# Need Help in finding ID or VGS for N-mos

Discussion in 'Homework Help' started by woon_h88, Mar 17, 2014.

1. ### woon_h88 Thread Starter Active Member

Mar 25, 2009
46
2

http://postimg.org/image/q5gx4qhxb/

Kinda stuck in solving either ID or VGS...

Its gave with Kn=2mA/Vsq, VTN =1V, RD=4KΩ, RG=100KΩ, VSS=-5V and VDD = 5V..

The current Is and Rs of the DC currnt source are 1mA and 1MΩ respectively..

Please slowly hint me as i can think properly..

Thank you...

Last edited: Mar 17, 2014
2. ### WBahn Moderator

Mar 31, 2012
18,087
4,917

Please show your work, as much as you can, so that we can see where you are going right and where you are going wrong.

3. ### BytetoEat New Member

Mar 5, 2014
25
5

current for a mosfet is:
Id=k/2Is[(Vgs-Vt)^2](1+λVds)

First tell me what is Rg going to do? Does any current flow into the gate?

Using the formula, if you set 1mA as Id, what variables do you need to solve for Vgs?

Edit: note the Is in the formula is for Isaturation, NOT Isource of your current source..

Last edited: Mar 17, 2014
4. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,505
512
They wrote that RG is 100 kOhm.

5. ### woon_h88 Thread Starter Active Member

Mar 25, 2009
46
2
Hi,
From Vds=Vdd-IdRd
i convent to Vgs=5-Kn/2 (Vgs-Vtn)^2, But soon i notice that my Vds is not equal Vgs..
They will be no current flowing in the gate, so Rg can be ignore...
if i use the formula, i still have Is and Vds to solve cos λ was given as 0.01V^-1. (sorry for didnt put the λ as i normally didnt use it often).
The most confusing to me is the current source part..So on this question i need to solve thru saturated stat?

Last edited: Mar 17, 2014
6. ### BytetoEat New Member

Mar 5, 2014
25
5
Good that you caught Rg is useless! My professor would put them in tests to throw students off who didn't understand no current flows into the gate.

Vds is not Vdd-IdRd, rather that is the voltage at the drain terminal of the FET.
The source terminal will be 0-Vgs
Vds is the difference across the drain and source, so it will be (Vdd-IdRd) - Vgs

The current source simply makes a constant current flowing through the FET and can be ignored for the most part. Don't worry about the parallel resistance from it, its super large.

Also, my appologies but i was mixing up the Bipolar formula and MOSFet one! Isat is only for BJTs.

This is the right formula.. Where UnCox/2*W/L = k/2(which you have)
Also, ignore Vdssat completely, just use Vds there

woon_h88 likes this.
7. ### woon_h88 Thread Starter Active Member

Mar 25, 2009
46
2
Sorry , suddenly my mind catch the link..Thank Thank..

Just to ask, which region is it?
I count Vds =3V and Vgs = -2V

For saturation, Vds>Vgs-Vtn, so i got 3V>-3V

so its in saturation region...just kinda stange cos its have a negative...on the right side...

Last edited: Mar 17, 2014
8. ### BytetoEat New Member

Mar 5, 2014
25
5
Ah thats cause i made another mistake.. its (Vdd-Id*Rd)-(-Vgs), making vgs add to it not subtract. This FET should be in saturation.. try to solve again and see if you get a positve number this time for Vgs. Sorry for my mistake