# how to determine the small signal parameters(gm, ro) of nMOSFET from Datasheet?

#### TarikElec

Joined Oct 17, 2019
98
as I want to calculate the transfer function of circuit that is meant to generate source current using an Op amp as control loop, I need the gm and r0 of the mosfet(to determine the transfer function of the Op amp current source with nMOSFET) here is the picture of MOSFET with showed parameters

and here are the link for the datasheet
https://docs.rs-online.com/c5c9/0900766b80dcb3d2.pdf

#### crutschow

Joined Mar 14, 2008
32,062
You now see the large gap between the theoretical device equations and the info on a real world data sheet.
The only parameters given are gfs(gm) and Vtn (VGS(th).
The rest would need to be estimated or extracted.
The main reason for this lack of data is that power mosfets are usually used in switching applications were the small-signal AC parameters are not used.

#### TarikElec

Joined Oct 17, 2019
98
You now see the large gap between the theoretical device equations and the info on a real world data sheet.
The only parameters given are gfs(gm) and Vtn (VGS(th).
The rest would need to be estimated or extracted.
The main reason for this lack of data is that power mosfets are usually used in switching applications were the small-signal AC parameters are not used.
Yes, I always fell I lack so much knowledge wen it comes to practical design and feel frustrated. I do so much google search and looks into books but not happy in the end.
as I will use with it an op amp and others cap. I think I dont need the small signal but just a gain(please correct me if i am wrong). because in small signal, all the caps will be considered a short circuit which is not correct for calculating the transfer function

#### Jony130

Joined Feb 17, 2009
5,445
Show us a circuit so that we can help you with the transfer function.

#### crutschow

Joined Mar 14, 2008
32,062
I think I dont need the small signal but just a gain(please correct me if i am wrong). because in small signal, all the caps will be considered a short circuit
No.

In small signal analysis all reactive components are included, but a linear model is used for all active devices.
That analysis is normally used to determine the circuit gain/phase response.

In large-signal (transient) analysis, reactive components are included and a non-linear (large signal) model is used for all active devices.

In a DC (or initial DC bias) calculation, capacitors are open and inductors are shorts.