Bipolar Junction Transistors BJT BJTs

The bipolar junction transistor (BJT, BJTs) can either be used as a on/off switch for a circuit or to amplify a signal.

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NPN 2N2222 Bipolar Junction Transistor BJT current gain demonstration beginning learning electronics video below! Click here to watch directly on YouTube!

NPN bipolar junction transistor switch introduction guide by Electronzapdotcom of YouTube and Electronzap of Twitter
NPN bipolar junction transistor switch introduction guide by Electronzapdotcom of YouTube and Electronzap of Twitter

Transistor circuit 1 NPN BJT 2N2222 switch for beginner DIY electronics hobbyists video below Click here to watch directly on YouTube!

– There are 2 types of BJT, the NPN and the PNP. They both have the same terminals names of Collector (C)  – Base (B) – Emitter (E). Make sure to check the pin layout of a particular transistor by looking at it’s datasheet. To find a datasheet for a particular component, do a google search of any component’s part number and the word “datasheet”.

  • NPN: Collector voltage (Vc) is higher than emitter (Ve) – (Vce). Base V (Vb) should not drop below Collector V (Vc) – (Vbc)
  • PNP: Collector voltage (Vc) is lower than emitter (Ve) – (Vce). Base V (Vb) should not rise above Collector V (Vc) – (Vbc)

– Gain: A small amount of base to emitter current (Ibe) controls a larger amount of collector to emitter current (Ice). If there is 100mA of collector current while there is 1mA of base current, then that transistor gas a gain of 100. 200mA of collector current per 1mA of base current would be a gain of 200. And so on.  The actual gain of a BJT transistor changes a lot during use, and therefore real life circuits aren’t made based on a specific transistor’s gain.

– 3 modes of Operation: A low voltage across and current through the base to emitter allows a larger voltage across and current through the collector to emitter (amplification).

  • Cutoff – The BJT collector to emitter is not conducting. Off.
  • Active region – The BJT collector to emitter is conducting, but no fully.
  • Saturated – The BJT is fully conducting as well as it can. On.

– Common biasing (how voltages are applied to the 3 terminals)

  • Base biased
  • Emitter biased
  • Voltage divider biasing
Understanding transistor pin layout focusing on bipolar junction transistors diagram by electronzapdotcom of YouTube and Electronzap of Twitter
Understanding transistor pin layout focusing on bipolar junction transistors diagram by electronzapdotcom of YouTube and Electronzap of Twitter

Voltage test 2N3904 NPN Bipolar Junction Transistor collector emitter pins connected backwards video below. Click here to watch directly on YouTube!

Some circuits in no particular order

  • Saturated switch
  • Schmitt trigger
  • Current Source/sink – Driving an LED – Mirror
  • Cascading NPN and PNP
  • Parallel BJTs
  • Push-Pull
  • Darlington
  • Sziklai
  • Transistor as a diode
  • Pulse
  • Emitter follower/common collector – Voltage regulator
  • Differential amplifier
  • Feedback

Voltage test 2N3906 PNP Bipolar Junction Transistor BJT collector emitter connected backwards video below! Click here to watch directly on YouTube!


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