physics investigatory project on transistor as a switch pdf

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The NPN and PNP type bipolar transistors can be operated as an "ON / OFF" solid state switch by polarizing the transistor base terminal differently from a signal amplifier.

The solid state switches are one of the main applications for the use of the transistor to switch a DC output "ON" or "OFF". Some output devices, such as LEDs, only require a few milliamperes with logic level DC voltages and, therefore, can be controlled directly by the output of a logic gate. However, high-power devices such as motors, solenoids or lamps often require more power than that supplied by an ordinary logic gate, so transistor switches are used.

If the circuit uses the Bipolar Transistor as a Switch, then the polarization of the transistor, either NPN or PNP, is arranged to operate the transistor on both sides of the characteristic curves "I-V" that we have seen previously.

The areas of operation for a transistor switch are known as the Saturation Region and the Cut Region. This means that we can ignore the operational Q-point polarization and the voltage divider circuits required for the amplification, and use the transistor as a switch when driving it back and forth between its "fully-OFF" (cut) and "fully" - IN "saturation regions" as shown below.

Operating regions