[sdiy] 2SK170 FET alternative

[Seb Francis]

Thanks Jay, this is helpful, but looking at the circuit it seems to me 
that the transfer function is very non-critcal ..
http://www.thatcorp.com/datashts/dn115.pdf

The capacitor C18 charges or discharges to whatever voltage is needed to 
get the output of the FET (-ve comparator input) to match the 
compression level voltage at the +ve comparator input.  At the point 
when these voltages are equal the comparator is 0V, so both the the 
lower (attack) and upper (release) current mirrors output flows only to 
the comparator and not to/from the capacitor.

When the compression level voltage changes, the comparator sources/sinks 
the current to/from the lower/upper current mirror, 'steering' the 
current from the other mirror to the capacitor C18 which 
charges/discharges at a rate determined by the current mirror (i.e. 
attack or release rate).  This occurs until the output voltage of the 
FET stabilises to the compression level voltage.

Aaaah!  Having just written this I now think I see the problem!  The 
gate-source voltage must always be less than a diode drop away from 0V, 
otherwise the diodes don't work to steer the current away from the 
capacitor C18.  This would explain why I'm getting a nice high frequency 
oscillation going on in my circuit when using a 2SK30A.

So I think I can make it work by just changing the resistor R34 to allow 
sufficient voltage swing at the FET output without the VGS getting too 
negative ... hmm, just off to calculate some values!

Seb


jays at aracnet.com wrote:

>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:
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>
>>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.
>>>
>>>      
>>>
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