# MOS Gate and Drain Shorted Problem solution

#### hoyyoth

Joined Mar 21, 2020
312
Dear Team,

May I know how to find the voltage across 1K resistor in the below circuit without using simulation

#### Ramussons

Joined May 3, 2013
1,417
The data sheet of that MoSFET must indicate what is the Vsd of the Device in Saturation.

#### ronsimpson

Joined Oct 7, 2019
3,149
The voltage across R1 is very dependent on what MOSFET is used. Also depends on temperature. The data sheet for this part is needed but even then the voltage is only approximant.

If R1 was across V1 you will have 40mA. I know from experience that you will get closer to 35mA. So lets say M1 D-S current is about 35mA. In the data sheet there is a Gate Threshold voltage listed for current of 10uA (or some very low current). There should be a graph of Gate Voltage, Voltage D-S, and Drain Current. There is probably not a graph-line for 35mA. You will have to guess.

Is this a school project?
If you use SPICE to find the answer, pick a real MOSFET. Using a "NMOS" may not work well.

#### hoyyoth

Joined Mar 21, 2020
312
The voltage across R1 is very dependent on what MOSFET is used. Also depends on temperature. The data sheet for this part is needed but even then the voltage is only approximant.

If R1 was across V1 you will have 40mA. I know from experience that you will get closer to 35mA. So lets say M1 D-S current is about 35mA. In the data sheet there is a Gate Threshold voltage listed for current of 10uA (or some very low current). There should be a graph of Gate Voltage, Voltage D-S, and Drain Current. There is probably not a graph-line for 35mA. You will have to guess.

Is this a school project?
If you use SPICE to find the answer, pick a real MOSFET. Using a "NMOS" may not work well.
Thank you .
This is an interview question

#### Papabravo

Joined Feb 24, 2006
21,264
Thank you .
This is an interview question
I don't think the answer is particularly revealing regardless of how it is answered. You or they need better questions.

#### ronsimpson

Joined Oct 7, 2019
3,149
The simplest answer is that it takes a "Gate Turn on Voltage" to get the MOSFET to turn on. It takes 4 or 5 volts across G-S to turn on the part. That voltage must come from the 40V. So the Source must be below the 40V by that 4 volts.

Another way to look at it. Think of a junction transistor. They take 0.6V (approx) to turn on the transistor. So the Emitter must be below the supply by about 0.6V.

#### hoyyoth

Joined Mar 21, 2020
312
The simplest answer is that it takes a "Gate Turn on Voltage" to get the MOSFET to turn on. It takes 4 or 5 volts across G-S to turn on the part. That voltage must come from the 40V. So the Source must be below the 40V by that 4 volts.

Another way to look at it. Think of a junction transistor. They take 0.6V (approx) to turn on the transistor. So the Emitter must be below the supply by about 0.6V.
View attachment 274741
Thank you.I think he is expecting something like this. I was not able to give any answer. Try to play around MOSFET saturation
condition equations.But did not reach any conclusion.