Output impedance of opamp

[countmybones]

Hi all, what we have learnt is that the output impedance of an opamp is very small.
I cannot seem the find this information in the datasheet of the opamp I am looking at.
May I know how can I measure the output impedance of an opamp using only spice?

 

[FvM]

Are you talking about the output impedance of the OP or the closed loop output impedance of a feedback OP circuit?

The output impedance of the OP itself is not very small, usually in a several ten to hundred ohm range. Both values can be measured in AC simulation like any impedance e.g. by injecting a current. The measurement method is basically the same as discussed for OTAs in this recent thread:

https://www.edaboard.com/threads/267627/

 

[countmybones]

Hi FvM, is the attached figure the correct way to do it?
The op-amp inputs are shorted to ground because if we want to evaluate the effects of the current source, all other sources must be made 0, from superposition theorem, right?
Then plotting the output voltage/current is the output impedance of the op-amp, 42 Ohm in this case.
Is this value accurate compared to the physical measurement?

And then going by the same logic, I can find the input impedance by putting the current source to the Vin+ and Vin- of the opamp.

 

[FvM]

Normally, the impedance measurement setup will need a DC bias. It depends on the OP model what happens with shorted inputs. With a real OP, this would cause output saturation.

In any case, it's wrong to set 1 A DC for the current source.

Additional comment, a simulation can only reveal what's been put into the model. I won't expect correct output impedance numbers with simple behavioral models like LTC2054.

 

[Junus2012]

Hello.
you are talking about simulating or measuring ? . for simulating , FvM has discussed this concept very well in the post he mentioned also countmybones gave you the exact circuit.

this method will work very fine for simulation but I am not sure if you can apply it for real measurement setup, what you say ?

Hi all, what we have learnt is that the output impedance of an opamp is very small.
I cannot seem the find this information in the datasheet of the opamp I am looking at.
May I know how can I measure the output impedance of an opamp using only spice?