of heptodes.


Cheers,

[Karen O]

I wonder if it will ever be possible to make our own?

I thought that nixie tubes would go extinct but a talented chap in the Czech Republic (I think) is making them once again.

Maybe a home silicon fab is not too far away?

[Karen O]

Another thought:

I opened one of those aerial boosters and found two SMD transistors. I looked up the markings and concluded that they were dual gate mosfets.

Maybe worth picking some up at car boot sales?

[Keith G8IXN]

Originally Posted by kellys_eye

are they not still available as smd devices?

bf998

[Radio Wrangler]

BF998 has also gone. There will be stocks around the place still, of course. But once those are used up, that's it. OK for fixing things for a while. No good for continued manufacture, I'm afraid.

David

[Al (astral highway)]

Wonder what Heathkit would use now? Weren't they enamoured with dual-gate FETS for RF work?

[MrBungle]

They were pragmatic using the most cost effective tool for the job. They'd use a full subsystem IC now.

As for Heathkit being enamoured with something, I don't think they were. That was usually because they had bought stupid large quantities of parts and reused as much existing stock as possible when designing new products.

Incidentally this is what I do as well

[Al (astral highway)]

Originally Posted by MrBungle

As for Heathkit being enamoured with something, I don't think they were. That was usually because they had bought stupid large quantities of parts and reused as much existing stock as possible when designing new products.

Interesting to hear that Heathkit were so pragmatic...Ahaha! I wish I did that! I seem to acquire loads of components that I intended to use for a project but remain unused!

[MrBungle]

I think we all suffer from that affliction to some degree, especially as about an hour ago I sat staring at my large crate of unsorted resistors and thought "I can't be bothered to find a 10 ohm one" and just ordered some from RS of which 9 will immediately go in that box