PNP Bipolar Junction Transistors can be used as a digital switch on the high side of a load, whereas an NPN BJT switch, switches from the low side of a load. NPN BJTs behave like PNP transisistor, but polarities are reversed. The 2N3906 is a well known PNP BJT, so I primarily use it.
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- Cutoff: BJT Collector to emitter (C-E) not conduction at all.
- Active region: BJT (C-E) conducting but not fully.
- Saturated: (C-E) conducting fully.
- BJTs are normally off/open.
- Load to be switch is connected between PNP BJT collector and ground/negative supply.
- PNP BJT emitter is connected to the more positive side of the circuit (usually directly to the Vcc/positive supply).
- When the PNP base is more negative than emitter by approx. 0.6 – 0.7V, then current will start flowing. Base-Emitter current should be limited, but not lower than what will saturate the transistor. A current limiting resistor is usually added the base terminal and signal voltage.
- Base-emitter current allows a large multiple amount of current to flow from collector to emitter. That same amount of current (C-E) will flow through a series load on the collector side. The load will actually be what sets the current once the BJT is saturated.
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