The load resistor is connected directly across the transistor's collector and the power supply, thereby positioning it within the path of the transistor's main current. The input voltage, Vin, controls the switching actions-opening and closing-of the transistor. When the transistor is in the "open" state, the load current is blocked; conversely, when the transistor is in the "closed" state, current is permitted to flow.
More specifically, when Vin is at a low voltage, the absence of base current results in no current flowing through the collector; consequently, the load connected to the collector terminal receives no current. This condition is equivalent to the switch being "open" (in the off state), during which the transistor operates in the cutoff region.
Conversely, when Vin is at a high voltage, the presence of base current induces a significantly amplified current flow through the collector. As a result, the load circuit becomes conductive-equivalent to the switch being "closed" (in the connected state)-and the transistor operates in the saturation region.