Transistor Switch Example The switch is open. Close the switch There is no current to the base, so the transistor is in the cut off condition with no collector current. All the voltage drop is accoss the transistor. Transistor operation for switch conditions Transistor Switches	Index Electronics concepts Digital Electronics Reference Horowitz & Hill p52
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Transistor Switch Example

The switch is closed. Open the switch The base resistor is chosen small enough so that the base current drives the transistor into saturation. In this example the mechanical switch is used to produce the base current to close the transistor switch to show the principles. In practice, any voltage on the base sufficient to drive the transistor to saturation will close the switch and light the bulb.

Transistor operation for switch conditions

Transistor Switches