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Impedance evaluation in large signal analysis

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Mircusx

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Hi everyone!
I'm having the following problem.
Referring to the circuit attached, a simple voltage amplifier in non inverting configuration, I have to evaluate the impedance at the node V2 and V0 when a large signal is applied at the node V1.
R2 is 580K and R1 is 5.8K, thus the theoretical gain is 101.
As the large signal analysis can only be done in transient simulation, when, with the calculator, I plot VT/IT (in order to evaluate the impedance) at, for example, the node V0, I only get as impedance 580K, but I expect to see a different value.
Attached the results of the simulation (Y axis has log scale).
My question is: are there other ways to calculate the impedance when performing a large signal analysis?
 

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To evaluate the output impedance you add a load and then determine the difference in output voltage with and without the load.
From that you can calculate the output impedance.
 

The impedance at V2 is 5.8k || 580k which is just somewhat less than 5.8k.

The output impedance isn't answered by the information provided as it depends on the amplifier, not the resistors shown. An ideal opamp for example would result in zero output impedance for that circuit.
 

In my approach, Output impedance (i.e. at node V0 ) = R2 =580k
Impedance at node V2 = R1 = 5.8 k (considering ideal OP Amp i.e. Gain -> infinity)
 

The open loop output impedance of an opamp is shown on its datasheet and is usually around 75 ohms. The negative feedback from the resistors in the circuit reduces the output impedance a lot at DC and low frequencies IF(!) the output is not clipping. If the open loop gain of the opamp is infinity then its closed loop output impedance is zero ohms in the circuit.
 
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    CataM

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As the large signal analysis can only be done in transient simulation, when, with the calculator, I plot VT/IT (in order to evaluate the impedance) at, for example, the node V0, I only get as impedance 580K, but I expect to see a different value.
I wonder what VT and IT are, they aren't indicated in your schematic. But obviously, 580k isn't the correct output impedance.

As already explained, circuit output impedance depends on open loop OP output impedance and loop gain. It's frequency dependent according to the loop gain characteristic. Caused by OP non-linearity, the output impedance can be expected to change slightly in large signal operation.

I guess, you didn't realize the complexity behind your question.

Due to this non-linearity, the AC output impedance has to be evaluated for a well defined output voltage and current waveform.
 
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    CataM

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What is your definition of "large signal analysis"?
Normally, we speak about "large signals" when a "small-signal" analysis is not allowed anymore because non-linearities must not be neglected.
That means: For a sinusoidal input the output is NOT a sinus anymore. This applies to all voltages and currents.
However, Amplification (gain), input and output impedances are defined using a RATIO of the signals.
How would you calculate the ratio of two signals having a different shape?
 
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