What is the MOSFET bias point?

[Jalo]

Homework Statement

Given the following circuit find the bias point (VDS,IDS).

Homework Equations

The Attempt at a Solution

The first thing I did was find the value of VG, the voltage from the gate of the mosfet to the -VDD source.

VG = VDD * R2/(R1+R2)

To find the current IDS I needed the voltage from the gate to the source, VGS, which is:

VGS = VG - RsID

Since we're operating in the saturation region I can replace ID by:
VGS = VG - Rsk(VGS-VT)^2 (1+λVDS)

where VDS is the voltage between the drain and the source.
I also know that:
VDS = 2VDD - (Rd+Rs)ID

This is as far as I can get. I'm a little confused regarding what I should do next.

If anyone could help I'd appreciate.

Thanks.
Daniel.

 

[rude man]

Homework Statement

Given the following circuit find the bias point (VDS,IDS).

Homework Equations

The Attempt at a Solution

The first thing I did was find the value of VG, the voltage from the gate of the mosfet to the -VDD source.

VG = VDD * R2/(R1+R2)

Not a good start. Need to fix that first.

 

[Jalo]

Not a good start. Need to fix that first.

Sorry, I meant to write 2V_DD instead of only V_DD.

 

[rude man]

Sorry, I meant to write 2V_DD instead of only V_DD.

Better, but still not right.

 

[rude man]

To find the current IDS I needed the voltage from the gate to the source, VGS, which is:

VGS = VG - RsID

That equation is also wrong.

You're having trouble with that "-V_DD".

 

[Jalo]

That equation is also wrong.

You're having trouble with that "-V_DD".

Thanks! I finally got it right.


V_G = V_DD (R₂-R₁)/(R₂+R₁)
V_GS = V_G - R_sI_D + V_DD

I_D = k(V_GS-V_T)²

Replacing V_GS in the expression (1) I managed to find the correct solution. I still have one doubt tho.
The value of λ is very small, only 0.02 V^-1. When the value is that low can I simply ignore it and solve the problem as if it was zero?

 

[rude man]

Thanks! I finally got it right.


V_G = V_DD (R₂-R₁)/(R₂+R₁)
V_GS = V_G - R_sI_D + V_DD

I_D = k(V_GS-V_T)²

Replacing V_GS in the expression (1) I managed to find the correct solution. I still have one doubt tho.
The value of λ is very small, only 0.02 V^-1. When the value is that low can I simply ignore it and solve the problem as if it was zero?

OK now.
Since you were given a number for lambda I would retain those values in your answer.