Hello everyone, I am working on designing a TIA and performed an AC simulation using an AC current as the input and measuring the AC voltage as the output. However, I noticed that the frequency response of the output appears unusual at higher frequencies. Does anyone have any insights or suggestions? Could I be overlooking something? Thank you.

Show us your circuit. Maybe it is unstable.

@Luis Henrique Rodovalho Here are the amplifier and stability summary.

Your circuit gain margin is very low. You should simulate it with an output load also for the stb analysis and your TIA config.

@Luis Henrique Rodovalho Thank you for taking the time to answer. I simulate the circuit with the output's load, but still the results are somehow wired. I removed the ac couple capacitor and simulate the TIA with different capacitor's sizes.

Now run the stb sim with those large capacitive loads. 5 pF is too small compared to your 100 nF. You can also run your stb sim in your TIA configuration. You should also run a common mode AC sim in your TIA configuration.

@Luis Henrique Rodovalho Thank you again for your suggestion. I did a stb simulation with 5pF load and 100nF feedback capacitor and here is the results. I just not sure that should I placed the probe in another location or not. but the problem is the DC gain is does not match to the feedback resistor's size (R=150k), DC gain now 96 dB. for the common mode simulation what should the circuit's configuration looks like?

First you should replace your resistor and capacitor values with variables, then run parametric sweep sims. You should experiment with lots of capacitor values, preferably with different order of magnitudes.