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Old 29th July 2012, 07:35 PM   #141
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Thank you for this careful explanation Dave.
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I think to have understood that if one is in the lucky situation of having sufficient FETs to pick from, one can "simply" do SK170/SJ74 Idss matching with 5R degeneration for the SJ74. Is this correct?
Cheers,
Nic
Hi Nic,

My pleasure. Very nice to see some activity here after such a long time. Hope to see more once FETs are available!

The intent essentially was the opposite. As Patrick points out a single point match for all devices should be the absolute minimum for each quad. Pick the Idss for the K170s and J74s as listed in the table and use the 5R resistor. These are datasheet values and get you close enough, bare minimum quality of match. This would apply mostly to those who are buying JFET sets from a vendor pre-matched.

If you have many JFETs to choose from build a fixture similar to the one described with resistors and jumpers or switches and do the three point match.

I took it a step further and did curve matching on the input devices to closely match what Patrick had previously done. I chose devices initially on static Idss based on the chart then ran curves on each group. I matched N/P sets with the best curves. Once matched I adjusted the resistor for the J74 for best Idss balance in circuit. Achieved near perfect curve matches but had to look through several date codes of each and many transistors of similar Idss to find them. Considering I did only 5 point matches on the output devices and had a limited number to choose from, the front end matching may have been overkill...

Dave
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Old 29th July 2012, 07:45 PM   #142
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One more point - The K170s were more consistent as far as true curve matching goes. Even lot to lot they were pretty close when testing within a given Idss range.

The J74 were more varied, even within the same date code. Probably more time (and possibly devices necessary to choose from) needed here if doing true curve matches.

This data is from a fairly limited sample set. 3-4 date codes each and only few hundred total curve tested once selected for Idss in the correct ranges. This may not be represenative of parts you have...

Hope it helps,
Dave
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Old 29th July 2012, 08:46 PM   #143
NicMac is offline NicMac  Italy
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Hi Dave,
I hope you will indeed see some more action here in the not so far future.
I should have a nice heavy MOSFET package from Davide landing my desk tomorrow

The K170/J74 is more a private thing as I don't want to run out of these useful quasi unobtanium FETs in the near future.

You write: "Once matched I adjusted the resistor for the J74 for best Idss balance in circuit.". Will this not also affect the matching? Does that mean that every J74 requires a different resistor value

Anyway, I'm sure that I will understand a lot more once I have done a few thousand measurements

Cheers,
Nic
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Old 29th July 2012, 10:08 PM   #144
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Hi Nick,

Good questions.
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Will this not also affect the matching?
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Does that mean that every J74 requires a different resistor value
Possibly, though maybe for a different reason than you expect - The best curve matches N-to-P I found did not follow Patrick's chart exactly for Idss. That chart is based on "typical" characteristics from the datasheet. Real parts will vary some from these "ideal" characteristics. My best sets were also slightly lower Idss ratio N-to-P so to get the best matched P/N devices and to achieve good current balance once in circuit I needed 4R degen resistors.

IMPORTANT NOTE*** - don't overlook the fact that the 5R resistor suggestion from the chart is specifically for the 6.5/7.6mA combination. If the Idss for your matched sets are 8/8.9mA, for example, the datasheet values would dictate 3R3 ohm Rdegen, not 5 ohms. Consult the chart for a good starting value of Rdgen based on the Idss parts you have.

As I said, I measured the static Idss without any Rdegen (maybe this point was not clear) to sort what I have into Idss groups. Only then proceeding to use the chart for an Rdegen starting value. Once in the ballpark either use the enhanced version of this fixture for 3 point matching (with the extra resistors to fine tune things) or curve match with the same type of selectable Rdegen resistors in the fixture.

If you have to you can fine tune the Rdegen resistor on the board after assembly while dialing it in (as see here) to fine tune the bias balance of the front end top to bottom before the output devices are installed.

Not sure it is clear but once you work through it, it should become more so…

Dave

Last edited by fitzfish; 29th July 2012 at 10:14 PM.
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Old 30th July 2012, 04:37 AM   #145
EUVL is offline EUVL  Europe
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This is of course fine tuning to the last 1% perfection, Dave.

For most DIYers who already have matched FETs according to the table I published, you can set the value of the degen resistor for the J74 as follows :

Measure Id of the 2SK170 using a degeneration resistor of 11R, as the JFET will actually see in the circuit. Note down this Id (Drain Current) value.
Using a 20R trim-pot as degeneration resistor for 2SJ74, repeat the same Id measurement. Note reversed power supply polarity of the P-JFETs.
Adjust the trim-pot until you get the same Id as the 2SK170s with 11R degeneration.
Measure and note down the resistance of the trimpot.
Subtract 11R from this value will give you the R_degen value for the J74s (nominally 5R).
Use the nearest standard value you can get for R_degen. Don't worry if it is not perfect.
Any imperfection will slightly increase second harmonics on one half of the circuit (Single Ended F5) which will then be cancelled by the balanced circuit.

That is the beauty of the X.

Of course you can also follow the fine tuning action of Dave for 100% perfection, but you need to know what you are doing.


Patrick
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Last edited by EUVL; 30th July 2012 at 04:59 AM.
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Old 30th July 2012, 05:58 AM   #146
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Absolutely true. Thanks for the clear method as well Patrick.

Dave
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Old 30th July 2012, 07:05 AM   #147
NicMac is offline NicMac  Italy
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I might try first with my matched SJ74 from Spencer and fit some SK170 to these according to instructions. No need to overcomplicate things here with the X topology of the F5X
Cheers,
Nic
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Old 30th July 2012, 08:03 AM   #148
EUVL is offline EUVL  Europe
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I should correct myself first.
The 11R quoted in Post #145 should have been 9.1R (9.167R if you wish to be 100% accurate).
Forgotten to take into account the 55R feedback resistor as well.

Why did I say this is the last 1% pefection :

A K170 has an effective source resistance (= 1/ Yfs) of about 30R, and varies a bit with actual Id.
A J74 has about 25R. Hence we add 5R degen extra. It also varies a bit with Id, by about the about same amount.

But since this is degenerated in the circuit also by the feedback network, the total eq R_source is then 30+9.167 ~ 40R.
Dave mentioned that in the perfect situation, you might wish to use 3R3.
This is a delta of 1.7R from the nominal value.
But seen in the circuit, it is 1.7R / 40R = <5%.
And that is difference in open loop gain between top and bottom halves.
Further reduction is then given by negative feedback, and balanced topology.


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Old 30th October 2012, 10:11 PM   #149
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Default Matching FET

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Old 30th October 2012, 10:54 PM   #150
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About 300 x k170 and 200 x J74, have gone 24 NNNN and 21 PPPP, but 1% only 6 NNNNPPPP (9.10 to 9.50 mA.) Rdegen of 2.5 to Oh.

We use 3 point Idss match 0, 5, 10 Oh. for k170.

And 3 Rdegen 2,5, 3,3 5 Oh for J74:
2,5 7,5 12,5 Oh.
3,3 8,3 13,3 Oh.
5 10 15 Oh.
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