How Can I measure MOSFET ro(output resistance), ID(DC current) at LTSpice?

[GMSong]

The subject is to make ro=500k, ID=100u by setting VG3,VG4 and W/L of MOSFET.

And I got the exact values from Hand-calculation, and am trying to test it by using LTSpice.



I did make this schematic. and DC currents of two mosfet was around 74uA (I don't know it is right way to measure...)

And the second, I tried to make V/I curve for measuring R values(blue line) It seems that it remarks something meaningful.. but I don't think it is values of output resistance because it is DC resistance...

How can I measure ro(output resistance) and DC current of PMOS-current-source?

 

[FvM]

You can measure impedance e.g. with .tf command. Circuit voltages and currents can be displayed with .op command or by pointing on the nodes and component terminals with cursor.

 

[GMSong]

You can measure impedance e.g. with .tf command. Circuit voltages and currents can be displayed with .op command or by pointing on the nodes and component terminals with cursor.

View attachment 169194

You can measure impedance e.g. with .tf command. Circuit voltages and currents can be displayed with .op command or by pointing on the nodes and component terminals with cursor.

View attachment 169194

Thanks a lot. I found the Oasis.
I got around 2700k ohm at this simulation, and the hand calculation value is 500k ohm.
Is it resonable to tolerate this difference?
(I thought it remarks quite right value because It soars up when I off the transistor)

 

[KlausST]

Hi,

"R" or "r"?
Do you want to know DC resistance "R"?
Or do you want AC (differential) resistance "r"? (At which operating conditions?)

Klaus

 

[FvM]

"R" or "r"?

The thread is clearly about ro (small signal), operation point is defined by the bias conditions and transistor parameters. SPICE .tf command also gives small signal impedance.

I got around 2700k ohm at this simulation, and the hand calculation value is 500k ohm.
Is it resonable to tolerate this difference?

You didn't specify complete transistor parameters respectively your way of hand calculation, thus can't comment.

 

[GMSong]

Hi,

"R" or "r"?
Do you want to know DC resistance "R"?
Or do you want AC (differential) resistance "r"? (At which operating conditions?)

Klaus

I wanted to know AC resistance r and I got my work Thank you!