Simply Put - 2N3904 NPN Transistor

By John LeDuc

Contributed By Digi-Key Electronics

The 2N3904 is an extremely popular NPN transistor that is used as a simple electronic switch or amplifier that can handle 200 mA (absolute maximum) and frequencies as high as 100 MHz when used as an amplifier.

Image of pin out of a standard TO-92 package ON Semiconductor 2N3904

Figure 1: An illustration of the pin out of a standard TO-92 package for Digi-Key part number 2N3904FS-ND: E = Emitter, B = Base, C = Collector.

Figure 2 shows a circuit example which turns on the collector emitter junction by energizing the base or biasing the transistor to turn it on by bringing 5 volts to the base via a slide switch. This example lights an LED which is considered to be the load in this case. The proper use of resistors to prevent over current draw is required when biasing the base. I used leaded parts in a bread board to test my example circuit. Most engineers will use surface-mount components (much smaller size then a TO-92 package) when it comes to using transistors in a new product design going to market. Here is a link that shows various package sizes for 3904 transistors.

Image of Scheme-it 2N3904 circuit example

Figure 2: 2N3904 circuit example - drawn in Scheme-it®.

Since the 2N3904 is an NPN transistor, that means the base needs positive biasing (appropriate voltage levels and resistance) to turn on the collector emitter junction for proper current flow. Use of a load resistor (R1 above) is also important so there is not too much current being driven through the LED and transistor. For the complete specifications on the 2N3904FS-ND, see the On Semiconductor datasheet associated with this part.

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About this author

John LeDuc

John LeDuc, Digital Business Project Manager at Digi-Key Electronics, started his career in 1984 assisting Digi-Key customers with technical questions, reviewing and adding products to the catalog. He specialized in supporting the INS8073 Tiny Basic demo board from National Semiconductor. John is now enhancing the digital experience of our engineering customers by collecting and generating unique ideas to make our website better. He holds an Associate degree in Electronics Technology and is a “Midnight Engineer”, tinkering with electronics in his spare time and creating unique designs with his 3D printer.

About this publisher

Digi-Key Electronics

Digi-Key Electronics, based in Thief River Falls, Minn., is a global, full-service provider of both prototype/design and production quantities of electronic components, offering more than six million products from over 750 quality name-brand manufacturers at Digi-Key.