JFET confusions...

[Thomas Juth]

Hi there!

I am currently trying to learn more about JFETs, their operation and characterisics, but have encountered a few confusions.

1. The first one is the relation between the effective Drain resistance and the Output characterisics graphs. Does this resistance (when kept in the ohmic region) increase if we follow the graphs to the right (against Vp) or to the left (down towards the 0 point)?

2. I read that an increased negative gate results in a increased channel resistance, which in turn leads to a decreased conductance of the channel (Id). But I also read that an increased R(fet) (also increased with more negative gate) gives an increased V(out). What is the relation between these two? Are they two different resistances?

3. Another issue is the fact that an increased Vds (according to the books) decreases the Id (the channel conductance). But when I look at these graphs I see that the more Vds (more to the right), the more Id you get.

I would be very happy if someone could explain this to me...

Cheers!

/Thomas

ps. thank you very much AD202! ds.

 

[Thomas Juth]

 

[PRR]

> What is going to the o/p?

Well, usually the input audio, since you seem to be talking about a FET attenuator.

Post or point-to a schematic of what you are looking at, or else someone may answer questions you are not asking and confuse the discussion.

 

[Mark Burnley]

Hi Thomas,

Welcome to The Lab!

I found this paper from Interfet useful:

http://www.interfet.com/pdf/App_Notes.pdf

...there's a bit near the end about using FETs as voltage-controlled attenuators.

Have you studied the Gyraf 1176 schematic? Check out here for schems and info.

(Glad you found us okay :wink: )

:thumb:

Mark

 

[Thomas Juth]

Thank you very much for that JFET article...very useful!

- From the arcticle I got that Id decreases with increased Vg, while the Rds increases. But what about Vds, does it alwas follow the i/p (Vgs), as it is said to be propotional to the o/p voltage? Or is it set around, let say, 100 mV without changing much? What's its relation to a change in Vgs?


Cheers!

/Thomas

 

[Thomas Juth]

:shock: :sad: :grin:

 

[PRR]

http://www.eecg.toronto.edu/~kphang/papers/2001/martin_AGC.pdf page 8