output impedance of common source amplifier

[tempus fugit]

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

 

[Tim Wescott]

On Wed, 18 Feb 2009 16:56:20 -0500, tempus fugit wrote:

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

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

 

[Fred_Bartoli]

"Tim Wescott" <tim@seemywebsite.com> a ĂŠcrit dans le message de
news:s-qdnU8Rr6hZGwHUnZ2dnUVZ_vCdnZ2d@web-ster.com...

On Wed, 18 Feb 2009 16:56:20 -0500, tempus fugit wrote:

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

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.

--
Thanks,
Fred.

 

[tempus fugit]

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.

Thanks Fred.

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

 

[Tim Wescott]

On Wed, 18 Feb 2009 17:46:31 -0500, tempus fugit wrote:

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.

Thanks Fred.

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

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

--
http://www.wescottdesign.com