OTA transient simulation: decay problem

[aboriginemm]

Hi,

The transient simulation of a open loop OTA shows that the output current is not proportional to input voltage, also the output voltage has decaying. The OTA has high linear range so the linearity shouldn't be the problem.
I put one simulation waveform. Check the lower waveform.
The input signal is sinewave and load is capacitor.
I think it is something related to output impedance.
Is there any model for describing this effect?
Thank you!

 

[FvM]

shows that the output current is not proportional to input voltage

Whre did you get this from.

Obviously the effective load (output impedance || external load) is a RC circuit. The transient response should be expected then like it's shown.

 

[aboriginemm]

shows that the output current is not proportional to input voltage

Whre did you get this from.

Obviously the effective load (output impedance || external load) is a RC circuit. The transient response should be expected then like it's shown.

What I expected is the output current of OTA is proportional to input voltage since
Iout=GM*Vin
And then the current Iout is integrated on the capacitor
Vcap=integral(Iout)/C

In the figure I attached, the first waveform is the capacitor voltage which uses ideal current source and the second waveform uses OTA.

What have I missed here?

 

[Tre]

Why are you running the OTA in open loop? What is regulating the common mode voltage? It is hard to be helpful without seeing a schematic/testbench.