With a zero signal applied to the base of the transistor, it rotates “OFF” by acting as an open switch and zero collector current. The easiest way to change moderate to big power is to use the transistor with an open collector output and the transistor emitter terminal directly connected to the ground.
When used as an AC signal amplifier, the transistors the basic polarization voltage is applied so that it always operates in its “active” region, ie the linear part of the output characteristic curves are used.
However, both NPN and PNP bipolar transistors can be used to operate as a “ON / OFF” solid state switch by moving the base terminal of the transistors differently from the one for a signal amplifier.
Solid state switches are one of the main applications for using the transistor to switch a DC output “ON” or “OFF”. Some output devices, such as LEDs, require only a few milliamps at logic DC voltages and can therefore be driven directly by the output of a logic gate. However, high power devices, such as motors, solenoids or lamps, often require more power than that provided by a regular logic gate, so the 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 I-V characteristic curves that we have seen before.
The operating ranges for a transistor switch are known as the Saturation Region and the Cutting Region. This means that we can ignore the circuit of influence of the operating point Q and the voltage divider circuit necessary for amplification and use the transistor as a switch, leading it back and forth between its “cut-off” and ” ON “(saturation).