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NJFETs for Source Follower Applications
NJFETs for Source Follower Applications
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Old 25th October 2018, 01:55 PM   #11
scott wurcer is offline scott wurcer  United States
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The HD looks to have approximately the same slope for all, 6dB/10dB for 2nd's and 3rd's with doubling. Any thoughts on a small source resistor and trim on the current source for 0V output?
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Old 25th October 2018, 02:29 PM   #12
EUVL is offline EUVL  Europe
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Post #1, "Each of the N-JFET pairs used in the testing was matched to <10μA in Idss."
So DC was < 1mV in all cases.

But yes, if one can measure upfront and use the JFET with higher Idss for the CCS, a small source resistor will allow easy trimming of DC offset.
I normally do not use source resistor for the upper JFET.
Somewhere I read before it is supposed to create more H3.

2SK117 can still be bought, 2SK209 for sure.
So no need to be desperate, for now at least.


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Old 25th October 2018, 03:25 PM   #13
scott wurcer is offline scott wurcer  United States
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Just curious, since the numbers imply a possible simple mathematical rule might underlie this.
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Old 25th October 2018, 03:33 PM   #14
EUVL is offline EUVL  Europe
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Do you have an explanation for the increase in H2 but significant decrease in H3 for the GR grade ?


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Old 25th October 2018, 03:38 PM   #15
scott wurcer is offline scott wurcer  United States
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Quote:
Originally Posted by EUVL View Post
Do you have an explanation for the increase in H2 but significant decrease in H3 for the GR grade ?


Patrick
Looking into it.
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Old 25th October 2018, 03:48 PM   #16
obh is offline obh  Norway
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NJFETs for Source Follower Applications
If 2sk2145 used as a single device then expect voltage noise reduced by 1.4 (about -3dB)
relative to 2SK209GR? Would relation H2/H3 be same?
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Old 25th October 2018, 05:22 PM   #17
EUVL is offline EUVL  Europe
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No, because the load now appear as 20k for each FET.
So both H2 and H3 will drop, probably to K170 levels.


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Old 25th October 2018, 05:28 PM   #18
EUVL is offline EUVL  Europe
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Note that 2SK2145 is NOT specified as a matched pair as e.g. 2SK389.
There is no specification on the datasheet as to how well the match is.
In my experience can be as bad as > 5% in Idss.

You can read Samual Groner's article on his Low Noise Amplifier regarding the effects of mismatch in paralleling devices.
In that amplifier, he was using 8x BF862's, but from a noise instead of distortion perspective.


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Old 25th October 2018, 07:26 PM   #19
scott wurcer is offline scott wurcer  United States
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Looking into it.
Nothing conclusive, seems that secondary effects come into play at these levels. -90 or so dB is pretty good.
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Old 25th October 2018, 08:24 PM   #20
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