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Old 2nd November 2006, 04:57 AM   #1
rsbay is offline rsbay  Germany
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Default Aleph X and matching power MOSFETs

Hello,

I want to build an Aleph X with 8 MOSFETs and I think it is not needed to have
two pairs of 4 matched MOSFETs.

My idea :
You have to match only the types in the same position on both halfs of the
amplifier. With this type of matching you get two identical output stages
and that is enough.

What kind of errors this idea have ?

Regards

Ralph
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Old 2nd November 2006, 06:55 AM   #2
EUVL is offline EUVL  Europe
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8 FETs per amp means paralleling 2 MOSFETS at each of the four corners of the circuit.

It is correct that the devices in parallel do not have to be matched perfectly, but if they are too different in Vgs, you get unequal current sharing, so that one gets more dissipation than the other.

And if I am not wrong, all 8 matched will give you minimum distortion and thermal stability. Just assume a polynomial as transfer function (Id vs Vgs) and do some simple algebra.


Patrick
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Old 2nd November 2006, 03:26 PM   #3
gl is offline gl  United States
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Hi rsbay,

You need to match the transistors that are connected in parallel as closely as you can. As EUVL says, the match does not need to be perfect. There is a great deal of information on this site and on Passdiy about the meaning of "as closely as you can" as used here.

It helps to reduce offset if the gain transistors on either side of the circuit are matched to each other. Matching the CCS transistors to the gain transistors is not important IMO.

Cheers,
Graeme
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Old 3rd November 2006, 04:44 AM   #4
rsbay is offline rsbay  Germany
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Hello,

I fear you didn't understand what I want :

I want to match at least only a pair at a time and put both MOSFET's at the
same position in the both halfs of the amp.

Lets say I'm able to match a pair perfectly, than at least both sides are
equal and I have no differential offset.

Regards

Ralph
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Old 3rd November 2006, 05:28 AM   #5
steenoe is offline steenoe  Denmark
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Quote:
I fear you didn't understand what I want
What you want maybe isnt exactly what the amp want's I am sure the 2 gentlemen, answering your question understood the question
Q2 and Q11 must match even if using pairs (or triples, whatever)
If you plan to use pairs you need 4 matching fets for those positions (gainfets).
The CCS fets doesnt necessarily need matching across Q1 and Q10, although it wouldnt hurt. The pairs for those positions would still need matching.

Steen
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Old 3rd November 2006, 06:22 AM   #6
EUVL is offline EUVL  Europe
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Ralph,

I understood exactly what you wanted.
I am afraid that you do not understand what I told you.

But the amplifier will work fine using the matching method you described.


Patrick
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Old 3rd November 2006, 03:39 PM   #7
gl is offline gl  United States
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Hi Ralph,

Yes I understood the question too and I agree with the statements made by the others.

Graeme
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Old 3rd November 2006, 05:16 PM   #8
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Quote:
Originally posted by rsbay
I want to match at least only a pair at a time and put both MOSFET's at the same position in the both halfs of the amp.

Lets say I'm able to match a pair perfectly, than at least both sides are equal and I have no differential offset.
In that case, simply match all of them.
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Old 4th November 2006, 06:29 AM   #9
rsbay is offline rsbay  Germany
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Sorry Gentlemen,

I don't want to insult someone, all I want to know is if my idea is worth to try. I have lying arround 4 quartets of matched IRFP044N to build 2 amps with 8 MOSFET's. But during the development of my PCB boards I had this idea.

Regards and thanks for the infos

Ralph
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Old 4th November 2006, 10:44 AM   #10
EUVL is offline EUVL  Europe
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> I have lying arround 4 quartets of matched IRFP044N

If I were you, I would use 2 Quartets for each amp, one quartet used for the driver FETs, and the other quartet for the current source.

But it is your choice in the end.
The amplifier will function one way or another.


Patrick
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