Transistor as a Switch

Operated in its saturation and cut-off regions, a transistor behaves like an electronic switch that a small control signal at the base can turn fully ON or fully OFF. When a HIGH input voltage (e.g. $V_{in}=+5$ V) is applied at the base, the base current $I_B$ increases, which in turn increases the collector current $I_C$; the transistor moves into SATURATION (turned ON), and the resulting large voltage drop across the collector resistor $R_C$ pulls the output voltage down close to zero -- the transistor now behaves like a CLOSED switch, i.e. an ON condition. When there is instead NO (or LOW) input voltage at the base ($V_{in}=0$), $I_B$ and hence $I_C$ both drop, the transistor moves into CUT-OFF (turned OFF), and with negligible drop across $R_C$ the output voltage rises close to the full supply, +5 V -- the transistor now behaves like an OPEN switch, an OFF condition. Because a HIGH input consistently produces a LOW output and a LOW input consistently produces a HIGH output, this switching action is an INVERSION -- exactly the behaviour of a digital NOT gate -- which is why a single common-emitter transistor stage is routinely used as the physical circuit implementation of a NOT gate/inverter in digital logic hardware. …