NPN Bipolar Junction Transistors (BJTs)

NPN bipolar junction transistor (BJT) is the first transistor that most electronics students and hobbyists learn about. It has the chemical property of a P type material sandwiched between two N type materials. That is why it is called NPN. Each of those 3 areas has a terminal for connecting to the outside world. They are called the collector (N), the base (P), and the emitter (N).

General NPN BJT electrical properties

Always remember that datasheets must be consulted to get the most accurate specific properties of the particular component you are using. Only use this, and other introductory material as a starting point of understanding.

  • Terminal polarity – Swapping collector with emitter is an easy mistake to make. Both are N type material for an NPN. Whereas PNP BJT collector and emitter are both P type material. If B-E are connected backwards, then the transistor will fail to operate properly (almost always for the worse), when higher voltages are used.
  • Gate to emitter current controls Collector to Emitter conduction.
  • Cutoff region – Collector to Emitter is not conductive.
  • Active region – Collector to Emitter is conducting but not fully.
  • Saturation – Collector to Emitter is conducting as well as it can. Almost no impedance.


Bipolar junction transistor basics diagram by electronzap electronzapdotcom
Bipolar junction transistor basics diagram by electronzap electronzapdotcom

Collectors of NPN BJTs are almost always wired to be more positive than the emitter. This connection is normally off (open), which means that it does not naturally conduct. The base is usually somewhere between the collector and emitter voltage. If the base voltage is about 0.6V more positive than the emitter, then current will start flowing through the base to emitter junction. If current is flowing from base to emitter, then the collector to emitter will be able to start conducting. Thus, the base-emitter current controls the collector-emitter current.

  • Gain (aka beta or hFE) (covered in paragraph below)
  • Vcc is commonly used in schematics to indicate the power supply voltage of undermined value on the collector side of the NPN BJT circuit. Depending on the components used in the circuit, make sure you use enough voltage to provide power, but not too much where you damage something. NOTE: Many people have extended the use of Vcc to being the positive power supply of any circuit they draw, even without transistors.

The current that can flow from collector to emitter is generally a large and unstable multiple amount of current in relationship to the base current. For example, a transistor that has 100 times as much current flow from collector-emitter as is flowing through base-emitter, is said to have a gain/beta/hFE of 100. Unfortunately, different voltages and currents will alter the gain some. The people designing circuits you are interested in, will usually list a particular transistor that they recommend to make sure it has enough gain, and other electrical properties needed for that kind of circuit.

NPN BJT switch circuit

The bipolar junction transistor switch is a simple circuit that demonstrates the effectiveness of transistor gain. A small base signal (low voltage and current) can easily cause the normally off collector to conduct fully, and thus switch a load on.

Electronics course 8 NPN Bipolar Junction Transistor BJT switch plus capacitor fade off 2N3904

Clicking the diagram below brings you to a page that explains the diagram in more detail.

Basic electronics circuit fragments and simple circuits 6 NPN Bipolar Junction Transistor BJT Switch diagram by electronzap electronzapdotcom
Basic electronics circuit fragments and simple circuits 6 NPN Bipolar Junction Transistor BJT Switch diagram by electronzap electronzapdotcom

This kit has most of the diodes and transistors I use, or could have used in my videos.

Multiple power sources

2N3904 NPN BJT 30V power source controlled by 3 point 3V breadboard supply electronics circuit

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