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Old 2nd December 2004, 04:30 AM   #1
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Angry MOSFET gate voltage

I am designing an amp that will run off of +/-75v rails. I want to use MOSFETs for the outputs, but I don't know much about them. How much voltage do I need at the gates to get the FETs to swing from rail to rail (or close to the rails)?
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Old 2nd December 2004, 04:41 AM   #2
Mr Evil is offline Mr Evil  United Kingdom
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Depends on the configuration and the devices. If they are in common-source configuration then it's easy to get them to swing to the rails with Vgs of maybe 5-10V or so, limited only by the on resistance. If they're configured as source-follower then you will probably need to take the gate voltage a couple of volts above the rails to get maximum swing, also limited by on resistance.
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Old 2nd December 2004, 05:09 AM   #3
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What's the difference between common-source and source-follower?
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Old 2nd December 2004, 10:04 AM   #4
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Common source = CFP (complementary feedback pair / Sziklai pair), n-ch device in lower half and p-ch device in upper half.

Source follower, n-ch device in upper half and p-ch device in lower half.
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Old 2nd December 2004, 08:04 PM   #5
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Attached is the amp I am working on. I need some help chosing output FETs. Which ones would be good? Would it be better to use N-channel on both rails? If so, how would I go about modifying this circuit to use all N-ch output FETs?
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Old 3rd December 2004, 04:21 AM   #6
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That's a really small amp...microscopic. in fact.
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Old 3rd December 2004, 04:34 AM   #7
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Thanks for pointing that out. I haven't realized that I didn't attach it.
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File Type: jpg national.jpg (72.0 KB, 206 views)
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Old 3rd December 2004, 08:25 AM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
I don't follow the workings of this schematic. It seems unconventional to me.
Comments anyone??
Try looking up previous threads here for suitable versions, there are a couple running just now in solid state.
regards Andrew T.
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Old 3rd December 2004, 09:24 AM   #9
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I've seen this kind of thing before, it's not common. Common base topology may be a way to describe it. It may be similar to Jan Dupont's (ACD) Lynx amp.

The output stage consisting of Q3,4 M1,2 is wrong as there's no method of biasing the output devices. R4 is doing nothing. If it makes it look OK in simulation that's because the output stage is incorrectly set up in the first place.
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Old 3rd December 2004, 10:10 AM   #10
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All this schematic is uncorrect, please read something about design of amps - I recomend to you for example pages of Erno Borberly .
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