The switch is open.

There is no current to the base, so the transistor is in the cut off condition with no collector current. All the voltage drop is accoss the transistor.

The switch is closed.

The base resistor is chosen small enough so that the base current drives the transistor into saturation.

In this example the mechanical switch is used to produce the base current to close the transistor switch to show the principles. In practice, and voltage on the base sufficient to drive the transistor to saturation will close the switch and light the bulb.

For switching currents less than an ampere, the transistor switch can be used. Instead of a mechanical switch in the base circuit, an op-amp could be used. When the switch is open, no current flows in the base so the collector current is cut off. The resistor R_C must be small enough to drive the transistor tosaturation so that most of the voltageV_cc appears across the load.

In a configuration where the output is taken below the load resistor, this kind of switch can function as an inverting buffer in digital circuits.

Though more commonly in the common emitter configuration, the emitter follower can also be used for switching. The resistor R_C must be small enough to drive the transistor tosaturation so that most of the voltageV_cc appears across the load.