output impedance of common source amplifier

Hi;

How do I determine the output impedance of a common source (or other JFET) amplifier? I know that it is roughly 1/gms, but how do you deal with the units? E.g., if a certain JFET has a gms of 5000 uS, the output Z would be

0.0002, but surely that's not ohms is it? Or do you divide out the micro and get an answer of 200 ohms?

Thanks

Reply to
tempus fugit
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Yes, you divide out the micro. 1 S = 1/(1 ohm), 1 micro = 10^-6; just substitute that into the "5000 uS" expression, and solve.

Note that the forward transconductance changes with bias current; make sure you're using the right figure, and don't forget to account for device variation.

--
http://www.wescottdesign.com
Reply to
Tim Wescott

But the output impedance of a common *source* amplifier isn't 1/gm. That one is the output impedance of a common drain stage.

For a common source stage you use 1/go

Also note that parameters depend on the bias conditions and that those given in datasheets and suffixed by a 's', like gfs, gos, are at VGS=0 bias conditions.

And for a R loaded common source stage the output impedance is often dominated by the load resistor value.

-- Thanks, Fred.

Reply to
Fred_Bartoli

one

given

Thanks Fred.

My mistake - I meant common drain; I was thinking source follower.

Reply to
tempus fugit

And you sucked me right in, because that's what I was thinking, too.

--
http://www.wescottdesign.com
Reply to
Tim Wescott

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