With a zero signal applied to the Base of the transistor it turns "OFF" acting like an open switch and zero collector current flows. Then the transistor operates as a "single-pole single-throw" (SPST) solid state switch. For a PNP transistor, the Emitter potential must be positive with respect to the Base. Then we can define the "saturation region" or "ON mode" when using a bipolar transistor as a switch as being, both junctions forward biased, VB > 0.7v andIC = Maximum. Therefore the transistor is switched "Fully-OFF". Cut-off Region Here the operating conditions of the transistor are zero input base current ( IB ), zero output collector current ( IC ) and maximum collector voltage ( VCE ) which results in a large depletion layer and no current flowing through the device. Both these transistor regions are defined as:ġ. The pink shaded area at the bottom of the curves represents the "Cut-off" region while the blue area to the left represents the "Saturation" region of the transistor. This means then that we can ignore the operating Q-point biasing and voltage divider circuitry required for amplification, and use the transistor as a switch by driving it back and forth between its "fully-OFF" (cut-off) and "fully-ON" (saturation) regions as shown below. The areas of operation for a transistor switch are known as the Saturation Region and the Cut-off Region. If the circuit uses the Bipolar Transistor as a Switch, then the biasing of the transistor, either NPN or PNP is arranged to operate the transistor at both sides of the " I-V " characteristics curves we have seen previously. Solid state switches are one of the main applications for the use of transistors, andtransistor switches can be used for controlling high power devices such as motors, solenoids or lamps, but they can also used in digital electronics and logic gate circuits. However, both the NPN & PNP type bipolar transistors can be made to operate as "ON/OFF" type solid state switches by biasing the transistors base differently to that of a signal amplifier. The Transistor as a Switch When used as an AC signal amplifier, the transistors Base biasing voltage is applied in such a way that it always operates within its "active" region, that is the linear part of the output characteristics curves are used.
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