What is the correct Idss of jfet? - diyAudio
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Old 9th January 2011, 02:42 AM   #1
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Default What is the correct Idss of jfet?

When I measure Idss, the current various for some time before it get stable. I should that the initial startup current as Idss or to the point when the current becomes stable?
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Old 9th January 2011, 05:22 AM   #2
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Make sure you don't touch them with your fingers before measuring and all will be stable.
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Old 9th January 2011, 06:55 AM   #3
Calvin is offline Calvin  Germany
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if You look at datasheets youŽll find, that several parameters among these the Idss vary with temperature, some varying over a quite wide range.
IŽd test preferrably under the voltage and current conditions the part will undergo later when built into the circuit. In any case give the JFET time to warm up. So You could read one set of parameters at turn on, when the JFET is still cold and another set when it has reached and stabilized at working temperature. In any case its a time consuming work to match JFETs.

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Old 9th January 2011, 06:58 AM   #4
EUVL is offline EUVL  Europe
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If you are using your Curve Tracer, you should measure at steady state under working conditions, as Calvin suggested above.
You need to set both the working current range (which is not always Idss) and the Vds in the actual circuit.

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Old 9th January 2011, 11:21 AM   #5
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Thanks for the input. I am travelling in China that I will post details of my question when I returned Hong Kong later this week.

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Old 9th January 2011, 12:31 PM   #6
AndrewT is offline AndrewT  Scotland
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as said above the device Idss varies with junction temperature. That junction temperature is dependent on Ta, ambient temperature, Pd, dissipation at the junction and time since the dissipation last changed.
The standard Idss specified in the datasheets fixes the variables by testing at Tj=25degC and dissipation very short term so that Tj does not change.
We, poorly resourced amateurs, cannot usually replicate that.

If we need precise Id, then in-circuit testing is the best. Next best is modeling so that operating conditions are replicated as close as possible.
Worst is using fingers to place a device in a 3pin socket and applying an unknown voltage at unknown Ta for an unknown time. These measured Idss are almost useless, if precision is required for the circuit to operate properly.
However, many circuits are specifically designed to accept a range of device tolerances and still perform to specification. It is, in my view, bad design to require absolute precision in device parameters to get specification performance.
regards Andrew T.
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