A question about transistor small signal analysis ?

For large signals the effect of that parameter makes little difference in the outcome

If you analyze a circuit both ways...one using (r pi) and then do it again without it

you will see that the difference is negligible

They have nothing to do with frequency, except the frequency has to be low enough that capacitive and inductive effects are negligible. 1. True 2. No, high frequency signals (what you mean, I assume, by large frequency) require different models for analysis. That is, if the frequency is high enough that the effects start to change. In general the amplitude has to be low enough and the frequency low enough, such that reducing the amplitude or reducing the frequency does not change the reading by an appreciable amount. Large amplitude is where any small increase (or decrease) in amplitude changes the output.