[sdiy] 2SK170 FET alternative

[Seb Francis]

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.
>
>-----Original Message-----
>From: owner-synth-diy at dropmix.xs4all.nl
>[mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of Seb Francis
>Sent: Tuesday, December 13, 2005 5:16 PM
>To: Synth DIY
>Subject: [sdiy] 2SK170 FET alternative
>
>
>Hi,
>
>I'm building a compressor based on this circuit:
>http://www.thatcorp.com/datashts/dn115.pdf
>
>It uses a 2SK170 FET as part of the adjustable attack/release bit in 
>between the RMS level detector and VCA.
>
>Neither of my 2 regular shops stock this FET, and while I'm sure I'd be 
>able to track one down, I'm wondering if a 2SK30A-Y would work instead. 
>They look fairly similar from the datasheet, but unfortunatly my 
>experience of FETs is pretty non-existant, so I wonder if some kind 
>person could cast a quick glance at these specs and tell me in what 
>situations the differences would matter (or not)..
>
>2SK170
>Gate cut-off current IGSS = −1 nA (max) (VGS = −30 V)
>Drain current IDSS = 2.6 to 20 mA (the FET suffix isn't specified on the 
>schematic so I can't be more specific about this)
>Gate-source cut-off voltage VGS (OFF) = -0.2 to -1.5V
>Forward transfer admittance Yfs = 22mS typ
>Input capacitance Ciss = 30pF typ
>Reverse transfer capacitance Crss = 6pF typ
>Noise figure NF = 0.5dB typ or 1.0dB typ
>
>2SK30A-Y
>Gate cut-off current IGSS = −1 nA (max) (VGS = −30 V)
>Drain current IDSS = 1.2 to 3 mA (this is for the Y suffix that I have)
>Gate-source cut-off voltage VGS (OFF) = -0.4 to -5.0V
>Forward transfer admittance Yfs = 1.2mS min
>Input capacitance Ciss = 8.2pF typ
>Reverse transfer capacitance Crss = 2.6pF typ
>Noise figure NF = 0.5dB typ
>
>The max ratings look very similar apart from the drain power dissipation 
>which is 4x higher (400mW) for the 2SK170, but I don't think I'm going 
>to be running the device anywhere near its max ratings.
>
>Seb
>
>
>
>
>
>  
>