A question about transistor small signal analysis ?

The dynamic resistance, or the input shunt resistance, is related to the small signal current gain.

This gain is the change in collector over the change in base current. For large signal analysis the change in collector current is zero, so the input resistance is also zero.

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.