Bipolar Junction Transistor (BJT)

Bipolar Junction Transistors (BJTs) have three terminals called Collector (C), Base (B), and Emitter (E).

How to read schematic diagrams 04 basic bipolar junction transistor symbol
How to read schematic diagrams 04 basic bipolar junction transistor symbol
  • Collector
  • Base
  • Emitter

A small amount of current flow from Base to Emitter allows a large amount of current to flow from Collector to Emitter.

There are 2 types of bipolar junction transistors, NPN and PNP. They have opposite polarities (the direction that current flows), but otherwise operate the same way.


Nice assortments of semiconductors that I use in my videos. Includes 2N3904 NPN BJTs and 2N3906 PNP BJTs. Amazon affiliate link ad.

Base Current (Ib)

NPN and PNP Bipolar Junction Transistor BJT base current examples diagram by electronzap
NPN and PNP Bipolar Junction Transistor BJT base current examples diagram by electronzap

Base to Emitter acts like a diode. The BJT doesn’t control the amount of current that flows from B-E, other than having an approx. 0.7V forward voltage for almost all BJTs. Remember that a diode is forward biased when the Anode is more positive than the Cathode. Anode is P type material, while Cathode is N type material.

When a signal (voltage and as little current as possible) is applied to the base, then the amount of current that flows from the B-E determines how much current will be allowed to flow from C-E based on the transistor’s gain.

  • 5V – 0.7V diode drop = 4.3V across current setting resistor at the base.
  • 4.3V/10,000Ω = 0.00043A , which is the same as 0.43mA of current through B-E.
  • Conventional current (imagined as flowing from more positive to negative, +5V to ground in these examples), flows through PNP BJTs in the opposite direction of how it flows through NPN BJTs.
  • Electrons really do flow from negative to positive.

Collector current (Ic) and gain:

Maximum current through NPN BJT Collector if it has a gain aka beta of 10 or 100 illustrative diagram by electronzap
Maximum current through NPN BJT Collector if it has a gain aka beta of 10 or 100 illustrative diagram by electronzap

For every charge that flows through base to emitter, many times as many charges will be allowed to flow from collector to emitter. This is known as gain.

  • Gain, hFE, and beta (ß) are 3 commonly used ways to refer to multiple amount of Collector current flow in relationship to the Base current flow of a bipolar junction transistor.

If a NPN BJT has a gain of 100, and you put 1mA of current through it’s B-E, then it will allow 100mA of current to flow from collector to emitter.

Lots of BJTs have a gain that may vary from around 200 to 300 depending on many factors explained in their datasheets.

Since gain varies depending on what a particular bipolar junction transistor is being subjected to, you just want to make sure that the transistor that you want to use will always have more gain than you need. Other circuitry will ultimately set the current. You will learn about that other circuitry while you studying BJT circuits.

This is probably confusing for most people who are starting to learn about it. But, it will make a lot more sense as you study simple BJT circuits. Especially if you build and multimeter measure BJT circuits on a breadboard.

Topics that will be added later:

Important topics, most that I will cover in more detail on this page later.

States:

  • Cutoff region.
  • Active region.
  • Saturation.

Common BJT circuit fragments:

Keep in mind, that there are a lot of transistor circuit fragments that you might not build yourself, but will encounter in one way or another. Therefore, by understanding them, you will better understand their limitations, and/or modify other circuits.

Low side NPN BJT switch versus high side PNP BJT switch using bipolar junction transistors schematic diagrams by electronzap
Low side NPN BJT switch versus high side PNP BJT switch using bipolar junction transistors schematic diagrams by electronzap

Basic schematic diagram of bipolar junction transistor switch circuits.

I’ll probably devote a page to each of the circuits below, instead of making this page a lot longer.

  • Switch: C-E drops almost no voltage while on, drops all voltage when off.
  • Current source/sink: C-E drops undesired supply voltage as needed to maintain a certain voltage across a resistor. ::I will insert basic schematic here::
  • Emitter/Voltage follower. C-E drops base voltage with a difference of approx. 0.6V
  • Inverted amplifier/switch. C-E takes away all voltage when on, and takes away no voltage when off.
  • Schmitt trigger.
  • Current mirror.
  • Differential amplifier.
  • Rubber diode – Adjustable zener.

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  • Information on this site is not guaranteed to be accurate. Always consult the manufacturer info/datasheet of parts you use. Research the proper safety precautions for everything you do.
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