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Old 4th July 2011, 09:14 AM   #1
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Default Preferred JFET in Constant current source?

Hello all,

From all the wisdom and experience built up over time, is there a preferred JFET to use as a constant current source for say 3-10mA?

What about if the CCS is cascoded, does it mean the CCS quality (sound, PSRR, etc) is less dependant on the JFET?

Last edited by Dr.H; 4th July 2011 at 09:20 AM.
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Old 4th July 2011, 01:18 PM   #2
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Personally, I like the 2SJ170, this little one will cover the whole range.
I have e.g. used it as a CCS for a phase inverter (6SL7); as well as a tube bias for a cascoded 6SN7/ECC82 CCS.
The 2SK170-CCS in a simple form is a little dependent on voltage (say 3-50 volt), which does show up on the work bench, specifically for the higher currents. That said, this is the converted resistance. But that is true of the majority of simple forms.

As well this FET is temperature sensitive; even the 2SK30 is to some extent (which I prefer for the low currents).

I have made complex CCS's (with small mosfets; led gate voltage) but that does make sense only if enough 'headroom' is available, plus the auxiliary current for the CCS bias. Swinging from 5 to 50 volt will even then show a minor increase of current.
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Old 4th July 2011, 03:48 PM   #3
AndrewT is offline AndrewT  Scotland
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if low volts drop across the CCS is not a priority then avoid the expensive k170.
There are lots of much cheaper and more easily available jFETs. bf862, bf244/5, 2n5457/8/9, 2n5460/1/2,

Read Walt Jung on CCSs. He tells us that min Vdrop must be >=2*Vp. This is important.
A lower Idss jFET will have a lower Vp than a higher Idss from the same family. If you need exceptionally low Vdrop then a 2sk170gr is hard to beat.
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Last edited by AndrewT; 4th July 2011 at 03:50 PM.
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Old 4th July 2011, 05:51 PM   #4
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THanks guys
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Old 11th July 2011, 05:39 AM   #5
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I have seen BF256 and BF245 used in Elektor designs.
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Old 11th July 2011, 06:45 AM   #6
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Ideally you would like to use a high input impedance jfet for a constant current source, so a 2SK246 is prefered over the 2SK170. It is, however, a large pinch-off voltage jfet, so voltage drop is larger.

If one's interested in further reading, Erno Borbely discussed this issue nicely.

Hannes
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Old 11th July 2011, 01:56 PM   #7
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Can you provide a Borbely document title?
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Old 11th July 2011, 06:58 PM   #8
h_a is offline h_a  Europe
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Hi,

jeez, sorry I just had a look at Borbely's Jfet-the new frontier articles and these do not exactly contain what I meant.

I think I (correctly ) remember now it was Walt Jung. Sorry for that, but I'll try to dig out a reference. Don't mind reminding me for that as I'm pretty busy currently.

Hannes
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Old 11th July 2011, 07:48 PM   #9
jcx is online now jcx  United States
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Default "long channel" geometry jfets best for ccs

http://liveweb.archive.org/http://ww...0596/70596.pdf

those intersted will probably want all of the Siliconix app notes: JFETs Application notes
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Old 11th July 2011, 10:26 PM   #10
AndrewT is offline AndrewT  Scotland
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thanks Jcx
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