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Old 13th December 2002, 01:07 AM   #1
Duo is offline Duo  Canada
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Default New to mosfets, need help!

I finally broke down and decided to learn about mosfets because of their ease of use in class A designs. I don't really know anything about them so could anybody fill me in here?

I need to know how to bias them and that sort of thing. I made a rendition of the Zen amplifier with an IRFZ34 N-ch Enhancement mosfet and it sounds excellent. I want to make some of my own designs so I need to know how to set these puppies up..


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Old 13th December 2002, 02:05 AM   #2
eLarson is offline eLarson  United States
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The search function in this forum or the Pass
Labs forum should turn up some hits.

See also the Nelson Pass article on building the
A-75. It gives some nice, practical information on
biasing MOSFETs and even how to match them.
The article is available from passdiy.com as a PDF.

Erik
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Old 13th December 2002, 02:26 AM   #3
Duo is offline Duo  Canada
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Thanks, I'll go look up Mr. Pass' stuff and see what I find...
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Old 13th December 2002, 02:30 AM   #4
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Hi,

Just search around on IRF's web site. They have some really good info.

...Try This doc (It may be a little on the technical side)
http://www.irf.com/technical-info/appnotes/mosfet.pdf

...or other docs here.
http://www.irf.com/technical-info/papers.html


Basically, a mosfet is a voltage controlled current device (or transconductance device). That means that a voltage from the gate to source will cause a current to flow from the drain to source. Very similar to BJT transistors except that a voltage is needed at the Gate, where in a BJT current is needed at the Base to cause current to flow. The source is basically the same as the emitter, wher the Collector is the same the Drain.

The Z34 is a switching mosfet. You may try to use a mosfet designed for linear applications. I don't know how this will affect sound, but the switching fet may be unstable at times.

-Dan
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Old 13th December 2002, 06:16 AM   #5
Duo is offline Duo  Canada
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Thanks for the info. I see what you mean about how the mosfet works. Actually, this z34 sounds absolutely excellent in the configuration I'm using right now. It's very clean and expresses everything describing the qualities of a class A amlifier. I guess perhaps a linear mosfet may sound even nicer. I'll have to get some...
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Old 13th December 2002, 07:18 AM   #6
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I think switching mosfets can sound really excellent. You can get really big ones too. N-channel for the most part though, not so many really big P-channels. My current amp uses a single switching fet 500v 32A TO-247max per channel and it sounds clean as a whistle. I cannot hear any sonic deficiencies. Nor do I want to!
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Old 13th December 2002, 08:22 AM   #7
Duo is offline Duo  Canada
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Well, a transistor is a transistor theoretically. I've used germanium bipolar switching transistors in an audio amp and they sounded fine too... What topology is your mosfet amp? Is it class A? I hope so! How much power output? How many output devices?
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Old 13th December 2002, 01:07 PM   #8
Jeff R is offline Jeff R  United States
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I think that some of the newer switching MOSFETs should work just fine in a feedback amplifier. Switching MOSFETs are often designed to be used in swithing power supplies, when very low Rdson and gate capacitance is needed. A low capacitance equates to higher bandwidth, too.

MOSFETs can be harder to stabilize (you may need gate resistors and ferrite beads), and the leads should be kept as short as possible - much more so than BJTs. In Audio Amature magazine in the mid 80's, I think, astronaut Norm Thaggard joined with another author, I forgot, to design an all FET amplifier. I bet you can get a reprint of that article - it had a lot of great info about using FETs in an audio application.
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Old 13th December 2002, 06:23 PM   #9
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Ah yes, I remember that Norman Thaggard designed the A75 with Nelson Pass. Someone referred to that discussion they wrote up in this very thread earlier. It's very informative about mosfet audio design.
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Old 13th December 2002, 06:55 PM   #10
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Quote:
Originally posted by Circlotron
I think switching mosfets can sound really excellent. You can get really big ones too. N-channel for the most part though, not so many really big P-channels.
That's not to say that switching mosfets won't work in a linear amplifier, it's just that they may be harder to use. I'm convinced that for the mostpart any mosfet will work in audio applications. They do make some really big RF-Amp fets. I've often wondered about these being used as audio amplifiers, however I fear they would tend to oscillate.

Myself, I tend to use the IRFP240/IRFP9240 fets. They have good current and voltage capability, along with a nice power rating (125W ea, at Tc = 25C or so)

Jeff R is right about them being harder to stabilize. Becuse they require only voltage to change current, any noise coupled to the gate will be amplified greatly. In the case of a IRFZ34 mosfet, that change in current will be almost double that of an IRFP240 because the 240 has a lower I/V characteristic. (Both devices gate to source voltage around 5 volts). However, if the Z34 works, use it, it should have yield a better frequency response.

-Dan
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