# N-Channel Mosfet (Find: Equivalent Resistance)

#### Fotic Pap

Joined Aug 24, 2019
11
Hi everyone,
After searching on internet and this forum i didn't find what im looking for, so i will make a second question and if anyone can help me i would be glad.

I have the below recuit (image attached).

Goal:
Find Equivalent Resistance when the switch is in position 1 and on position 2.

VDD = 3 V
VT = 0.2 V
K = 0.41 mA/V^2

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#### Jony130

Joined Feb 17, 2009
5,176

Also remember that:

Req = VDD/ID

Where:

ID = K*(VGS - VT)^2

#### Fotic Pap

Joined Aug 24, 2019
11

Also remember that:

Req = VDD/ID

Where:

ID = K*(VGS - VT)^2
Hello, thanks for the effort.
How can i find VGS?

Do i need to use this types below?
Switcher on position 1:
R1= VDD/ ID

Switcher on position 2:
R2= VDD/ IDSS

Last edited:

#### Jony130

Joined Feb 17, 2009
5,176
How can i find VGS?
Simply, just answer yourself a question:
What is the voltage between the gate terminal and the source terminal when the switch is in position 1 and 2?

Switcher on position 1:
R1= VDD/ ID
Yes.

Switcher on position 2:
R2= VDD/ IDSS
Yes.

Also, you should check if you MOSFET is in a triode region or in a saturation region.

#### Fotic Pap

Joined Aug 24, 2019
11
Simply, just answer yourself a question:
What is the voltage between the gate terminal and the source terminal when the switch is in position 1 and 2?
VGS = 3V ?
How can i find IDSS ?

#### Jony130

Joined Feb 17, 2009
5,176

#### Fotic Pap

Joined Aug 24, 2019
11
So the solving of this exercise, is the below?

ID = K*(VGS - VT)^2
= 0.41*(3 - 0.2)^2
= 3.21 mA

Switcher on position 1:
R1= VDD/ ID
= 3/ 3.21
= 0.91 kohm

Switcher on position 2:
R2= VDD/ IDSS
= 3 kohm

Do i need anything else?

#### WBahn

Joined Mar 31, 2012
25,911
So the solving of this exercise, is the below?

ID = K*(VGS - VT)^2
= 0.41*(3 - 0.2)^2
= 3.21 mA

Switcher on position 1:
R1= VDD/ ID
= 3/ 3.21
= 0.91 kohm

Switcher on position 2:
R2= VDD/ IDSS
= 3 kohm

Do i need anything else?
You need to track your units through each step of your work -- it will save you serious time and grief if you do.

ID = K*(VGS - VT)^2
= (0.41 mA/V²)*(3 V - 0.2 V)^2
= 3.21 mA

Switcher on position 1:
R1= VDD/ ID
= 3 V / 3.21 mA
= 0.91 kΩ

What is Idss? Why don't you put it in your final equation? You are implying that it is 1 mA, where does this value come from? What did Jony130 say about what we generally assume it to be if not provided?

#### Fotic Pap

Joined Aug 24, 2019
11
You need to track your units through each step of your work -- it will save you serious time and grief if you do.

ID = K*(VGS - VT)^2
= (0.41 mA/V²)*(3 V - 0.2 V)^2
= 3.21 mA

Switcher on position 1:
R1= VDD/ ID
= 3 V / 3.21 mA
= 0.91 kΩ

What is Idss? Why don't you put it in your final equation? You are implying that it is 1 mA, where does this value come from? What did Jony130 say about what we generally assume it to be if not provided?
Hello, thanks for the effort to help me out.

i really dont know what is this IDSS... i found a similar exercise that is using it when the switcher is on Position 2.
(i dont know if it is correct)
Thats why im asking how to find IDSS... and as Jony130 said if it not given assume it is 0(zero).