Electronic – Why doesn’t BD139 act as a switch like the 2N3904

transistors

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?

Best Answer

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.

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