Closed loop output resistance of an OP-AMP.

[Mad I.D.]

Hi.

I'm using an opamp as a buffer (G=+1) and simulating its closed loop output resistance in LTspice.
OpenLoop gain is depicted in the picture attached. What bothers me is the rate of increase in the output resistance. If you take a look at the OpenLoopGain, you can see that at 100k, gain drops approximately 2dB.

Now if you take a look at the same 100k frequency in the output resistance graph, you can see that output resistance increases by a mere 4-5dB. How is this possible?
I know from theory that output resistance is equal to Ropen_loop/ (1 + LoopGain).

That is, if a loop gain drops 2dB, one would expect 2dB increase in the output resistance. What's wrong with my reasoning ?

Thank you.
Ivan from Belgium.

 

[LvW]

...................
If you take a look at the OpenLoppGain, you can see that at 100k, gain drops approximately 2dB.

Please, can you verify/explain this statement?

 

[Mad I.D.]

...................
If you take a look at the OpenLoppGain, you can see that at 100k, gain drops approximately 2dB.

Please, can you verify/explain this statement?

Of course, first, sorry for the typo lopp->loop.
Picture OpenLoopGain.jpg contains the graph of the open loop gain. At the frequency of 100kHz, the gain drops approximately 2dB with respect to the gain at DC.
Another picture shows the output resistance. At the same frequency, i.e. 100kHz, the resistance already increases by 4-5dB. One would except increase of 2dB, well, at least with my reasoning.

I hope the matter is more clear know. Thank you for trying to help.

Ivan

 

[LvW]

Picture OpenLoopGain.jpg contains the graph of the open loop gain. At the frequency of 100kHz, the gain drops approximately 2dB with respect to the gain at DC.

I am afraid, you mix up 100 Hz with 100 kHz, don't you?

 

[Mad I.D.]

Picture OpenLoopGain.jpg contains the graph of the open loop gain. At the frequency of 100kHz, the gain drops approximately 2dB with respect to the gain at DC.

I am afraid, you mix up 100 Hz with 100 kHz, don't you?

LoL. You are right. I have to sleep more.
Sorry for such a stupid question.

Ivan