Transistors

[Kalee20]

One of the most important things to understand about transistors is that they are voltage amplifiers, not current amplifiers. Yes a change in base current will give a bigger change in collector current but base current is actually a nuisance, something that we have to put up with. Designers have to make their circuits work despite base current which will vary a lot with teperature and between different devices.

Many people have written that transistors are current amplifiers. They are wrong. Any article that claims this should be taken with a large pinch of salt as the author clearly doesn't understand how transistors work.

I'm going to have to disagree with Jeffrey!


Fundamentally, I think of transistors as current-operated devices. You bias the collector-base junction in reverse, and no current flows.

You then put a bit of current into the emitter, and lo and behold almost all of it gets attracted to the collector. The ratio is alpha and is virtually 1 - highly predictable!

I have designed amplifiers and level-shifters using the predictable and stable property of alpha.

Once we move away from the fundamental connection (common base) to common emitter - admittedly the most frequent manner of use - then yes, beta is not to be relied on, and base-emitter voltage is the quantity to work with (and base current is a nuisance). But Vbe varies with temperature too, as does gm and re.