Output and input impedance of buffer simulation

[Supersnail]

Hi everyone,

I need to characterize a buffer (OTA rail-to-rail with cascode stage and class AB output stage) but I have not understood how to correctly make the simulation to obtain input and output impedance.

So the amplifier is in feedback configuration and for input impedance I have connected an ac current source (1V) to the input. I also connected a DC source for my operating point. But when I launch an ac simulation and plot voltage at the desired node, this does not work. When I remove the DC source I have an input impedance from Gohms to ohms over the frequency range.
In a datasheet that I need to take as an example, there is only one value so I am a bit confused on how to obtain it and how to make sure it is accurate.


As for the output impedance, I also plan to connect an AC current source but should I keep the loads that I have ? (inductance and RC filter) or characterize the output impedance without any load ? I imagine also that it varies with frequency.
So what is the best method to have these characteristics ?

Thanks

 

[erikl]

... for input impedance I have connected an ac current source (1V) to the input. I also connected a DC source for my operating point. But when I launch an ac simulation and plot voltage at the desired node, this does not work.

When I remove the DC source I have an input impedance from Gohms to ohms over the frequency range.

Put the DC bias source and the ac current source in series!

As for the output impedance, I also plan to connect an AC current source but should I keep the loads that I have ? (inductance and RC filter) or characterize the output impedance without any load ? I imagine also that it varies with frequency.
So what is the best method to have these characteristics ?

If you want to measure the own output impedance of the amp, remove the loads.

 

[Supersnail]

Thanks for your answer
I have connected the DC bias source and ac current in series and I still have a plot that goes from Gohms to ohms over the frequency range.
However, for output impedance, I have a plot that seems correct w/ 150mohms

 

[erikl]

I have connected the DC bias source and ac current in series and I still have a plot that goes from Gohms to ohms over the frequency range.

This might be correct: The gate of a MOSFET - with an input capacitance of e.g. 10fF - could have an input impedance of GΩ at low frequencies (due to leakage current), and 1Ω at 160 GHz.

However, for output impedance, I have a plot that seems correct w/ 150mohms

For your class AB output stage OTA, this seems to be a good value for low frequencies.