Choosing JFET bias voltage from transfer curve

Thread Starter

SamR

Joined Mar 19, 2019
5,019
(subscript)

I(DSS)= 14.8mA
V(GS(off))= -3.2V

It's not a linear curve so V(GS(off))/2 doesn't seem right or I(DSS)/2? Do I just SWAG it? Eyeball the curve and pick a number about the middle?
 

Audioguru

Joined Dec 20, 2007
11,248
The written spec's for a Jfet show minimum, typical and maximum I(DSS) in a pretty big range. The V(GS(off)) has even a larger range.
The curves show different amounts of spec's and maybe you do not know if your Jfet has minimum or maximum spec's.
 

Thread Starter

SamR

Joined Mar 19, 2019
5,019
The problem states for me to determine a value near the middle of the curve. So I assume it's a SWAG by just picking a value somewhere around the middle of the curve I was given. However the curve is exponential so it won't be dead in the middle of the value range? Or just go with -1.6V? which is V(GS(off))/2? Or am I overthinking this?

Thanks for answering.

Sam
 

Audioguru

Joined Dec 20, 2007
11,248
Why chose a value in the middle of the range when you do not know if your Jfet is minimum, middle or maximum? Design the circuit so that negative feedback allows the circuit to work properly anywhere in the range of spec's.
 

Thread Starter

SamR

Joined Mar 19, 2019
5,019
Because I don't have a JFET, it's a book exercise to learn about JFET amps. I finally got smart and peeked at the answer in the back of the book. They used the V(GS(off))/2 = -1.6V value so I guess that answers my question. Even though it's NOT the middle of the transfer curve. I tend to be too literal at times and overthink it... Anyway back to my "homework". Thanks for the input and the amp feedback lesson is still to come.
 
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