JFET specifying

[dogears]

This is a basics question: how does one go about specifying a JFET for a specific circuit? Specifically in a traditional shunt or variable resistor type compressor. I see different JFETs used in different circuits, but often the devices themselves are very broadly or underspecified, so it's hard to understand why one particular JFET is used in one or another application. It becomes more relevant as more and more vintage devices are obsolete and unavailable - how do you go about making substitutions, short of trial-and-error?

 

[JohnRoberts]

Back in the good old days there were device data sheets that specified "family" characteristic designs for JFETS (including pictures of the dies)...

There were popular devices already being used and occasional new devices that needed to be bench tested to see how they work in a given application.

Caveat, some new device that is dramatically better for one parameter (like noise) may be worse in some other (like capacitance), bench testing will generally help sort out the wheat from the chaff.

JR

 

[Matador]

how does one go about specifying a JFET for a specific circuit?

It's similar to how tubes are specified (IMHO). You have AC drain impedance (like rp for a tube), gm (transconductance, again just like a tube), and mu (u, or amplification factor), again, just like a tube. I kind of think you can think of different JFET's just like different tubes. Some have lower amplification factors, less gain, however can swing between wider gate voltages without hitting a supply rail.

I'm not sure there is a general way to answer.

 

[dogears]

yeah, it is an admittedly vague question. i suppose the challenge is the fact that a lot of the datasheets are extremely brief.

for example - I've seen BF245A, 2N5457/8, and 2N3819 all used in various compressors, more or less in the same application - gate voltage as variable resistor. I'm too ignorant to figure out why one vs the other were selected by the designers. What would I look at in the datasheet there as to why one or the other was picked?

https://www.onsemi.com/pdf/datasheet/bf245a-d.pdfhttps://www.onsemi.com/pdf/datasheet/2n5457-d.pdfhttps://www.mouser.com/datasheet/2/68/2n3819-31474.pdf

 

[merlin]

The two important criteria are usually:
1: Does it need really low 'ON' resistance or is a couple of hundreds ohms nothing to worry about?
2: Is the gate control voltage range restricted (like in a 9V guitar pedal) in which case you probably need a very small Vgs(off), or is Vgs(off) nothing to worry about because you have an enormous 15V control voltage window or whatever?

 

[JohnRoberts]

This is a basics question: how does one go about specifying a JFET for a specific circuit? Specifically in a traditional shunt or variable resistor type compressor. I see different JFETs used in different circuits, but often the devices themselves are very broadly or underspecified, so it's hard to understand why one particular JFET is used in one or another application. It becomes more relevant as more and more vintage devices are obsolete and unavailable - how do you go about making substitutions, short of trial-and-error?

OK when specifying a JFET for shut applications one criteria of interest is the Vgs law (on resistance vs gate voltage). As I recall there were short channel and long channel JFETs a crude characterization of the gate laws. A good start is to look for popular devices already used for similar applications.

JR