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Old 10th August 2009, 03:07 PM   #691
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Quote:
Originally posted by AndrewT



I have matched k170 at various <=Idss and find that similar or even identical Idss is worthless when the current (Id) is reduced slightly or a lot below Idss.
The slope of the transconductance curve must be matched at all reasonable currents. I try for 100% of Idss down to ~ 25%Idss.
My experience varies, with respect to the graph from the 2SK170 DS, gm (dId/dVgs) is a very weak function of Idss. In fact the lines are not separable at 2mA operating current. I posted a fully differential current mirror loaded JC self biasing quad where trimming the mirrors independently gave 0.0 offset and 1ppm THD2 for badly (20%) matched FETs. THD3 is generally not affected much. A weak servo just to keep the offset 0 might be helpful to add.

Yes the trim process was a manual search "algorithm" the point being that at some ratio Id1/Id2,Id3/Id4 the offset and distortion both cancelled. Without help with isothermalness I don't think any technigue holds up perfectly.
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Old 10th August 2009, 03:25 PM   #692
h_a is offline h_a  Europe
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Hi Scott,

I remember your circuit very well

It's just a pity that the general trend is to ignore such novel information and keep to old dogmas.

@AndrewT: it's a pity to see that now also you started to present your opinion as a fact, while you haven't even checked it. That's not proper engineering, but happens all the time here.

Well, what you do is that you merely assume that transconductance in the (arbitrary) range you specified is dominant in a distortion cancellation circuit. While that may be correct, it is completely unclear wether other mechanisms and manufacturing tolerances allow to see any advantages in real life.

I want just to remind you I have offered to measure a magic matched pair on an Audio Precision long time ago and nobody bothered to take the opportunity.

Maybe you can post some figures in the near future, I am of course interested.

Have fun, Hannes
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Old 10th August 2009, 03:41 PM   #693
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Please remember this is meant to be a disscussion of technical issues not a confrontation. I have not tried it but a single diffpair would probably have the same properties (0 offset/0 THD2) if the trims were the current ratio and a small pot between the sources.

It would be nice to see how different techniques perform in large signal open loop circuits.

Single ended THD2 trimming is another set of problems, I see Salas has somone tweeking the simplistic RIAA. The perfect null does not yield to analysis easily.
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Old 10th August 2009, 03:51 PM   #694
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by h_a
@AndrewT: it's a pity to see that now also you started to present your opinion as a fact, while you haven't even checked it. That's not proper engineering, but happens all the time here.
that is not the case,
in the context of matching and whether Vds matching is more appropriate than Idss matching.
I have stated that Idss selecting is almost worthless when looking for pairs that perform similarly at less than Idss current compared to Vds matching for near equal Id.
That has been done by test and measurement. It is not simply "an opinion"

Quote:
Originally posted by h_a
Well, what you do is that you merely assume that transconductance in the (arbitrary) range you specified is dominant in a distortion cancellation circuit. While that may be correct, it is completely unclear wether other mechanisms and manufacturing tolerances allow to see any advantages in real life.
I have never stated that distortion cancellation is due to the dominance of any particular parameter. That is your conclusion.
I have stated that matching by one method does little to get the devices to perform on similar parts of the curve. I claim there are better ways to get the components to match. That is quite different from what you claim I have said.

Now let's extend this a little further.
Take two jfets with identical Idss (<0.05% tolerance)
set them up as an LTP and connect them to flow Idss through each by using zero for the gate resistor.

now apply a Vgs to the gate rather than using the shorted gate to source.
Increase the Vds until the Id equals Idss. You will find that with the gates connected and the sources connected that the Drains will flow identical currents and this will result in zero differential voltage between the drains.
Now reduce very slightly the the common Vgs applied to the jfets.
We have two alternatives.
Either the drain diff Voltage remains at zero.
or
the diff voltage becomes greater than zero.
In the first case we can imply thet the transconductance of the two devices match at Idss.
In the second case we can imply that the transconductance at Idss and for that tiny range just below Idss that the transconductance does not match.

Now take those two devices with unmatched transconductance and show me that they perform as identical amplifiers.
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Old 10th August 2009, 03:54 PM   #695
h_a is offline h_a  Europe
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Quote:
not a confrontation.
Of course, I apologize.

It's just a pity to see that even if there's solid information on a popular topic the general trend is to ignore it although everybody claims to be here to learn.

Well, we need myths I guess.

Have fun, Hannes

EDIT: Andrew, I didn't see your post before. I can only repeat myself, the magnitude of the effect is completely unclear and may be not seen in real life. That's why it has to be checked. I will let this topic rest as nothing usefull can emerge from this discussion. You are of course free to do whatever you like to improve your amps.
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Old 10th August 2009, 03:57 PM   #696
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Quote:
Originally posted by hitsware
>It's not a resistor value, it's a current,
>that for IRF whatever needs to be too
>high for them to be temperature stable.
>That means, when they are temperature
>stable they are already too hot.

Then you can't use source resistors
the way you use emmitter
resistors with bi-polars ?

Sure, much smaller voltage drops on emitter resistors are needed to set emitter currents closer to each other than would be needed for MOSFETs. Millivolts of a negative feedback are needed to change base currents of paralleled transistors so this feedback would define their emitter currents, while Vgs variations are greater than such small voltage drops on similar resistors.
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Old 10th August 2009, 03:59 PM   #697
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Quote:
Originally posted by scott wurcer
with respect to the graph from the 2SK170 DS, gm (dId/dVgs) is a very weak function of Idss.
I think we are saying the same thing.
and when I apply it to BJTs I find the same.
hFE is a very weak function of Vbe.
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Old 10th August 2009, 04:00 PM   #698
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Try, build, compare - and then speak about myths.
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Old 10th August 2009, 04:02 PM   #699
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by h_a
the magnitude of the effect is completely unclear and may be not seen in real life. That's why it has to be checked.
I agree completely.
I am not competant to state nor measure what that magnitude is.
Similarly I cannot check whether it makes a big or small or any difference.

Do not try to put non existent words/claims into my posts.
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Old 10th August 2009, 04:05 PM   #700
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Quote:
Originally posted by AndrewT


Now take those two devices with unmatched transconductance and show me that they perform as identical amplifiers.
Andrew your matching method involving VDS is so alien to me that I would like to inquire as to what you consider "performing as an identical amplifier".

To be clear the school of detailed device matching and no trims or adjustments is a perfectly acceptible path. Certainly taking pots out of the direct signal path might be a goal to some. And the old trick of trimming and replacing the pot with R's can be as much work as the matching in the first place.
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