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blogroll
Fri, 21 Oct 2005
[ permalink | 2 comments ]
the interviewee has become the interviewer
Sweet! I did my first interview today. It was kind of cool.
Last night, while trying to come up with reasonable interview questions, I got to thinking about how an emitter follower can have a negative real input impedance when driving a capacitive load. In particular, I wanted to show that it was the case, so I busted out the small-signal model and started burning through some lead. It only took two pages of engineering paper, but I decided that it made an exceptionally poor interview question (because it was all math and no intuition).
Instead, I went with a simple question about BJT device physics and a basic switched-capacitor integrator.
The BJT question is cute and quick: say we have an NPN transistor. OK, now consider that a diode is just PN, so if I have two of them back-to-back, I have NP-PN—somewhat similar, but does it act like a transistor? No. Why? Ohmic contacts; more specifically, what we get is infinite recombination velocity in the base, which reduces β to 0. So what does that imply about the relationship between base doping and β? Lower doping gives higher beta, because it increases the time before the minority carriers we inject recombine. We could also change β by making the base region narrower.
[ permalink | 0 comments (add one you lazy bastard!) ]