In the first case the circuit acts as a current follower, as understood as follows: for "RS" Â» 1/"gm" the voltage source can be replaced by its Norton equivalent with Norton current "vThév / RS" and parallel Norton resistance "RS". Because the amplifier input resistance is small, the driver delivers by current division a current "vThév / RS" to the amplifier. The current gain is unity, so the same current is delivered to the output load "RL", producing by Ohm's law an output voltage "vout = vThévRL / RS", that is, the first form of the voltage gain above.