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Old 6th April 2005, 11:31 AM   #1
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Default How to use Jfet?

I've bought 2sk30Y, I intended to use it for differential pair.
In the original sch, the ccs feeding the differential is about 5.5mA.
But when I look at datasheet and check the device (by connecting D-S together and measure the current with current meter with 12VDC supply) is only 2mA. This is Idss (max drain current) that I'm measuring.

If the mosfets are 2mA max, how many mA should be in the differential's CCS? I read that with positive VGS Jfet acts like bipolars (sucking current from Gate?)

Many shematics write Jfets without specification about the type of Jfet (Y, GR, BL, V)? How to choose which one for certain schematic?
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Old 6th April 2005, 11:49 AM   #2
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Old 6th April 2005, 12:07 PM   #3
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You didn't measure Idss, at least not if you did what you said. To measure Idss, you connect the gate and the source, then apply the voltage to the drain- being careful, of couse, not to exceed the FET's ratings.
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Old 6th April 2005, 12:38 PM   #4
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Hi, SY,

How careless I am in writing. The ones I connected is G-S, not D-S like I wrote in post#1.

For K30 (Y) it is only 2mA. What happened if this device is put in place where the current is more than 2.5mA?
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Old 6th April 2005, 12:54 PM   #5
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In that case, you're right, those FETs will not work in that diff amp. For a 2.75mA current, you'd want FETs with Idss>4-5mA.
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Old 7th April 2005, 03:12 AM   #6
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Quote:
For a 2.75mA current, you'd want FETs with Idss>4-5mA.
Thanks for the info. Do you know what is the rule of thumb between Idss and practical working point of 1 Jfet?

Working point current = 1/2xIdss?
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