# output impedance of common source amplifier

T

#### tempus fugit

Jan 1, 1970
0
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

F

#### Fred_Bartoli

Jan 1, 1970
0
Tim Wescott said:
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.

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.

T

#### tempus fugit

Jan 1, 1970
0
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.

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

Replies
16
Views
2K
Replies
14
Views
3K
Replies
2
Views
1K
Replies
1
Views
1K
Replies
5
Views
1K