BD140 PNP Audio Driver Power Transistor

This is Q6, a PNP TO-126 package power transistor. It is used here in the lower half of the class B complementary push-pull output stage. When active on the negative going half of the audio cycle, the transistor "pulls" the speaker voltage down towards the 0V rail by sinking current in the direction of the arrow.

A BD139 and BD140 complementary pair are quite strong power transistors to be used in a small transistor radio like this one. They can pass 1 Amp without problems, which is more than is needed here. You might choose to use something specified at 400mA, which would be a bit easier for the previous stage to drive. Classic examples would be the SS9012 and SS9013, far eastern transistors specifically made for this purpose. I chose BD139 and BD140 because the larger cases dissipate some heat without the need for a heatsink, whereas the SS9012 and SS9013 are in the much smaller TO-92 cases.  That's OK if you can ensure that the biassing diodes are in close contact with the cases, and you can blob them down to a piece of metal with some heatsink grease. That's not possible in this kit. BD139 and BD140 are also very difficult to destroy on a 9V battery supply if plugged in the wrong way round, or flipped in location.

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