[sdiy] 2SK170 FET alternative

[jays at aracnet.com]

Seb,

There are two things that I think that you need to look at. First is where the linear range is of the JFET and next is the amount of signal on the input. The 2SK170 is higher gain so the slope of the transfer function is steaper so it's input requirements are less than a 2SK30. It's input goes from ~0.5V to 0.0V VGS (off to on) so if you pick the center point in its VGS curve ~0.15V. For the 2SK30 its VGS range is ~-2.8V to 0V with a center at ~-0.8V.

So you'll have to change the input level and the bias point.

Also I don't know how they are using the JFET in this case. It could just as a buffer or it could be more. They maybe relying on the fact that JFETs don't clip as hard as bipolar transistors and using that as part of the compression.

When I do something like this I just breadboard that part of the circuit and get it to the point where I think it works. Then I go on and do the rest and see how it really behaves. What looks good on the scope may not sound so good.

Hope that makes sense.

Jay S.

Seb Francis wrote:

> Thanks Jay, those articles look very useful.  I also went to bed last 
> night with The Art Of Electronics chapter on FETs, so I'm feeling a bit 
> less ignorant today!
> 
> The 2SK170 is used as a buffer with the input voltage connected to the 
> Gate, V+ connected to the Drain, and a resistor between V- and the 
> Source.  Am I right in thinking that to compensate for using a JFET with 
> a different transfer curve (VGS) I can just change the resistor value?
> 
> Seb
> 
> 
> Jay Schwichtenberg wrote:
> 
> >They have different transfer curves (VGS) so I wouldn't consider them exchangeable.
> >
> >Here are a couple of good articles on JFETS:
> ><http://www.borbelyaudio.com/special_articles.asp>
> >
> >Check out JFETs, The New Frontier Parts 1 and 2.
> >
> >Jay S.
> >