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Old 23rd May 2002, 03:53 PM   #1
rbroer is offline rbroer  Netherlands
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Default Paralleling jfet CCS's

Is there any objection against paralleling 2 terminal jfet constant current sources (J111 for example, 25mA @ 10Vds) to lower power dissipation for each device ?
I assume output impedance gets lower.

Regards, Rudolf.
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Old 23rd May 2002, 04:06 PM   #2
jam is offline jam  United States
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No. Works great.
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Old 23rd May 2002, 05:15 PM   #3
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Lightbulb JFET Constant Current Sources

Hi Rudolf,
Jfet's make much poorer and less predictable current sources than bipolar transistors. To quote from Horowitz: "It is important to realize that a good bipolar transistor current source will give far better predictability and stability than a JFET current source" .
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Old 23rd May 2002, 05:29 PM   #4
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Default Stable current

They are fine for audio circuits and the value of current can be tuned by selecting the right source resistor. They can be simpler and made as a two terminal devices which is not easy for a BJT current source. Oh yeah.... they sound better. Keep in mind that Howowitz comes from an instrumention backgrund where precision is much more important than in most audio applications.

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Old 23rd May 2002, 06:12 PM   #5
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Default Fets Rule!

Sorry Elso but Harry is right. Have you compared a fet current source wiyh a bipolar sonically?

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Old 23rd May 2002, 08:19 PM   #6
rbroer is offline rbroer  Netherlands
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The idea is to feed LM431 shunt regulators for several supplies.
I've read that feeding them with a current source has some sonic benefit above just a series resistor (at least for analog supplies).

Another reason I want to use a 2 terminal CCS is, I sometimes use unregulated supplies; so using resistors they turn out big and the LM431 might be overloaded when coping with voltage
variations=>current variations.

For larger currents, say 50mA at Vds=10~15V one jfet can't handle all the power, so I want to parallel two jfets...

Any suggestions for types ? I think one needs jfets with low "goss". Are the J111/112 good candidates for idds 20 and up ?

Rudolf
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Old 23rd May 2002, 08:37 PM   #7
Geoff is offline Geoff  United Kingdom
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Rudolf

If you haven't already seen it, have a look at AN103 at:

http://www.vishay.com/brands/siliconix/SSFan.html

Geoff
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Old 24th May 2002, 07:50 AM   #8
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For higher currents, bipolar is the way to go. Perhaps you can use a FET to bias a bipolar.
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