I got this low power transistor that I want to make a class-A amplifier out of, but I'm having a really hard time with it. It seems not to behave much like what I expect from a transistor.

The transistor came in a TO-126 package. I've never seen one like it, so I had no idea which side was the emitter. To figure this out, I tried to build a switching circuit.

Here's the test circuit I built: enter image description here

When I unplug the base resistor going to the Vcc (+12 V), I expect Q1 to stop conducting. It doesn't though. I thought maybe I messed up the orientation. Flipping it around, however, did nothing. It does not matter how I wire it.

I replaced it with a 2N3904, and that worked just as I expected. The LED starts off not being lit. Then I hit the NO switch, and it lights up. But in the circuit above, it doesn't matter what I do, the circuit is always on.

What gives guys? Why can't I use this transistor as a switch?

  • 2
    \$\begingroup\$ You know, the BD139 has his base terminal on the right side, not in the middle. \$\endgroup\$ Dec 14, 2018 at 21:16
  • 1
    \$\begingroup\$ Please, type BD139 datasheet into your favourite search engine and read that very first PDF document it offers: st.com/resource/en/datasheet/cd00001225.pdf \$\endgroup\$
    – Janka
    Dec 14, 2018 at 21:19
  • \$\begingroup\$ It's embarrassing... I did read that datasheet. Many times. Each time I went on to check which side the emitter was on, but I assumed that the base followed right after.... I cannot believe I was this blind. \$\endgroup\$ Dec 14, 2018 at 21:21
  • \$\begingroup\$ I rolled back the weirdish "answer". Also upvoted the question. \$\endgroup\$
    – pipe
    Dec 14, 2018 at 21:28

3 Answers 3


It works with 2n3904 because it has the following pinout.

enter image description here

But the pinout of the BD139 is not the same.

enter image description here

So you have to change the breadboard connections.


Look at the datasheets for the two transistors. The BD139 and the 2N3904 have different pinouts, that simply reversing the device does not fix.


I think this is the best picture you can find.

I think this is the best picture you can find for the BD139


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