Non-feedback parametric linearisation in FETs

[x-pro]

Ok, actually I felt from the verry first beginning that the lower JFET must be "bigger" than the upper device to achieve correct working point without manipulating the working point for each device without to add circuit complexity, but i might be wrong in my assumption.
But if we would manipulating the working condition for each device I think it must be possible to use the same device for both upper and lower, but this should be studied closer so I'm not sure of this idea, but again circuit complexity grows.

Michael

Michael,

the size of the FET is not really important, actual curves are. In my headphone amplifier I use a "smaller" device as the "lower" and a "larger" as the "upper". However it is not a rule - It could be other way around and I even did manage to get a good linearisation effect with two devices of the same type but with different "pinch off" voltages.

Alex

 

[Ultima Thule]

Alex,

it's obvious you have made a detailed work!

I get a feeling that the choice of lower JFET in itself is not as critical as the upper JFET (yes?), anyhow what is the special property of the upper device needed to give the lower device suitable working condition?

Have you noticed other diffrences the way your circuit work compared to the general cascoded JFET circuit such as noise, input capacitance, bandwith etc.?


Mike and Alex,

you both mentioned "pinch-off voltage, I guess if I interpret Alex idea right and if we would use those JFET's mentioned by Mike to adopt the idea then 2SK246 would be used as the lower device and 2SK170 as the upper device as an example.
I noticed for both JFET's mentioned here that the pinch-off voltage @ Vds = 10V and Id = 0,1 uA is varying a lot noticing Min and Max value.

Thanks guys!

Michael