In many applications, the transistor is used as a switch. Consider the output characteristics of a transistor in the CE configuration as shown in Fig. 6.8. Condition A corresponds to the switch in the ON state and in saturation. Condition B corresponds to the switch in the OFF state.
Consider the transistor switch as shown in Fig. 6.9.
As long as the driving signal amplitude is zero, the base current is zero; and hence, the collector current is ideally zero. Therefore, the drop across RC is also zero and the voltage at the collector, VCE = VCC. The resistance between the collector and the emitter terminals is the OFF resistance of this static switch. Ideally:
FIGURE 6.8 ...