Part 4: Transistor Switching

In computers and other digital devices voltages are used to represent binary values: a high voltage represents the number 1, a low voltage represents the number 0. Consequently digital transistor circuits are designed to act as switches which are either on or off.

When the base voltage (VBB)is less than 0.6 volts, the transistor is off and no current flows through the collector. Because there is no voltage drop across the collector resistor, the collector voltage (VCC) is equal to the supply voltage. When the transistor is not conducting, it is said to be cut off.

Digital circuits are designed so that the base voltage is always below cut-off, or high enough to turn the transistor fully on. When the base voltage is about 0.85 volts, sufficient base current flows to turn the transistor fully on. The collector voltage drops to approximately half a volt because of the voltage drop across the collector resistor. A transistor which is conducting the maximum current is said to be in saturation.