Electronics > Beginners
JFET mysteries
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David Hess:

--- Quote from: T3sl4co1l on December 31, 2018, 12:29:40 pm ---
--- Quote from: David Hess on December 31, 2018, 11:28:59 am ---The 2N7000 is 60pF input, 25pF output, and 5pF reverse maximum capacitance which is actually very high for a small signal application.  A 3N part or JFET will have 1/5th to 1/10th that much capacitance and can be 10 times faster.

For switching LEDs this does not matter but even the smallest vertical power MOSFET is useless for something like a sample and hold application.
--- End quote ---

Small, usually-intended-for-RF MOSFETs are both damned hard to find, and also hard to use with respect to ESD -- even if you're quite good about ESD normally!

They're also in a bit of a death-valley market segment: too small to handle any real power (like a jellybean 2N7000), too large and too slow to do any serious RF (where PHEMTs, GaNFETs and such dominate).  Everything else is ASICs, even in modest quantities (100k's?) so there's no one buying discretes in quantity.
--- End quote ---

Pricing and availability are definitely a problem but I linked two manufaturers to show that they (not the dual gate parts) are still available.  They are useful for choppers and electrometer applications where a JFET is not quite good enough.

I never really noticed the connection between the RF and chopper/electrometer 4-pin MOSFETs until going through my old Signetics book where they specified them both ways.

PNP RF transistors are beyond the valley of death.


--- Quote ---The smallest I know of is the RUM001L02 and brethren.  It's about 1/4th the size of a 2N7000, electrically speaking (for that matter, mechanically too -- it's a tiny bugger, hope your soldering iron tip is sharp).  This comes at the price of Vds(max) and Pd.  It's definitely a newer process, with Rds(on) being quite comparable, while Qg being so much smaller.

Any smaller (electrically speaking), and you should be looking at arrays (e.g., CD4007?), analog switches, oh or those "precision zero offset" MOSFETs by ALD ($$$), or doing it in an IC (which may involve a boring ADC-DSP-DAC scheme, that still wins on power consumption -- it's dumb, it works, get over it?..).
--- End quote ---

It depends on the application; not all are suited to an integrated part.  I usually see these part in old precision mixed-signal designs like electrometers and high resolution multimeters.  They show up sometimes in high performance active mixers in place of JFETs.


--- Quote from: Kleinstein on December 31, 2018, 11:55:46 am ---Those 4 PIN MOSFETs with separate substrate are difficult to get now. But the same element is commonly used in switch chips.  For some experiment is might be even worth getting the old CD/HEF 4007 chip (old CMOS Logic) - it includes such FETs with no too many connections.
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This becomes more apparent looking through the Signetics datasheets.  Not only are they the same element, they are exactly the same part which gives a rare look into the performance of individual transistors found in old CMOS processes.


--- Quote ---There are also depletion mode MOSFETs, that in many respects behave similar to a JFETs (turn on at 0 gate voltage, and turn off with a negative gate voltage). However they still have the reverse diode.
--- End quote ---

There were also depletion mode 4-pin MOSFETs but I do not know the part numbers or if anybody makes them now.


--- Quote from: Kleinstein on December 31, 2018, 03:42:21 pm ---For RF use there are dual gate MOSFETs (e.g. BF 998).  In principle they should also work for logic switching if needed. One does not really need to actively use the 2 nd gate - it's quite common to have it at a fixed potential and than behave quite normal, like a small low voltage MOSFET with low input capacitance.

A slight problem is that they are so fast that they tend to oscillate quite easy at frequencies the entry level scope may no even show.
--- End quote ---

Dual gate MOSFETs are not a substitute for 4-pin source-gate-drain-substrate MOSFETs.  I have never seen them used in a chopping application but I am sure someone tried it.
T3sl4co1l:

--- Quote from: David Hess on December 31, 2018, 03:54:05 pm ---Pricing and availability are definitely a problem but I linked two manufaturers to show that they (not the dual gate parts) are still available.  They are useful for choppers and electrometer applications where a JFET is not quite good enough.
--- End quote ---

Yeah, I shouldn't say unavailable; and yeah, definitely not mainstream (they're not carried by Digikey; though Calogic is carried by Future which isn't too bad :) ).



--- Quote ---PNP RF transistors are beyond the valley of death.
--- End quote ---

Get your BFT92s now. Last call! :-BROKE

Tim
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