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-   -   NJFETs for Source Follower Applications (https://www.diyaudio.com/forums/pass-labs/329131-njfets-source-follower-applications.html)

EUVL 25th October 2018 07:14 AM

NJFETs for Source Follower Applications
 
1 Attachment(s)
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 :
https://www.diyaudio.com/forums/pass...ives-list.html
https://www.diyaudio.com/forums/pass...170-2sj74.html

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.
https://www.diyaudio.com/forums/pass...ml#post5501795

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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EUVL 25th October 2018 07:16 AM

2 Attachment(s)
Here the results for 2SK170BL.


Patrick

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EUVL 25th October 2018 07:16 AM

2 Attachment(s)
For 2SK117BL.


Patrick

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EUVL 25th October 2018 07:17 AM

2 Attachment(s)
For 2SK209GR.


Patrick

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EUVL 25th October 2018 07:19 AM

While both 2SK170 & 2SK117 are obsolete, the SMD version of the latter (2SK209) is still active.
Available at e.g. Mouser or Digikey.


Patrick

obh 25th October 2018 07:47 AM

Measurement of 2SK2145 (active part, dual 2SK209) would be interesting.
Do you have any idea how it would behave?

EUVL 25th October 2018 09:35 AM

Why should it be different to 2SK209 ?


Patrick

marconi118 25th October 2018 12:11 PM

What about bf862?

EUVL 25th October 2018 12:27 PM

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.


Patrick

Mark Johnson 25th October 2018 12:35 PM

1 Attachment(s)
Feel free to use this image, it might be easier to interpret than unformatted ascii text.

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https://www.diyaudio.com/forums/atta...6&d=1540470919


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