How do i calculate VDS? Is there an equation for it?

KeithWalker

Joined Jul 10, 2017
2,409
Yes, there is an equation for it. It is called "simple addition and subtraction".
On your diagram, mark the drain and source of each FET. Take a closer look at the voltages marked on the diagram and use the "equation" that I quoted.
Next time, try using your brain before you panic and post.:)
 

Thread Starter

subatomic particle

Joined May 8, 2018
65
Yes, there is an equation for it. It is called "simple addition and subtraction".
On your diagram, mark the drain and source of each FET. Take a closer look at the voltages marked on the diagram and use the "equation" that I quoted.
Next time, try using your brain before you panic and post.:)
I think you misunderstood my question. I meant how do i calcuate them by myself without using the simulator xD
I mean the picture was taken from a software that calcuated them. I want to calcuate by myself
 

dl324

Joined Mar 30, 2015
14,704
What type of devices do you think you're using? They're not standard symbols JFETs or MOSFETs. What do the colors on the symbols mean?

Your schematic drawing style is weird. Why do you have so many unnecessary wire jogs?
 

Thread Starter

subatomic particle

Joined May 8, 2018
65
What type of devices do you think you're using? They're not standard symbols JFETs or MOSFETs. What do the colors on the symbols mean?

Your schematic drawing style is weird. Why do you have so many unnecessary wire jogs?
These are two MOSFETS NMOS (the lower one) and PMOS (the upper one)
The colored triangle just means that they are in saturation
My question is: Lets suppose we have a random main Voltage and the two transistors are identical (same dimensions ..etc) and they have different Gate Source voltages. How can i determine the Drain Source voltage of each one? Is there an equation?
 

dl324

Joined Mar 30, 2015
14,704
These are two MOSFETS NMOS (the lower one) and PMOS (the upper one)
No they aren't. The arrow points in on N channel and out on P channel. When the vertical line representing the channel is solid, that indicates a depletion mode device. The line is dashed for enhancement mode devices.

1655774765394.png
 

Audioguru again

Joined Oct 21, 2019
4,805
Your Mosfets have no part number then you cannot see the wide range of conduction they have in their datasheet.
A simulation simply guesses that your Mosfet has "typical" specs. When you buy one then it might be very sensitive or have very low specs.

There are many opposing Mosfet symbols that are used:
 

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Thread Starter

subatomic particle

Joined May 8, 2018
65
Your Mosfets have no part number then you cannot see the wide range of conduction they have in their datasheet.
A simulation simply guesses that your Mosfet has "typical" specs. When you buy one then it might be very sensitive or have very low specs.

There are many opposing Mosfet symbols that are used:
If we assume that both MOSFETs are identical, how can i know how much voltage would drop on each of them? (We know Vgs1, Vgs2 and V of the voltage source (current is not known). How would you calculate the voltage in this case?
 

KeithWalker

Joined Jul 10, 2017
2,409
If we assume that both MOSFETs are identical, how can i know how much voltage would drop on each of them? (We know Vgs1, Vgs2 and V of the voltage source (current is not known). How would you calculate the voltage in this case?
If you were designing a MOS FET circuit, you would refer to the data sheet for the devices you plan on using.
In your diagram, there is not enough gate to source voltage on either device to allow any apreciable source to drain current to flow.
 

Thread Starter

subatomic particle

Joined May 8, 2018
65
1655817180825.png
Both mosfets are in saturation and the same current is flowing and the parameters where chosen exactly the same
SO why is the voltage drop on both mosfets different?
shouldnt it be 1.5V on each one?
 

KeithWalker

Joined Jul 10, 2017
2,409
Maybe the simulation is wrong but i am asking a general question about how to calculate the drain source voltage of two transistors connected as above
I am not familiar with the simulator that you are using. The "data sheets" that you show must be the device definitions that the simulator uses. They are meaningless to me. You will have to study how the simulator works to get the answer to your question.
In the real world, we use a data sheet like this to get the information on specific devices:

http://www.datasheet-pdf.com/PDF/IRFZ40-Datasheet-Vishay-966217
 

Jony130

Joined Feb 17, 2009
5,351
The hand calculation will be a pain in the ass. Because we have two current sources connected in series. Thus, the transistor which tries to "make the lowest current" will set the current in the circuit.
https://forum.allaboutcircuits.com/...tant-current-source-load.147146/#post-1253377
In the first iteration, we can assume that both transistors are in the saturation region.
Thus:
Id1 = Id2
So we need to solve this for Vds1.
W/L *Kp/2 * (Vgs1 - Vt)^2 (1 + λVds1) = W/L *Kp/2 * (Vgs2 - Vt)^2 (1 + λ(Vdd - Vds1))
Next, we need to check if our assumption about saturation was wrong or not. And if it turns out that one of the transistors is not saturated we need to repeat the calculations. Again assuming Id1 = Id2 but we need to plug the triode region equation for the one that is not in saturation.
 

Thread Starter

subatomic particle

Joined May 8, 2018
65
The hand calculation will be a pain in the ass. Because we have two current sources connected in series. Thus, the transistor which tries to "make the lowest current" will set the current in the circuit.
https://forum.allaboutcircuits.com/...tant-current-source-load.147146/#post-1253377
In the first iteration, we can assume that both transistors are in the saturation region.
Thus:
Id1 = Id2
So we need to solve this for Vds1.
W/L *Kp/2 * (Vgs1 - Vt)^2 (1 + λVds1) = W/L *Kp/2 * (Vgs2 - Vt)^2 (1 + λ(Vdd - Vds1))
Next, we need to check if our assumption about saturation was wrong or not. And if it turns out that one of the transistors is not saturated we need to repeat the calculations. Again assuming Id1 = Id2 but we need to plug the triode region equation for the one that is not in saturation.
Thank you :D that was a helpful answer
 
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