Very, very helpful! I'm a vacuum tube guy trying to understand transistors, be them a BJT, FET, JFET, MOSFET... and the others. I've never heard BJTs described this way before. It's a lot of food for thought. Thank you!
I've done a lot of work in electrical circuits, but thanks to you, today I found out (basically) how an air-conditioner works! Very simply explained. Thanks!
Another terrific explanation, Bob! Yet, if I may offer a different point of view, the way I always understood it, is that the base-emitter (input) current directly controls the collector-emitter (output) current. Thanks to Ohm's law, increasing base voltage causes a rise in base current, which in turn, raises collector current far higher. Finally, thanks again to Ohm's law, this rise in collector current causes a substantial drop in collector voltage. And, conversely, dropping base voltage would have the opposite effect on collector voltage! (P.S. - I should point out that this explanation applies specifically to common- emitter circuits!)
This is probably a oversimplification but here goes. The collector has a voltage applied to it. The emitter in the video example is tied to ground. The collector to emitter basically looks like a infinite resistance with no base voltage. Applying a small voltage to the base allows current to flow through the collector to emitter. The higher the base voltage the higher the collector to emitter current. So in a nutshell, the small variable base voltage controls a large collector to emitter current. Mike KC3OSD
Not to get into the whole "Conventional current vs. Electron flow" debate, but the truth of the matter, is that electrons flow from negative to positive. So, with an NPN type transistor, as shown in the example, electrons actually flow IN to the emitter. From there, due to the atomic structure of the semiconductor layers, approximately 5% of those electrons flow OUT of the base, while the remaining 95% flow OUT of the collector. This proportion is maintained regardless of how many electrons (thus, how much "current") are actually flowing at any given time. As a result, a small change in the base current creates a much larger change in collector current, thereby amplifying the current.
Short answer: Because that's how they work. There is always a relationship between voltage and current, and the operation of a BJT is best characterized by the current relationship.
