amplitude in the frequency response

Hi wang

when you are using  ![](http://latex.codecogs.com/svg.latex?hl(w)=1/(1-e^{jw\tau}) ), how to deal with the case that  ![](http://latex.codecogs.com/svg.latex?f\rightarrow0 )? and ![](http://latex.codecogs.com/svg.latex?hl(w)\rightarrow\infty)?
