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another output Z question

Hey all

I'm still trying to find the output Z of a low gain JFET preamp I'm working with. I'm using an MPF102, in common source configuration. I have a 1K resistor going from the 9v supply to the drain, with the source connected to ground. There is no spec for gO on the data sheet, so I can't just use 1/gO, and I figure the output Z is at least somewhat dependent on the drain resistor and the load Z (which is 10K min). I know that the common source configuration has a high output Z, but how high is high? Would it be 1000s of ohms, or 10s of 1000s?

Thanks

Reply to
tempus fugit
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working

to

1/gO,

Wait - I found a data sheet that gives the yOs as 200 uS which would give an output Z of 5000 ohms. Is this a close enough approximation or will the output Z vary much with different drain and load resistances?

Thanks

Reply to
tempus fugit 

--
Since you\'ve got a 1k pullup to the positive rail, with the JFET cut off
you\'re looking at 1000 ohms of resistance/impedance between the rail and
the load.

As you drive the JFET, its resistance will change from quasi-infinite
when it\'s cut off to quasi-zero when its fully enhanced, so the output
impedance of your circuit will vary with JFET drive.

    
JF
Reply to
John Fields

Zout will be about 1K in parallel with the fets's 5K, say 800 ohms.

John

Reply to
John Larkin

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