another output Z question

Feb 20, 2009 3 Replies
T

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


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T

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

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J

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
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J

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

John

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