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JFET vs MOSFET

Started by Unknown July 28, 2016
I am mostly an RF engineer , but our analog guy moved on and I am doing 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 an off state and must be turned on) 

SO I guess I am soliciting comments on when one would use a JFET vs a depletion mode MOSFET. If I am totally screwed up in my thinking I will don an asbestos suit and gladly take the abuse , all for my own betterment.

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

AFAIK the remaining depletion MOSFETs are high voltage things such as the Supertex LD250, whereas JFETs are all small signal devices. Generically JFETs 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. 

Cheers

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

AFAIK the remaining depletion MOSFETs are high voltage things such as the Supertex LD250, whereas JFETs are all small signal devices. Generically JFETs 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. 

Cheers

Phil Hobbs
On Thu, 28 Jul 2016 12:30:04 -0700 (PDT), bulegoge@columbus.rr.com
wrote:

>I am mostly an RF engineer , but our analog guy moved on and I am doing 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 an off state and must be turned on) > >SO I guess I am soliciting comments on when one would use a JFET vs a depletion mode MOSFET. If I am totally screwed up in my thinking I will don an asbestos suit and gladly take the abuse , all for my own betterment. > >Thanks
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. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com

<bulegoge@columbus.rr.com> wrote in message 
news:c601052b-f63e-484d-a3f4-ced0e6ec09d0@googlegroups.com...
> I am mostly an RF engineer , but our analog guy moved on and I am doing 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 an off state and must be turned on) > > SO I guess I am soliciting comments on when one would use a JFET vs a > depletion mode MOSFET. If I am totally screwed up in my thinking I will > don an asbestos suit and gladly take the abuse , all for my own > betterment.
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.
On Thursday, July 28, 2016 at 3:50:44 PM UTC-4, Phil Hobbs wrote:
> JFETs and depletion MOSFETs hardly overlap nowadays. Back in the day you could get dual-gate depletion MOSFETs but they've mostly gone away, as have all or almost all small-signal discrete single-gate ones. > > AFAIK the remaining depletion MOSFETs are high voltage things such as the Supertex LD250, whereas JFETs are all small signal devices. Generically JFETs 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. > > Cheers > > Phil Hobbs
Thanks for the insight
On Thursday, July 28, 2016 at 4:32:30 PM UTC-4, John Larkin wrote:
> On Thu, 28 Jul 2016 12:30:04 -0700 (PDT), bulegoge@columbus.rr.com > wrote: > > >I am mostly an RF engineer , but our analog guy moved on and I am doing 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 an off state and must be turned on) > > > >SO I guess I am soliciting comments on when one would use a JFET vs a depletion mode MOSFET. If I am totally screwed up in my thinking I will don an asbestos suit and gladly take the abuse , all for my own betterment. > > > >Thanks > > 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. > > > > > > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com
Actually it is PIN diode driver that switches between 50 and -12V, and he uses enhancement MOSFETS. But I am trying to get the whole picture about FETs. Thx.
On Thu, 28 Jul 2016 21:35:21 +0100, "Ian Field"
<gangprobing.alien@ntlworld.com> wrote:

> > ><bulegoge@columbus.rr.com> wrote in message >news:c601052b-f63e-484d-a3f4-ced0e6ec09d0@googlegroups.com... >> I am mostly an RF engineer , but our analog guy moved on and I am doing 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 an off state and must be turned on) >> >> SO I guess I am soliciting comments on when one would use a JFET vs a >> depletion mode MOSFET. If I am totally screwed up in my thinking I will >> don an asbestos suit and gladly take the abuse , all for my own >> betterment. > >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.
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. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On 2016-07-29 03:56, John Larkin wrote:
> On Thu, 28 Jul 2016 21:35:21 +0100, "Ian Field" > <gangprobing.alien@ntlworld.com> wrote: > >> >> >> <bulegoge@columbus.rr.com> wrote in message >> news:c601052b-f63e-484d-a3f4-ced0e6ec09d0@googlegroups.com... >>> I am mostly an RF engineer , but our analog guy moved on and I am doing 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 an off state and must be turned on) >>> >>> SO I guess I am soliciting comments on when one would use a JFET vs a >>> depletion mode MOSFET. If I am totally screwed up in my thinking I will >>> don an asbestos suit and gladly take the abuse , all for my own >>> betterment. >> >> 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. > > 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.
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
On 28/07/2016 20:30, bulegoge@columbus.rr.com wrote:
> I am mostly an RF engineer , but our analog guy moved on and I am doing 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 an off state and must be turned on) > > SO I guess I am soliciting comments on when one would use a JFET vs a depletion mode MOSFET. If I am totally screwed up in my thinking I will don an asbestos suit and gladly take the abuse , all for my own betterment. > > Thanks >
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