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xplod1236 2nd December 2004 03:30 AM

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)?

Mr Evil 2nd December 2004 03:41 AM

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.

xplod1236 2nd December 2004 04:09 AM

What's the difference between common-source and source-follower?

richie00boy 2nd December 2004 09:04 AM

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.

xplod1236 2nd December 2004 07:04 PM

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?

EchoWars 3rd December 2004 03:21 AM

That's a really small amp...microscopic. in fact.;)

xplod1236 3rd December 2004 03:34 AM

2 Attachment(s)
Thanks for pointing that out. I haven't realized that I didn't attach it.

AndrewT 3rd December 2004 07:25 AM

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.

richie00boy 3rd December 2004 08:24 AM

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.

Upupa Epops 3rd December 2004 09:10 AM

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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