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Old 12th April 2007, 10:00 PM   #1
Dusk is offline Dusk  United States
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Smile More op-amp/mosfet power amplifier

Hello Quasi and Others
I have the op-amp front-end/mosfet output stage amplifier up and running...........that problem i was having with the TL071 ic......was all my fault........i took it on my own to add a Zobel network to the circuit.........needless to say it did not like this at all..so i removed and everything was fine...........i could bias the amplifier with the 100ohm and 10 ohm resistor in the supply line with no problem,and it(the amplifier) sounds great.........that is for a guitar amplifier....very good bass and mid range........but there is one thing that i worry about....no short-circuit protection for the mosfet.......can i use a zener diode clamp from the gate to the drain(the drian are the output in this circuit) on each output mosfet.....will this work in this circuit......long-nuff for the supply line fuse to blow.
iAM So impressed with this amplifier.....that i now want to build an more powerful version........8 mosfet per channel......HahAHA........he's.........gone power Mad.....really beside the power supply increase(-/+ 70 v) and 8 mosfet per-channel what else in the circuit ( componets increase in voltage rating)and aheavey-duty heat-sink) do i need to change??? iam looking for 200watt/8 ohm per-channel.

http://www.diyaudio./forums/attachme...amp=1122897161
Thanks For your help
Dusk
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Old 12th April 2007, 10:13 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
the datasheet for your FETs should state the absolute maximum gate to source voltage (normally around 20V).
If you look at the Id vs Vgs curves you can select a Protection Zener voltage to suit the limiting current you think the FETs can survive passing until the fuses blow.

But the current limiting Zener must be fitted from gate to source to limit the Vgs to match the curve you just selected.
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Old 12th April 2007, 10:40 PM   #3
Dusk is offline Dusk  United States
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Smile More op-amp/mosfet

Hi Andrew
Iam no amplifier GURU.......but in this circuit.....itried to post it(circuit) it's on here under Op-amp based mosfet power amplifier for guitar rig....post#12 anyway the mosfet are in the circuit switched around......the Dain is the output and the source is the supply feed.....no amplifier GURU here......hope you can find post#12 to see the schematic.........Thanks for your help

Peace Dusk
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Old 12th April 2007, 10:52 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
I don't need to see the schematic.
The Zener goes from gate to source to prevent excess voltage across the FET gs.
If you want to use the same Zener to limit current then choose a lower value voltage as I said earlier. Look up the datasheet.
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Old 13th April 2007, 05:27 PM   #5
Dusk is offline Dusk  United States
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Smile Current Limiting

Hi Andrew T

Ok....the light bulb come on.........the path is before me....thanks
this circuit has a different mosfet set-up.....is it called CF?
I have seen more source-follower with the source being the output instesd of the dain........like i say ....iam no amplifier GURU.
so,this zener will limit voltage/current until the supply line fuse kick-in.


Great....thanks again
Dusk
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Old 13th April 2007, 05:54 PM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
yes, many FET output stage designers advocate this method of protection and point to how robust the FETs are.

Personally, I dislike this simple method of current limiting and would much prefer V-I limiting to be integrated on board.
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Old 13th April 2007, 06:43 PM   #7
Dusk is offline Dusk  United States
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Smile v-i limiting on circuit board

Andrew
Tell me can the v-i limiting circuit that you prefer work with this circuit.....that i have?
Thanks
Dusk
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Old 14th April 2007, 06:42 AM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
the cfp output needs source resistors to the supply rails.

Normal V-I sensors must sense the current across these resistors so the limiters must be arranged to tie the cfp inputs to the supply rails rather than the usual arrangement of to the output line.

I have only seen once version implemented correctly and I have no idea whose schematic it was.
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