# Output impedance of JFET source follower with constant-current biasing

I try to study the output impedance of a JFET amplifier biased by a current source, but I can't do it.

I know how to determine the output impedance of a JFET amplifier polarized by a voltage divider, but I am hampered by the current source.

How can I do it?

• Start by determining the output impedance of only the current source. Assume that the collector of the NPN is connected to a DC voltage source with value Vcc. Draw the small signal equivalent model and determine the impedance when looking into the collector. Jan 25, 2021 at 21:18
• I agree with Bimpelrekkie - once you determine the impedance of the current source, then replace the current source with its impedance, and you got a much simpler circuit to work with :) As a hint to make your life easy: sometimes it helps to start with idealized components first. How about you see what happens if the current source would be an ideal current source. That'll give you a limiting "best case". Then you can replace the current source with its opposite (the worst current source ever): an ideal voltage source. That's another limiting case. Finally, look at the non-ideal source. Jan 25, 2021 at 22:06
• Is this answer gives you any help? electronics.stackexchange.com/questions/342859/… and this electronics.stackexchange.com/questions/265046/…
– G36
Jan 26, 2021 at 14:53