PNP BJT switch – Bipolar Junction Transistor – 2N3906

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.

Diagrams

2N3906 PNP bipolar junction transistor BJT schematic symbol and pin layout diagram by electronzap electronzapdotcom
2N3906 PNP bipolar junction transistor BJT schematic symbol and pin layout diagram by electronzap electronzapdotcom

Kit includes 2N3906 and many other common transistors/diodes. As an Amazon Associate I earn from qualifying purchases.

 

Basic electronics circuit fragments and simple circuits 7 PNP Bipolar Junction Transistor BJT Switch and capacitor fade off diagram by electronzap electronzapdotcom
Basic electronics circuit fragments and simple circuits 7 PNP Bipolar Junction Transistor BJT Switch and capacitor fade off diagram by electronzap electronzapdotcom

Circuit description

  • 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.

Video

Electronics course 9 PNP Bipolar Junction Transistor BJT switch plus capacitor fade off

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NPN BJT emitter follower circuit – transferring a voltage minus a diode drop

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