We know that output cascade of Rail-to-Rail opamp use common emitter.Common emitter has very high output resistance because two collectors connect in output. But when I measured output resistance of Rail-to-Rail opamp in micro-cap 9,I get low out put resistance . I want to know , why I get this result? Please explain me.The measurement result of output resistance of Rail-to-Rail ( AD8031 ) is shown in below.

AD8031
​

 

I think Rout is low thanks to voltage negative feedback.
Negative feedback decreases the output resistance by a factor ( 1 + β A_ol)
A_ol is a open loop gain
β - feedback factor ( feedback gain )
So
Rout = Rout_open_loop/(1+A_ol*β)

 

thanks your answer. But it is without negative feedback. It is open loop output resistance measurement of rail-to-rail(AD8031).

 

thanks your answer. But it is without negative feedback. It is open loop output resistance measurement of rail-to-rail(AD8031).

Post the schematic of your measurement technique.

 

Post the schematic of your measurement technique.

Output measurement of rail-to-rail (AD8031) in micro-cap 9 is shown in below.

 

How did you come up with 531μV for the input offset voltage?

 

How did you come up with 531μV for the input offset voltage?

Input offset voltage effect the output resistance and other parameters.In this case, the offset voltage is so small and can neglect it. Output resistance measurement of rail-to-rail (AD8031) is shown in below, as we assume input offset voltage is zero.Clearly, I get same value of open loop output resistance.

 

I think your op amp is saturated at one of the rails, so you are seeing the resistance of a saturated transistor.
For the simulation, try running the op amp with unity DC feedback, but open loop for AC. You can do this by connecting a 10k resistor from output to -in, and a 1000 Farad cap from -in to GND. Connect +in to GND, and use the power supplies you show in your schematic.
I have done this with many op amps. LTspice does not converge for this op amp. Perhaps your simulator will.

 

I think your op amp is saturated at one of the rails, so you are seeing the resistance of a saturated transistor.
For the simulation, try running the op amp with unity DC feedback, but open loop for AC. You can do this by connecting a 10k resistor from output to -in, and a 1000 Farad cap from -in to GND. Connect +in to GND, and use the power supplies you show in your schematic.
.

I did simulation as you said , but result is same.If my simulation is wrong,please reply me.Thanks for your help.

 

Your sim circuit looks OK. How do you measure Rout? Is there an equation you use?