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The book says the rds of the 2N8461 JFET can be varied from 100 ohm to 10Meg. enter image description here

I went on and tried to verify this using the equation and plug values in it from the datasheet

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using the minimum values for vgs(off) and Idss and a vgs = 0.799 I only get a maximum a rds of 4k ohm why is that?

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Unfortunately, the gm equation you used is only valid for gm, and over values of Vds and Vgs that are appropriate when the FET is being used in its linear amplifier mode, that is, when it's useful to know gm.

If you want to know Rds, especially for large negative Vgs which makes for a high Rds, then you'll need a different equation.

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From what I recall, the Id of a JFET varies as the square of the gate-source voltage.

That means, taking the derivative, the "Ron" varies also.

To produce 10,000,000 ohms, the current variation is 0.1 uA per volt.

Thus your parameters need to bias the JFET on, but at Id near 0.5ua.

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