Consider a $p-n$ junction as a capacitor, formed with $p$ and $n$-materials acting as thin metal electrodes and depletion layer width acting as separation between them. Basing on this, assume that a $n-p-n$ transistor is working as an amplifier in CE configuration. If $C_1$ and $C_2$ are the base-emitter and collector-emitter junction capacitances, then :