FET- trouble with finding Id

[KUMARA SHP]

here is the question. I did it as

I used Id = Iss (1-Vgs/Vp)^2 (1+lemda(Vds))

used Vgs= 0.7-Id*680
Vds = 18 - Id*6800 -Id*680
after I solved it using wolfarm alpha
gaves me real solution Id=0.05869

but problem is when I try to find Vds and Vgs they are big negative values ,bigger than 18V

there was another complex solutions for Id

how to choose Right Id ???

if it complex what is the value of Id?

 

[Harald Kapp]

gaves me real solution Id=0.05869

0.05869 what? A, mA, µA?

The static solution (DC value) will definitely not be complex, but real.

 

[KUMARA SHP]

I got 3rd power equation for Id after solve it, answers were

59.39
-0.0829-1.01 i
-0.0829 - 1.01 i
all are in mA
but any value is not give correct things
59.39mA makes big -Vds
what is correct one?

 

[KUMARA SHP]

I have really strucked on that about 4hrs.
I redo same calculations , always same result
where is the wrong?

(also extra question, - for voltage follower mosfet , Vg=Vs? )

 

[KUMARA SHP]

there is something wrong. please check the solution without considering lemda part
it also gives two complex solutions for Id
how it's happens?

 

[Laplace]

Have you defined polarities such that the term Vgs/Vp is always a positive number?

 

[KUMARA SHP]

noo. I don't know that
do you mean - mark can be + in the equation (1-Vgs/Vp)

 

[KUMARA SHP]

it did for a JFET. Vgs should be - value for JFET, am I correct?
therefore I have to make - sign to plus?
is that you mentioned?

 

[Laplace]

No. Just that Vp is one particular value of Vgs so the ratio must be positive. The whole equation doesn't make any sense otherwise (in the region of interest).

 

[KUMARA SHP]

ok since this is JFET Vgs always negative (or zero)
therefore I should use Vp=1.372 given value as -1.372 when substitute in equation
it that all
thank you for help
really feel free now

 

[KUMARA SHP]

it worked master
Id is about 1mA
so it is correct
thank you all
really appreciate this
also I will try to answer others too