When a transistor is to be used as a switching device, it operates either as an open circuit (i.e. in the cut-off region) or as a short circuit (i.e. in the saturation region). Figure 3 shows the solenoid switch and an alternative transistor switch.
For a common base circuit, such as in figure, the output voltage taken from the collector is either equal to the supply voltage (saturated region), or zero volts. (cut-off).