JFET buffer utput impedance?

[neazoi]

Hi I was wondering what is the component that defines primarily the output impedance in this jfet buffer HF amplifier?

 

[FvM]

It's the JFET, rout = 1/gm.

 

[neazoi]

It's the JFET, rout = 1/gm.

While I was writing this, I have found an explanation https://www.muzique.com/lab/buffers.htm
I know this is a different question, but I was wondering why is the source choke needed in my buffer?
I have seen other designs where no choke is in place https://www.ab4oj.com/test/imdtest/main.html
Can I leave this choke out for a broadband (HF) jfet buffer?

 

[FvM]

Can I leave this choke out for a broadband (HF) jfet buffer?

Yes, but then the source resistor forms a voltage divider with the FET buffer output impedance.

 

[neazoi]

Yes, but then the source resistor forms a voltage divider with the FET buffer output impedance.

So the RF output voltage will be lower than the input then, despite this is a gain 1 buffer...
Correct me if I am wrong.
Thanks a lot!

 

[FvM]

Yes common drain buffer has always a gain below unity, only with high load impedance and high source resistor (or current source bias) it can achieve near unity gain.