Current source loading

My question concerns common source amplifiers (or common source with degenerative feedback) and their output impedance. From what I have read the output impedance, with the drain loaded with a resistor, is essentially the value of the drain resistor (drain resistor in paralell with the impedance looking into the drain).

If this is so, then how do I calculate the output impedance of a common source amplifier when the drain is loaded with a current source or current mirror? :confused:

Thanks,
SteveS
 
Common Source

Well as long as we are all in Texas. Since the mosfet gate input looks like a large capacitance, I would worry about having the current to get a decent slew rate. This kind of of a poor match for front end and output stage. A small mosfet follower to drive the output mosfet would be a great idea. Say a Zetex VP3110.....

H.H.

P.S. That Jocko guy don't do mosfets, I have been trying for years to get him to.....
 
Yes, Hairy, RCA made them, 3Nxxx versions. The single-gate versions were usually called IGFETs. They had low transconductance, poor linearity, and useless for anything but ham radio stuff.

The dual-gate versions used the extra gate for AGC or LO injection. You can find them in the tuner section of most TVs.

Thought you knew everything, didn't you?

Anyway back to the subject at hand........

Let me guess......triode diff-amp w/current mirrors.......high impedance with no current drive trying to drive........maybe...........a P-channel MOSFET? Even more capacitance than the N-channel ones.

OK.....obvious question: why?


Jocko