Transfer Function of the RC Filter

Thereafter, since we are looking at only steady state excitations (not transient impulses), we can set s = joo, and plot out a) the magnitude of the transfer function (i.e. its gain ), and b) the argument of the transfer function (i.e. its phase) — both in the frequency domain of course. This combined gain-phase plot is called a Bode plot 

Note that gain and phase are defined only in steady state as they implicitly refer to a sine wave ( phase has no meaning otherwise ). 

Therefore, the asymptotic approximation is a valid short cut that we will often use from now on to simplify the plots and their analysis. 

---