Dead Transistor

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  Miss biasing a transistor can definitely damage it, reverse biasing the BE junction in a wrong input connection of a LTP this isn't strange to happen. It behaves as a zener, but once it reaches that point once the transistor never is the same again (depending on the current and time) it's pretty common for low noise transistors to lose that property, or to have a protection diode to avoid it all together.

  While I'm at it, as this is a side question to an already running topic, it could be asked there to keep things tidy, there is no need to start a new topic for every single question, as there is a need to start a topic for a completely different conversation, just to keep things tidy!

JS

80hinhiding said:

I was experiencing some wonkiness with the differential amp I've been testing and I just found one of the transistors wasn't working.  Is it common for them to fail?  I must have abused that poor little guy. haha

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The transistor data sheet will suggest the range of operation they can tolerate.  One of my early technician jobs involved troubleshooting a prototype switching supply and that puppy ate transistors for lunch.

Common failure modes for transistors is #1) over current in the base, that would literally open the base connection like a fuse. #2 over voltage collector-emitter (the excess voltage would punch through creating a short between collector and emitter). #3 reverse biased junctions. The popular zener diodes are pretty much formed by reverse biased base-emitter junctions (typically 6.8V). This is called zenering and does not immediately kill the device, but will degrade performance characteristics like forward beta and junction noise. You will often notice clamp diodes across low noise transistor designs to prevent them from ever zenering, during turn-on transients or from faults elsewhere.

Back in the day doing my job I used to grind the tops off of failed metal power transistors to inspect the faults and you could literally see melted base wires, or where a collector voltage punch through melted the nearby silicon.

When working properly you should measure diode junctions between base and emitter and base and collector, with open circuits backwards (for modest voltage) and open circuit collector emitter.  Heat from over current is a common cause of failure.

JR

I'll raise..... ;D

Walrus said:

I'll raise..... ;D

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Yup it's a bad sign if they open themselves up... a lot of smoke trying to get out in very little time.

JR

Was in a Chevin power amp like this http://img.audiofanzine.com/images/u/product/normal/chevin-a-3000-25252.jpg
Even punched through 3 of the thin aluminium heatsink vanes that you can see in the middle of the picture!

Walrus said:

I'll raise..... ;D

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That's a keeper.

It looks like it struck an internal E-C (case) arc, and plasma cutter style, blew clean through the case. NPN I assume? Might have been initiated by a tin whisker from E growing towards the case, and when it got close enough....

Sorry for the off-topic veer, but that's really cool.

Gene