JFET vs MOSFET

JFETs and depletion MOSFETs hardly overlap nowadays. Back in the day you co uld get dual-gate depletion MOSFETs but they've mostly gone away, as have a ll or almost all small-signal discrete single-gate ones.

AFAIK the remaining depletion MOSFETs are high voltage things such as the S upertex LD250, whereas JFETs are all small signal devices. Generically JFET s are much quieter, especially at low frequency, but a bit harder to use--y ou have to worry about forward conduction in the gate, and their leakage is a much stronger function of temperature and V_DS.

Cheers

Phil Hobbs

JFETs and depletion MOSFETs hardly overlap nowadays. Back in the day you co uld get dual-gate depletion MOSFETs but they've mostly gone away, as have a ll or almost all small-signal discrete single-gate ones.

AFAIK the remaining depletion MOSFETs are high voltage things such as the S upertex LD250, whereas JFETs are all small signal devices. Generically JFET s are much quieter, especially at low frequency, but a bit harder to use--y ou have to worry about forward conduction in the gate, and their leakage is a much stronger function of temperature and V_DS.

Cheers

Phil Hobbs

What fet did his circuit use, and what does it do? More info would help.

One problem with jfets is that specifified Idss can range over a 3:1 and even 10:1 current range. The depletion mosfets tend to be a bit better.

The depletion mosfets are generally higher voltage, slower, noisier things than jfets. They don't make good RF amps.

Some folks here use depletion mosfets as current limiters; works great.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

A few years back, one of the electronics magazines (probably Elektor) published a high voltage power supply - there were PCB options to use either a valve (tube) or a depletion mode power MOSFET. AFAIK: there isn't much in the way of power JFETS, but I've heard mention of very recent silicon carbide parts.

Usually; depletion MOSFETs are low power types that you'd find in a TV tuner, and more often than not they're dual gate.

A while back I found some high frequency Hitachi TO126 enhancement MOSFETs in the casc-ode video outputs of a 19" monitor. So far; I haven't found a complete datasheet for the type, but a summary suggests they're good for about 1GHz.

could get dual-gate depletion MOSFETs but they've mostly gone away, as have all or almost all small-signal discrete single-gate ones.

Supertex LD250, whereas JFETs are all small signal devices. Generically JF ETs are much quieter, especially at low frequency, but a bit harder to use-

-you have to worry about forward conduction in the gate, and their leakage is a much stronger function of temperature and V_DS.

Thanks for the insight

a little bit of sustaining of his stuff. In looking at one of his circuits I have gone back to the basics on JFET and MOSFETS. I am seeing that the basic electrical characteristics of a depletion MOSFET is similar to a JFET (I see the difference in enhancement MOSFET - ie - they are generally in a n off state and must be turned on)

pletion mode MOSFET. If I am totally screwed up in my thinking I will don a n asbestos suit and gladly take the abuse , all for my own betterment.

Actually it is PIN diode driver that switches between 50 and -12V, and he u ses enhancement MOSFETS. But I am trying to get the whole picture about FE Ts.

Thx.

Supertex makes some great depletion mosfets; high voltage, pretty predictable Idss, SOT23 and SOT89.

Since Microchip acquired Supertex, most of the depletion fets are well hidden, as if Microchip is ashamed of them.

IXYS makes some gigantic depletion fets, like 1000 volts and 700 watts.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Funny, I got that same impression trying to find something on the InterFET web site. You'd almost think they don't *want* you to find the right device. Far too many categories without obvious distinctions, each with only a few devices. No parametric search and no sortable device lists. Very tedious to find anything, unless you already have the type number. I imagine that must hurt sales in the end.

Jeroen Belleman

The light bulb moment for me many years ago was the realization that enhancement and depletion mode fets work the same just that the gate threshold has been shifted to one side of the origin. It is the zero gate voltage behavior that defines the difference. In other words an n-channel enhancement device could be "further depleted" by driving the gate negative just as an n-channel depletion mode device could be "further enhanced" by driving the gate positive.

The jfet symbol gate arrow helpfully is a visual reminder of the gate-channel diode direction. And as with the insulated gate devices a jfet could be "further" enhanced by driving the gate into diode foward bias.

piglet

There are even some MOSFETs that are right in the middle..

Vgs(th) = 0V +/- 10mV @ 1uA !

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If you go products/power/mosfets, you get a list of eight parts. If you want a Microchip part, use the Digikey search engine.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Does negative bias on the gate of an n-ch enhancement fet reduce gate current below Idss? I've been meaning to try that.

2N7002 already leaks picoamps drain current at zero gate bias. Maybe it can leak even less?

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

I guess the technology is used in normally-on solid state relays.

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John Devereux

Not depletion triacs? ;)

Cheers

Phil Hobbs

Yep. It's all about the doping polarity in the channel.

...Jim Thompson

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             I'm looking for work... see my website.

Here's another entry in my Goofy Circuits Collection:

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Mesfets and phemts, nominally depletion devices, enhance a lot, up to the point where the gate begins to conduct.

Phemts typically turn on hard, twice Idss or so, and get really serious when you poke in a bit of gate current at roughly +0.75 volts. The gates are typically fragile, and won't stand a lot of current.

I conjecture a bipolar sort of effect in phemts, super drain conduction when the gate begins to draw current.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

The easiest way I found to locate them was to do a search on the word "depletion" on the Microchip web site.

For grins, download the data sheet for the DN2470. The data sheet all reads correctly on the screen at first glance, but Microchip seems to have glitched the PDF-generating process. The title and subject information embedded in the PDF claim that it's an MCP3421 18-bit ADC with I2C interface and onboard reference.

So, maybe you're right - they're so ashamed of these little analog parts that they're hiding them inside boxes with misleading labels!

Sure. Then you get a mess of search results, which have to be investigated one at a time.

If there is a tabular listing of parts and specs, I couldn't find it.

Their web site is a disaster. They make some nice parts, and could sell a lot of them if anybody knew that they existed.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com