I am a beginner in electronic design and want to learn about transistors.

I have an old TV and I decided to de-solder the following transistors. Transistors

Can you tell me what I can use them for, are they all the same, and if anyone has some literature where I can start, and some schematics if possible?

Here are the part numbers for the transistors I have de-soldered.

enter image description here

  • 6
    \$\begingroup\$ Start by googling those numbers with the magic word "datasheet". \$\endgroup\$
    – Majenko
    Oct 10 '14 at 17:55
  • \$\begingroup\$ Better still (often) is to use the datasheet lookup at www.digikey.com \$\endgroup\$
    – Russell McMahon
    Oct 10 '14 at 22:25
  • 1
    \$\begingroup\$ If you want data on "A844" look up "2SA844" - same will be true for some of the others : 2SC1815, 2SK2134, possibly 2SD1555; these are Japanese types. \$\endgroup\$ Oct 11 '14 at 0:39

The 2N3904 and 2N3906 are very common NPN and PNP transistors, and can be used in many circuits that calls for small-signal (as opposed to power) transistors. Both can drive 200 mA.

There are hundreds (or more) sites with circuits using these transistors. Google "2N3904 circuits" or "2N3906 circuits". Some are for amplifiers, some are for timers (blinking LED's), and some are for making various kinds of sounds -- sirens, musical notes, etc.

Here are a couple to get you started; the first is a Class A amplifier using a 2N3904 in a common emitter configuration:

enter image description here

and the second is a Class B amplifier using both the 2N3904 and 2N3906 in what's called a complementary configuration:

enter image description here

The values do not have to be exact; in partof the fun is changing them and seeing what effect that has. You can learn about how the initial values are determined by reading about "biasing" in articles like this.

You may also run into circuits using the 2N2222, which is like a 2N3904 but can switch three times as much power. So if you find a circuit using a 2N2222, as along as it doesn't need to handle more than 200 mA, you should be able to get by with a 2N3904 instead.

You can get the datasheets for any of these by Googling the part number along with the word datasheet, several ones will show up. You can also try looking them up in a distributor's webpage like Digi-Key.

  • \$\begingroup\$ Thank u very much, i will try with that u said.:)) \$\endgroup\$
    – Ricky
    Oct 10 '14 at 18:27
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    \$\begingroup\$ @Ricky: Please try to use proper English and do not write you=u and capitalize "i" when referring to yourself. Thank you! \$\endgroup\$
    – Rev
    Oct 10 '14 at 18:46

Since you de-soldered actual parts and want to use them I assume you want to learn with lab type experiments versus just reading about them. Here are some ideas in ascending difficulty.


I would suggest you start with trying to build a Common Emitter Amplifier. Change the values of the resistors to see if you can get the BJT to operate in each of it's four regions. This will be extremely helpful in understanding the basic operation of BJTs.


If you want to move onto something a little more complicated you can try to create a current mirror. You can do some research to figure out which two transistors would be best for a current source of 100mA. Does it change for 300mA?


If you've found the earlier experiments too easy then try to create an operational amplifier using only discrete components. There exists many guides on the internet for this application. Try to calculate and measure the open-loop gain using the datasheets of the transistors. If you can understand this then you will have a very firm grasp on how BJTs work and what they are commonly used for.

  • \$\begingroup\$ On this link en.wikipedia.org/wiki/Common_emitter, can u check my transistors i wrote and tell me which one to use for the "Common Emitter"? THanks \$\endgroup\$
    – Ricky
    Oct 10 '14 at 18:33
  • \$\begingroup\$ @Ricky Doesn't that take out the fun of learning? You can google these terms for additional help. As for your comment edit: try them all and see what the differences are. I'd suggest starting with the smallest ones you have pictured. Change out the transistor and see if anything changes, then try to figure out why if it did. \$\endgroup\$
    – ACD
    Oct 10 '14 at 18:38
  • \$\begingroup\$ I will.Sorry for my stupid question, i am just a beginner but can u tell me what is purpose of that circuit?Is that voltage amplifier? \$\endgroup\$
    – Ricky
    Oct 10 '14 at 18:39

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