How to simulate input and output impedance of OTA

[AMSA84]

As the title says, how can I get the output and input impedance of the OTA amplifier? For the case of the output impedance, I have connected both input of the OTA shortened and connected to a common input voltage. Then in the output I used a current source injecting 1A current. Don't know if there is something missing or it is ok like that.

However I don't have a clue how to simulate the input impedance.

Can someone give me a hand?

Regards.

 

[chris.mourad]

Hi

For input impedance, why don't you apply a common mode voltage on the inputs of your OTA, and check the current that is drawn on this voltage source: this will give you the DC component of the input impedance, which should be in the tens of MegOhms for classical OTA.

Are you looking for AC input impedance?

 

[AMSA84]

Well, actually I don't have a clue how the input impedance is characterized on the OTA. Don't know if it is AC or DC. For the output impedance I have been told that the best way to do it is through a AC analysis and a idc current source in the output. Regarding the input, don't have a clue.

 

[erikl]

It's the same for the input impedance: Choose a good operation point in the ICMR, apply a unit ac current between the differential inputs (look out that your ICMR bias doesn't short circuit this current!), run an ac (or xf) analysis and plot dB20(diff. input voltage) vs. frequency. This directly results in an input impedance vs. frequency plot (MV ≙ MΩ). For very low frequencies it will converge to a parallel line to the x-axis, which gives you the (differential) DC input resistance. The initial -20dB/decade decline vs. higher frequencies means the differential input capacitance.