# DC Current of MOSFETs

Status
Not open for further replies.

#### ali.mirvakili

##### Newbie level 3
Is there any simple way to get approximately close result in hand calculation to the result from simulation in Cadenec or Hspice?!

The result of hand calculation may never be the same as simulation! The main result is that, for instance in the case of a MOSFET, in hand calculation we just consider 3-4 parameters, while in simulation there are about 49 parameters!

We do know that using the common formulas, which are brought in reference books, does not provide a close result to that of simulation!

Thanks,

#### keith1200rs

##### Super Moderator
Staff member
Exact DC Current of MOSFETs

I don't understand the problem. The current in a MOSFET can be probed the same as the voltage or current in a resistor or other component, surely?

Keith

Hmm. It seems you changed the question after I answered!

#### ali.mirvakili

##### Newbie level 3
Yes, you are right. But the problem is that, the result of hand calculation is just a good point for start. This is different from the result from simulation in e.g. cadence.

In other words, softwares like cadence or pspice, consider lots of parametres in calculation, while in hand calculation we have just 3-4 parametres. That's why these two results are not the same.

#### FvM

##### Super Moderator
Staff member
I guess, you are refering to MOSFET drain current. It's mainly a function of Vgs, less sensitive to Vds and also depends on
temperature. You can refer to simplified principle MOSFET characteristics from literature, if appropriate.

The result will be different from SPICE simulation and even more different from the empirical results with a real component
unless you don't consider the individual Vth parameter. I guess, you know that already and in so far I wonder, if you intentionally

Obviously, "hand calculation" has a different purpose than simulation.

#### ali.mirvakili

##### Newbie level 3
OK. Thank for your comment. You are completely right.

My question was:

Is there any simple way to get approximately close result in hand calculation to the result from simulation in Cadenec or Hspice?!

This means that result of hand calculation and simulation is not the same. And it is asking about hand calculation since the reason why these two are not the same comes back to hand calculation.

Anyway, I have just recently found that one way is entering one more parameter like η in the DC current formula. In this way, the new formula could be like:

I = 0.5 µ cox (W/L) (Vgs-Vth)^2 * [1/η]

In other words, with comparing result of hand calculation and simulation result, we can find the value of η.

But, as you know, it is still prone to error.

I am wondering if anybody discover other better ways!

Anyhow, I thank you for your nice and good points.

Good Luck,

#### FvM

##### Super Moderator
Staff member
If these extended equations are of any use depends completely on the purpose of hand calculation. If you want to predict
a bias point, you're mainly stuck with unknown individual Vth. In this case, the extended formula is simply useless.

#### umesh49

##### Full Member level 4
You can't do hand calculation for each and every parameters, thats why people use simulation software. Offcource you must do some hand calculation to verify whether simulation is showing proper results. Those parameters could be some peak transient voltage/current, input/output power balance, KCL at nodes etc.

Umesh.

Status
Not open for further replies.