If you need assistance, please send an email to forum at 4hv dot org. To ensure your email is not marked as spam, please include the phrase "4hv help" in the subject line. You can also find assistance via IRC, at irc.shadowworld.net, room #hvcomm.
Support 4hv.org!
Donate:
4hv.org is hosted on a dedicated server. Unfortunately, this server costs and we rely on the help of site members to keep 4hv.org running. Please consider donating. We will place your name on the thanks list and you'll be helping to keep 4hv.org alive and free for everyone. Members whose names appear in red bold have donated recently. Green bold denotes those who have recently donated to keep the server carbon neutral.
Special Thanks To:
Aaron Holmes
Aaron Wheeler
Adam Horden
Alan Scrimgeour
Andre
Andrew Haynes
Anonymous000
asabase
Austin Weil
barney
Barry
Bert Hickman
Bill Kukowski
Blitzorn
Brandon Paradelas
Bruce Bowling
BubeeMike
Byong Park
Cesiumsponge
Chris F.
Chris Hooper
Corey Worthington
Derek Woodroffe
Dalus
Dan Strother
Daniel Davis
Daniel Uhrenholt
datasheetarchive
Dave Billington
Dave Marshall
David F.
Dennis Rogers
drelectrix
Dr. John Gudenas
Dr. Spark
E.TexasTesla
eastvoltresearch
Eirik Taylor
Erik Dyakov
Erlend^SE
Finn Hammer
Firebug24k
GalliumMan
Gary Peterson
George Slade
GhostNull
Gordon Mcknight
Graham Armitage
Grant
GreySoul
Henry H
IamSmooth
In memory of Leo Powning
Jacob Cash
James Howells
James Pawson
Jeff Greenfield
Jeff Thomas
Jesse Frost
Jim Mitchell
jlr134
Joe Mastroianni
John Forcina
John Oberg
John Willcutt
Jon Newcomb
klugesmith
Leslie Wright
Lutz Hoffman
Mads Barnkob
Martin King
Mats Karlsson
Matt Gibson
Matthew Guidry
mbd
Michael D'Angelo
Mikkel
mileswaldron
mister_rf
Neil Foster
Nick de Smith
Nick Soroka
nicklenorp
Nik
Norman Stanley
Patrick Coleman
Paul Brodie
Paul Jordan
Paul Montgomery
Ped
Peter Krogen
Peter Terren
PhilGood
Richard Feldman
Robert Bush
Royce Bailey
Scott Fusare
Scott Newman
smiffy
Stella
Steven Busic
Steve Conner
Steve Jones
Steve Ward
Sulaiman
Thomas Coyle
Thomas A. Wallace
Thomas W
Timo
Torch
Ulf Jonsson
vasil
Vaxian
vladi mazzilli
wastehl
Weston
William Kim
William N.
William Stehl
Wesley Venis
The aforementioned have contributed financially to the continuing triumph of 4hv.org. They are deserving of my most heartfelt thanks.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
As I increase base current I can gradually increase Collector-Emitter current. Is the same true for a MOSFET, but with voltage; or, is it an all-or-none thing? If I increase voltage there will be a point when the MOSFET allows current to flow like a short, or is there a gradual increase before full conduction is reached?
Registered Member #580
Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410
as gate voltage increases so does the current through Drain-Source, although it is a logarithmic graph, you can find this graph Vgs vs Id on the datasheet for example, here is the transfer characteristics for the BUZ11 MOSFET
Registered Member #1875
Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635
All MOSFETs have a linear phase but most aren't designed to operate while in it. Most are switch-mode type, which, as the name suggests, are meant to be used as ideal switches. There are linear-rated MOSFETs out there, though, which are meant to have their "gain" controlled by gate voltage.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yep, +1.
Here is part of a stereo amp I built years ago using those "linear rated MOSFETs". These particular ones are lateral MOSFETs that come in complementary pairs. They have lousy Rds(on), so they make less power than BJTs would in the same amp, and would be utterly useless in a SMPS. But they're easy to drive, don't need temperature compensation, and have very low distortion.
You can use the ordinary ("vertical") kind of MOSFETs in their linear mode, they are good enough for many applications. I've used them as load banks and regulator pass transistors, and I've seen people make audio amplifiers with the IRFP250/9250 and similar, but it's hard to get them to share current and idle stably. This article by Rod Elliott sums up the situation nicely.
If you get deeper into it, you'll find that MOSFETs for switchmode applications have a failure mode not unlike second breakdown in BJTs. The newer high-performance ones are worse than the older designs. IXYS addressed this by bringing out their "Linear L2" MOSFETs that are fully rated for linear applications and can dissipate scary amounts of power.
Registered Member #142
Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102
IamSmooth wrote ...
As I increase base current I can gradually increase Collector-Emitter current. Is the same true for a MOSFET, but with voltage; or, is it an all-or-none thing? If I increase voltage there will be a point when the MOSFET allows current to flow like a short, or is there a gradual increase before full conduction is reached?
Compare bjt transconductance to mosfet transconductance. Output current is a function of input voltage. A square law describes mosfet behavior above threshhold. Bjt collector current is an exponential function of base voltage. I think you should jet a textbook and start studying this stuff. The questions you're asking would require weeks of study and reading scores of pages in some EE textbook like Jaeger, or Sedra. You're asking an extremely broad question in a very naive way. Where you stand, you don't even know where to start looking, or how much there is to learn.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
kell wrote ...
The questions you're asking would require weeks of study and reading scores of pages in some EE textbook like Jaeger, or Sedra. You're asking an extremely broad question in a very naive way. Where you stand, you don't even know where to start looking, or how much there is to learn.
Do you realize you are responding to a question I posted 2 years ago? I think I have a basic understanding of mosfets at this point, although I still have more to learn.
This site is powered by e107, which is released under the GNU GPL License. All work on this site, except where otherwise noted, is licensed under a Creative Commons Attribution-ShareAlike 2.5 License. By submitting any information to this site, you agree that anything submitted will be so licensed. Please read our Disclaimer and Policies page for information on your rights and responsibilities regarding this site.
MOSFET vs BJT
