Electronics Fundamentals/Semi Conductor Devices/Transistor/Electronic Switch

Transistors are commonly used as electronic switches, both for high-power applications such as switched mode power supply and for low-power applications such as logic gates.

BJT used as an electronic switch, in grounded-emitter configuration.

In a grounded-emitter transistor circuit, such as the light-switch circuit shown, as the base voltage rises the base and collector current rise exponentially, and the collector voltage drops because of the collector load resistor. The relevant equations:

V_RC = I_CE × R_C, the voltage across the load (the lamp with resistance R_C)

V_RC + V_CE = V_CC, the supply voltage shown as 6V

If V_CE could fall to 0 (perfect closed switch) then Ic could go no higher than V_CC / R_C, even with higher base voltage and current. The transistor is then said to be saturated. Hence, values of input voltage can be chosen such that the output is either completely off, or completely on. The transistor is acting as a switch, and this type of operation is common in digital circuits where only "on" and "off" values are relevant.