# question about mosfet circuit homework

Discussion in 'Homework Help' started by omri karni, May 8, 2008.

1. ### omri karni Thread Starter Member

May 8, 2008
12
0
hello my friends

my teacher gave to me I must say very hard mosfet problem

firs of all I want to know if this site will help me with my question?

thanks for the helpers...

omri karni

2. ### recca02 Senior Member

Apr 2, 2007
1,211
1
I'm pretty sure the members here will help. It would e easier if you could post a schematic(if there is one) and your attempt at solving the problem.

3. ### omri karni Thread Starter Member

May 8, 2008
12
0
hello again
I put here the question ;so you can see that I need find the currents: Id Is Ig (as you can see Id and Is are equal) and the voltages: VRD VRS (this is the main problem)
thankes

omri karni

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4. ### pinnacle06 Member

Feb 9, 2007
16
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Ig is pretty much given to you. For Id, you should probably make an assumption about the region of operation the transistor is in and then start with the proper equation for Id. You will have 2 unknowns in your equation, so based on the given schematic, you will have to come up with another way to express one of them (ie Id). Then you can solve for a single unknown and then solve for the current and go back and check your orginal assumption. That's what I would do anyway.

5. ### omri karni Thread Starter Member

May 8, 2008
12
0
First of all thanks.
I tried to solve the id current with the saturation level equation and then I've got negative current it's unpossible for this condition
how come ??
Do you think that I need to use the Triode level equation ?
If you will try to solve it ,it's will be great.

6. ### pinnacle06 Member

Feb 9, 2007
16
0
I would try using that equation again and see what you get. If you post your steps, I will take a look at it.

7. ### Caveman Senior Member

Apr 15, 2008
471
1
Just solve it like this

g*(Vg - Vs)^2 = Vs/Rs = Is
Vg = 5V is given.
g = 1mV/V^2 is given
Rs = 6k is given
Solve for Vs

It comes out to a quadratic equation with two solutions.
Now using those solutions only one will work with the rest of the circuit.
If you still can't figure it out, please post your two solutions to the quadratic. That way we actually know if you are getting this far correctly.

8. ### omri karni Thread Starter Member

May 8, 2008
12
0
Thank you

I will wait for your answers .

It will be interesting .

omri karni

9. ### pinnacle06 Member

Feb 9, 2007
16
0
We are not going to give you the direct answer, caveman told you what to do. We have already solved these types of problems way more than once. If you post your steps, I will help you out.

10. ### omri karni Thread Starter Member

May 8, 2008
12
0
o.k firs of all I want to thanks you all for help.

I posted my answer steps as you can see I think that now will be easier for

you to help me to solve this question.

omri karni

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11. ### pinnacle06 Member

Feb 9, 2007
16
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looks like a math error.... it came out correctly for me

12. ### Caveman Senior Member

Apr 15, 2008
471
1
Your equation is right, but the answer is not negative. Multiply it all out and try the quadratic equation. If you still have trouble post your results.

13. ### omri karni Thread Starter Member

May 8, 2008
12
0
Are you shure that it's came out correctly for you?
I tried alot of time to solve it but it didnt work.

I'm realy dont know what to say.

If someone know what is the problem please help me to solve it.

Thanks;

omri karni

14. ### Caveman Senior Member

Apr 15, 2008
471
1
Multiply it out and put it online! Post your results.

15. ### omri karni Thread Starter Member

May 8, 2008
12
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Here are the reasults...

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16. ### Caveman Senior Member

Apr 15, 2008
471
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The line right after "technique it's become negative" is wrong. The last term should have Id^2.

17. ### omri karni Thread Starter Member

May 8, 2008
12
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As you can see I opened the brackets; it was (4-6*10^3id)^2

and after the opening it's became (16-48*10^3id+36*10^6id).

so I think that I opend it's well dont you think??

18. ### Caveman Senior Member

Apr 15, 2008
471
1
No, you messed it up.

(x-y)^2 = x^2 - 2xy + y^2
x = 4
y = 6*10^3*id
y^2 is wrong, it should be 36*10^6*Id^2.
You have to square the Id term.

19. ### omri karni Thread Starter Member

May 8, 2008
12
0
Thank you very much Caveman I succeed to solve this question.

I cant belive on my self that I did this stupid math mistake .

thanks

omri karni