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NJFETs for Source Follower Applications
NJFETs for Source Follower Applications
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Old 25th October 2018, 07:14 AM   #1
EUVL is offline EUVL  Europe
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Default NJFETs for Source Follower Applications

In recent years, since Toshiba has stopped production of their low noise JFETs, there have been a lot of discussion about possible alternatives to replace them,
and what penalties there might be :
B1 JFET alternatives list
Replacement For Toshiba 2SK170/2SJ74

The most recent discussion is probably the source follower used in the LX Mini Active Crossover.
Following the discussions there, we decided to put some substance behind that discussion.
LX-mini Crossover Article

With the help of Jan Didden, we obtained access to an Audio Precision 2722 Distortion Analyser for half a day.
A small test setup was prepared to test various (Toshiba low noise) N-JFETs in a source follower buffer circuit.
The circuit consisted of 2x 9V alkaline batteries as power supply.
This was further filtered by 4mH Fastron inductors, followered by Panasonic FC 120μF capacitors.
Each of the N-JFET pairs used in the testing was matched to <10μA in Idss.
The output was loaded by a 10k ohm Vishay S102 bulk foil resistor.

The JFETs tested were
a. 2SK170BL at 8.2mA Idss
b. 2SK117BL at 7.5mA Idss
c. 2SK209GR at 3.8mA Idss

Single-ended input signal was 1Vrms and 2Vrms at 1kHz from the AP built-in waveform generator.
The results can be summarised.

DUT 1Vrms __H2 __H3__ 2Vrms H2 __H3
2SK170BL -102dB -104dB -96dB -92dB
2SK117BL -99dB -100dB -93dB -89dB
2SK209GR -92dB -107dB -86dB -97dB


As can be seen, there is very little performance difference between 2SK170 and 2SK117 (= 2SK209 in SMD) at the same Idss; on average about 3dB.
Halving the Idss push up the second harmonics by so 6dB, in return for 7dB lower third harmonics. The latter result (with the low H3) was rather unexpected and interesting.


Patrick
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Old 25th October 2018, 07:16 AM   #2
EUVL is offline EUVL  Europe
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Here the results for 2SK170BL.


Patrick

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File Type: png 2SK170BL 1Vrms FFT.png (46.5 KB, 1041 views)
File Type: png 2SK170BL 2Vrms FFT.png (45.7 KB, 1017 views)
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Old 25th October 2018, 07:16 AM   #3
EUVL is offline EUVL  Europe
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For 2SK117BL.


Patrick

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File Type: png 2SK117BL 1Vrms FFT.png (46.3 KB, 1026 views)
File Type: png 2SK117BL 2Vrms FFT.png (45.6 KB, 998 views)
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Old 25th October 2018, 07:17 AM   #4
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For 2SK209GR.


Patrick

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File Type: png 2SK209GR 1Vrms FFT.png (43.8 KB, 232 views)
File Type: png 2SK209GR 2Vrms FFT.png (45.9 KB, 227 views)
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Old 25th October 2018, 07:19 AM   #5
EUVL is offline EUVL  Europe
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While both 2SK170 & 2SK117 are obsolete, the SMD version of the latter (2SK209) is still active.
Available at e.g. Mouser or Digikey.


Patrick
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Old 25th October 2018, 07:47 AM   #6
obh is offline obh  Norway
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NJFETs for Source Follower Applications
Measurement of 2SK2145 (active part, dual 2SK209) would be interesting.
Do you have any idea how it would behave?
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Old 25th October 2018, 09:35 AM   #7
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Why should it be different to 2SK209 ?


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Old 25th October 2018, 12:11 PM   #8
marconi118 is offline marconi118  Germany
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What about bf862?
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Old 25th October 2018, 12:27 PM   #9
EUVL is offline EUVL  Europe
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BF862 is obsolete. You can't get them anymore on Mouser & Digikey.
It has high Idss and therefore will heat up quite a bit more.
It also has lower dynamic range as the gate current increases significantly above 9V Vds.
So it was not included in this test.


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Old 25th October 2018, 12:35 PM   #10
Mark Johnson is offline Mark Johnson  United States
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NJFETs for Source Follower Applications
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