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Old 6th December 2011, 04:51 PM   #11
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There are no precise complementary matches for power Mosfets - just close as feasible for manufacture. The closest match for IRF/P 9140 is IRF/P 240 and not what might look more obvious.

Crossover distortion in Mosfets is not minimal at some particular bias level, as with transistors. Rather. it starts at worst with zero and trends slowly down until full class A bias is applied. However, we don't all want low power burning boxes so we find that for most builders, where adequate NFB is applied, a good compromise begins with around 150mA bias current per pair of these devices.

Check the many threads here discussing designs using these most popular VFET pairs and check the facts in Bob Cordell's book covering your concerns and interest well.
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Last edited by Ian Finch; 6th December 2011 at 04:58 PM.
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Old 6th December 2011, 05:58 PM   #12
CBS240 is offline CBS240  United States
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Originally Posted by LoveVirus View Post
irfp140N Qgs=94nc@Vds=80V
irfp240 Qgs=70nc@Vds=160

irfp240 Qgs=?@Vds=80V

What I mean is the parameter manual to different conditions, may mislead some of our judgments。
Qgs is not very important in this application. The datasheets reflect the intention of these devices as being designed for switching purposes. Here is a related post.
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Old 6th December 2011, 07:31 PM   #13
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Qgs is not very important in this application. The datasheets reflect the intention of these devices as being designed for switching purposes. Here is a related post.
Exactly, they're usually not intended for lineair operation, but can be used as such with (much) lower currents, the most important is their power derating stat to figure out tolerable dissipation with lineair operation. If the higher capacitance isn't an issue, one with a higher transconductance (and those HEXfets have a lot of it) is always better. These conditions makes them excellent as cascode devices since the parasitic capacitance has no effect in common gate configuration and the high transconductance reduces Vgs modulation making the cascode more robust.

I personally never liked the idea of using these as output devices because of the non-lineair behaviour at higher currents that are common to output stages. Laterals are so much better in that regard
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Old 6th December 2011, 10:51 PM   #14
jcx is offline jcx  United States
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completley wrong - the big gate, parasitic C of power MOSFET make them worse performing at "subthreshold" currents than properly sized MOSFETs running at a good fraction of their Idmax, properly chosen BJT Q are often better

this is textbook stuff - please don't "help" others with incorrect info

for better info try Bob Cordell Interview: BJT vs. MOSFET

Last edited by jcx; 6th December 2011 at 10:56 PM.
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Old 7th December 2011, 12:59 AM   #15
ilimzn is online now ilimzn  Croatia
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Originally Posted by Ian Finch View Post
Love Virus
There are no precise complementary matches for power Mosfets - just close as feasible for manufacture. The closest match for IRF/P 9140 is IRF/P 240 and not what might look more obvious.
Also note these are not the N-suffix parts. The 'improved' N suffix parts are improved for switching use, charge storage and transconductance is improved but the die is smaller, resulting in much lower peak power dissipation handling ability, exactly what you do not want in a linear application.
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Old 7th December 2011, 03:05 AM   #16
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Originally Posted by ilimzn View Post
Also note these are not the N-suffix parts. The 'improved' N suffix parts are improved for switching use, charge storage and transconductance is improved but the die is smaller, resulting in much lower peak power dissipation handling ability, exactly what you do not want in a linear application.
Your mean is none N-suffix parts better than N-suffix parts in audio?
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Old 7th December 2011, 06:58 AM   #17
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Originally Posted by jcx View Post
completley wrong - the big gate, parasitic C of power MOSFET make them worse performing at "subthreshold" currents than properly sized MOSFETs running at a good fraction of their Idmax, properly chosen BJT Q are often better

this is textbook stuff - please don't "help" others with incorrect info

for better info try Bob Cordell Interview: BJT vs. MOSFET
Jcx, you gotta stay realistic. With devices that have an Id leakage current of say 50uA, you're not going to run these at 100uA quiescent. The maximum rated continuous current as specified in the datasheet for these devices is practically meaningless if you want to exploit lineair operation as that is the current they can handle when fully saturated. For example, that IRF620 is still fine dealing with several 10s of mA which is still above the sub treshold for them. But, for lineaur small (current) signal operation the Vgs/Ids curve at that small scale is still linear. FYI I've experimented a lot with HEXfets in control loops and have quite a good understanding of how they behave for real.
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Old 7th December 2011, 01:40 PM   #18
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Since the tube is difficult to find the complementary pair is not complementary with better than the quasi-complementary, at least in the state amateur
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Old 7th December 2011, 06:24 PM   #19
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Default Quasi Mos?

I'm not clear on what your comment about tubes is there, LV, but some members build quasi-complementary Mosfet amplifiers for this very reason. It's not as common as approximate complementary pairs but there are some advantages to either topology.

You may need specific advice for the complements you wish to use as the degree of mismatch in the pair will affect the choice of best topology, as will the circuit you plan to use/adapt. Even if they are not best suited to Audio, Power Mosfets have some big advantages: they are cheap, tough and plentiful.

Practically, the choice is yours but you must read the theory behind these choices and learn how to make the best compromise yourself. Try to use the most common parts so that others will be familiar with the issues if you need assistance when it comes to designing or building things on your own. You can't learn much by starting a lot of Q & A threads other than a few random facts, recollections and references to documents, IMHO. The better choice for any newb, young or old, is to follow a construction thread (where the design is decided, posted and the guy(s) are buying parts and doing rather than just talking about more possibilities). Then research the topics as you follow the thread from the beginning with the search button, Wikipaedia or Google etc, if you have no access to a library or useful books on audio electronics.

For me, the best money I ever spent on electronics was on good books - not parts nor PCBs nor cameras to show things off even, though I've used a lot of stuff.
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Old 8th December 2011, 12:06 AM   #20
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Originally Posted by Ian Finch View Post
I'm not clear on what your comment about tubes is there, LV, but some members build quasi-complementary Mosfet amplifiers for this very reason. It's not as common as approximate complementary pairs but there are some advantages to either topology.

You may need specific advice for the complements you wish to use as the degree of mismatch in the pair will affect the choice of best topology, as will the circuit you plan to use/adapt. Even if they are not best suited to Audio, Power Mosfets have some big advantages: they are cheap, tough and plentiful.

Practically, the choice is yours but you must read the theory behind these choices and learn how to make the best compromise yourself. Try to use the most common parts so that others will be familiar with the issues if you need assistance when it comes to designing or building things on your own. You can't learn much by starting a lot of Q & A threads other than a few random facts, recollections and references to documents, IMHO. The better choice for any newb, young or old, is to follow a construction thread (where the design is decided, posted and the guy(s) are buying parts and doing rather than just talking about more possibilities). Then research the topics as you follow the thread from the beginning with the search button, Wikipaedia or Google etc, if you have no access to a library or useful books on audio electronics.

For me, the best money I ever spent on electronics was on good books - not parts nor PCBs nor cameras to show things off even, though I've used a lot of stuff.
Thank you for your suggestion, I think I should buy an oscilloscope, because often only our subjective hearing is not perfect.
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