Hi, can someone please help. I'm studying for my exam and i have no idea how to solve this Jfet circuit

 

Do you manage to identify the JFET type?
Do you know the equation which describe Id vs Vgs ?
Id = K*(Vgs - Vp)^2
And if you look at the circuit Vgs is equal to ??

 

Yes i did. its a P-channel Jfet. I do not no what vGS is equal to. I said the current, ID=vs/100 and VGS=Vs(vs is the voltage across the 100 ohm resistor ). plugged both into the equation and got a value for Vs. from that value i got the current ID, and then the voltage Vr. is this correct? is the 20V the value of Vss?

 

It look like:
VR1+Vsd+VR=20 volts
VR1+Vsd+R2*Id=20 volts
VR1+Vsd+R2*[K*(Vgs-Vp)^2]=20 volts
It looks to me like VR1 is Vgs. If so:
Vgs+Vsd+R2*K*(Vgs-Vp)^2=20 volts
R2 is known
K is known
Vp is known
If we knew Vsd, we could solve for Vgs, then plug Vgs into Id formula, then use Ohm's Law for VR=R2*Id.
I think I am missing some assumption somewhere. Either for Vgs or Vsd.

 

It look like:
VR1+Vsd+VR=20 volts
VR1+Vsd+R2*Id=20 volts
VR1+Vsd+R2*[K*(Vgs-Vp)^2]=20 volts
It looks to me like VR1 is Vgs. If so:
Vgs+Vsd+R2*K*(Vgs-Vp)^2=20 volts
R2 is known
K is known
Vp is known
If we knew Vsd, we could solve for Vgs, then plug Vgs into Id formula, then use Ohm's Law for VR=R2*Id.
I think I am missing some assumption somewhere. Either for Vgs or Vsd.

The idea is to find Id first using the known conditions with the FET in the saturation region - for (say) the case of R2=1K. In this case Id would notionally be independent of R2 for ideal behaviour. Since the circuit behaves very much like a current source one is then required to consider the limiting conditions as the load R2 increases and the FET enters the linear (triode) region.

 

The idea is to find Id first using the known conditions with the FET in the saturation region - for (say) the case of R2=1K. In this case Id would be independent of R2. Since the circuit behaves very much like a current source one is then required to consider the limiting conditions as the load R2 increases and the FET enters the linear (triode) region.

Isn't in saturation Vgs=0?
So Id=0.001(0-4)^2=0.016 A

 

Isn't in saturation Vgs=0?
So Id=0.001(0-4)^2=0.016 A

FET's are considered as operating in the active region when in saturation. Vgs is the controlling parameter which in relation to the pinch-off voltage determines the active region saturation current.

At Vgs=0 the device current is the parameter Idss - the saturation current at zero gate-to-source voltage.

 

Yes i did. its a P-channel Jfet. I do not no what vGS is equal to. I said the current, ID=vs/100 and VGS=Vs(vs is the voltage across the 100 ohm resistor ). plugged both into the equation and got a value for Vs. from that value i got the current ID, and then the voltage Vr. is this correct? is the 20V the value of Vss?

Very good except one small mistake.
Vs = Vss - Id*R1 and Vgs = Vss - Vs so from this we can say that Vgs = Id*RS.

So we have

Id = K*(Vgs - Vp)^2 = Vgs/R1

And after we solve this

http://www.wolframalpha.com/input/?i=10^-4*%28X+-+4%29^2%3DX%2F100

We have Vgs = 0.14835V and Id = 1.4835mA

 

Isn't in saturation Vgs=0?
So Id=0.001(0-4)^2=0.016 A

No; JFET is in saturation when Vds > (Vgs - Vp)

And also check your math

K = 10^-4 = 1/10000 = 0.0001

 

What happens if both Vgs values where smaller than vp? how will you know which one to choose?
thanks

 

What happens if both Vgs values where smaller than vp? how will you know which one to choose?
thanks

Very unlikely to happen. But maybe you have some example ?