Kā tranzistori PATIESĪBĀ strādā (1. lapa)

How transistors REALLY work (page 1)

Most introductory textbooks do a very poor job of explaining the details of how transistors work. First they assume that the Base current is somehow controlling the Collector current, then they try to explain how one current can affect another. These explanations always fail because Bipolar transistors, like FETs, are voltage-controlled devices. One current doesn't affect another. Instead, the Base-Emitter voltage controls the thickness of an insulating "depletion layer" which lies in the path of both the Collector current and the Base current.

When I first became interested in electronics as a kid, I sat down and figured out how bipolar transistors work.

Well, sort of.

I read many articles which explained the "Common Base" amplifier. Common-base is the setup which was used by the inventors of the transistor. In those explanations, the Base is grounded and the input signal is applied to the Emitter. Since common-base amplifiers are rarely used in transistor circuitry, I ended up having to dream up my own explanation. I based it upon the little bits I already knew about the Common Emitter configuration. Common Emitter the one where the Emitter is grounded, the Base is the input, and where the output is taken across a resistor connected to the Collector. My home-made explanation sort of worked, but I wasn't satisfied. I was full of niggling doubts. And why the hell were the textbooks using Common Base to introduce transistors to the newbies? It just didn't make any sense.

When I went into engineering school, I found it extremely odd that there were still no good explanations of bipolar transistors. Sure, there were detailed mathematical treatments. Just multiply the Base current by "hfe" to obtain the Collector current. Or, treat the transistor as a two-port network with a system of equations inside. Ebers-Moll and all that. But these were similar to black-box circuits, and none of them said HOW a transistor works, how can a small current have any effect on a larger one???? And nobody else seemed curious. Everyone else in the class seemed to think that to memorize the equations was the same as learning concepts and gaining understanding of the device. (R. Feynman calls this the Euclidean or "Greek viewpoint;" the love of mathematics, as opposed to the physicists' " Babylonian viewpoint" where concepts are far more important than equations.) I'm a total Babylonian. For me, math is useless at the start, equations are like those black box Spice programs which might work great, but they don't tell you any details of what's happening inside a device in the real world. Only after I've attained a visual and gut-level understanding of something, only then is the math useful to me for refining it and adding all the details. However, math alone is not a genuine explanation. Math is just a tool or a recipe, a crutch for those who want nothing except the final numerical result, and it certainly does not confer expert knowledge.