Find the MOSFET bias point

1. Dec 27, 2013

Jalo

1. The problem statement, all variables and given/known data

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

2. Relevant equations

3. 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.

2. Dec 27, 2013

rude man

Not a good start. Need to fix that first.

3. Dec 27, 2013

Jalo

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

4. Dec 27, 2013

rude man

Better, but still not right.

5. Dec 27, 2013

rude man

That equation is also wrong.

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

6. Dec 27, 2013

Jalo

Thanks! I finally got it right.

VG = VDD (R2-R1)/(R2+R1)
VGS = VG - RsID + VDD

ID = k(VGS-VT)2

Replacing VGS 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?

7. Dec 27, 2013

rude man

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