I was working on a simple 16MHz Pierce oscillator recently, built around a 2N2222 transistor. Nothing special, should work without any trouble, right?
Nope. The darn thing just wouldn’t oscillate! I found that it would oscillate below about 10MHz, but not above that. After some testing of the transistor, I found that it only had a beta of about 3, and its base-collector breakdown voltage was 8 or 9 volts!
Not only that, but I had 5 such transistors, all exhibiting the same behavior, and all with the same part markings:
Now I’ve bought a lot of 2N2222 transistors, so I wasn’t sure who the actual manufacturer was, but I was sure that somehow I’d gotten a bad batch of transistors. I was just about to Widlarize them all, when I decided to put it to the EEVBlog forums.
Several of the commenters asked if I was sure that I didn’t have the transistor in backwards. Well I’m quite familiar with the standard 2N2222 pin out, so I hadn’t even considered that to be an issue, and I was sure that wasn’t the problem. That is, until Kevin.D pointed out that what I was seeing was in fact indicative of swapping the collector and emitter pins: low beta, and low base-collector (which, if swapped, is actually the base-emitter) breakdown voltage.
Doh!! That was exactly the problem, and had I bothered to look it up, Wikipedia would have told me that too:
You’d think by this point in my life I would have learned to stop making assumptions… 😛