Thank you all for your comments.
Klauss, you have mentioned the output impedance and I haven't thought about it. Can I ask you something about this?
I am experiencing the following: when a step the output current, from zero to, say 80% of the bias at the output, the amplifier has a ring but it settles. However, when I step it with 90% of the bias current, the amplifier takes a longer time to settle. In both cases, either with a longer or shorter rise times (1us or 10us), with 90% of output bias current, the settling time still longer when compared to a step with 80% of the bias current at the output stage. It looks like that the transistor at the output goes to cut-off because I am taking away current from the biasing.
Now I wonder, if I want to see the stability of the amplifier, I can use the body plot. Nevertheless, if I want to use transient as well, what are the best ways to test the amplifier or take the amplifier to its limits?
I was thinking: dc voltage at the input and current step at the output as well as a current squarewave at the output with something around 80% of bias current.
Then the other way around. Squarewave voltage at the input and do the same at the output as mentioned above. All of this in unity gain mode.
Now, does this make any sense? How could I test an amplifier using transient?
Regards