This page provides an overview of various Power Amplifiers (PAs).
The output of a power amplifier does not depend on just its instantaneous input. It also depends on past inputs. This is due to the time constants of the analog components. When modeling transitors, we typically look at the parasitic capacitances, $C$, and resistances, $R$. These $R$s and $C$s contribute to the time constants, $\tau$.