How important is N to P ch Mosfet matching for an F5Tv3 - diyAudio
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Old 10th December 2014, 01:27 AM   #1
nashbap is offline nashbap  United States
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Default How important is N to P ch Mosfet matching for an F5Tv3

I understand the importance of matching Mosfets within an N channel or P channel but I am not sure whether it is important to match between N and P channels. Obviously this leads to a difference between N and P output bias voltages which you adjust to zero the offset but do the two numbers have to be within a range say 10% and why?

This is for a F5TV3 with 4 N and 4 P devices per channel.

Also when a supplier provides a Vgs value and a Yfs value which is the more relevant of the two to consider?

Thanks.

Nash
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Old 10th December 2014, 03:21 AM   #2
6L6 is offline 6L6  United States
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Not necessary at all. A difference from n to p is normal.

Vgs.
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Old 10th December 2014, 04:56 AM   #3
JMFahey is offline JMFahey  Argentina
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N and P mosfets are inherently different, so fine matching is not practical.
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Old 10th December 2014, 03:47 PM   #4
nashbap is offline nashbap  United States
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Quote:
Originally Posted by JMFahey View Post
N and P mosfets are inherently different, so fine matching is not practical.

Thanks both of you.

So for example lets say N channel bias voltage is .30V and P channel is .26V to obtain zero offset, is this much of a spread still acceptable? In a mono with N on one side and P on the other, wont this result in different heat sink temperatures?

Could someone please explain Yfs measurements. I cant find much on this anywhere.

Nash
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Old 10th December 2014, 04:23 PM   #5
6L6 is offline 6L6  United States
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Make bias as close as you can, zero offset is the real goal.
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Old 10th December 2014, 05:04 PM   #6
Zen Mod is online now Zen Mod  Serbia
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Quote:
Originally Posted by nashbap View Post
Thanks both of you.

So for example lets say N channel bias voltage is .30V and P channel is .26V to obtain zero offset, is this much of a spread still acceptable? .......

Nash
current goes (by nomenclature) from positive PSU to negative PSU

that means that upper and lower halves of amp are on same (and only ) route (disregarding slight current through feedback net) ........ so difference in voltage across source resistors is coming from difference in their value ( tolerance!) , not difference in current

so - read tutorials for F5 , several times and it will click eventually

Quote:
Originally Posted by nashbap View Post
......

Could someone please explain Yfs measurements. I cant find much on this anywhere.

Nash
as long you're in dark regarding more basic issues , there is no use of knowing how to measure Yfs

however , at FW site you'll find articles for everything , except 42






search for articles having test and mosfet in title
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Old 10th December 2014, 06:25 PM   #7
nashbap is offline nashbap  United States
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Originally Posted by Zen Mod View Post
current goes (by nomenclature) from positive PSU to negative PSU

that means that upper and lower halves of amp are on same (and only ) route (disregarding slight current through feedback net) ........ so difference in voltage across source resistors is coming from difference in their value ( tolerance!) , not difference in current

Thanks Zen Mod. I am learning. Understand the concept about the current going thru the two halves. Using the example of bias voltage 0.26V and 0.30V over 0.5ohms source resistance for zero offset gives me bias current of 0.52A and 0.6A; so is it correct to say that the bias current is an average of the two at 0.56A?
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Old 10th December 2014, 07:31 PM   #8
Zen Mod is online now Zen Mod  Serbia
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irrelevant

choose one side Rs to monitor Iq (with first DVM) , put second DVM at output (ref. to gnd) to monitor offset


you can put 3-rd DVM to adjacent side Rs , but just to confirm that physics is still valid in Universe ;

as long that's the case (physic's ruling) and you didn't dreked something substantial , amp will be functional and Smarty Current will find it's way where Pa intended ........ and difference in reading between 1-st and 3-rd DVM-s will be probability sum of tolerances of two Rs-es and two DVM-s , combined

inputs shorted , no load at output
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Last edited by Zen Mod; 10th December 2014 at 07:37 PM.
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Old 11th December 2014, 10:34 AM   #9
AndrewT is offline AndrewT  Scotland
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Originally Posted by nashbap View Post
Thanks Zen Mod. I am learning. Understand the concept about the current going thru the two halves. Using the example of bias voltage 0.26V and 0.30V over 0.5ohms source resistance for zero offset gives me bias current of 0.52A and 0.6A; so is it correct to say that the bias current is an average of the two at 0.56A?
your two resistors marked 0r5 are probably +-10% tolerance.
If you want or need to measure currents through them, then YOU need to measure them before assembly.

Now I know that I can't measure 0r5 accurately using a DMM set to resistance.
I know that I need to use a method that let's me get closer to the actual resistance.

Now you need to ask yourself:
"is it worth my effort to research the methods available to match, or measure, low value resistors?"
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Old 11th December 2014, 10:38 AM   #10
AndrewT is offline AndrewT  Scotland
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Alternatively,
you could use 9off 4r7 1% resistors in parallel and then add 12r 1% in parallel to give a final value of 0r5004 +-1%
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