(Beginner question) The different ways a transistor can be connected to power source

Question

I very recently just started exploring the world of electronics, and one thing is irritating me about transistors.

I know that if a current runs to the base, then the current will start to run from emitter to collector, and we could then take this current and use it for whatever we want, but I've seen setups where both the base and the emitter are powered. I don't understand why the emitter and base would both need to be powered, why not just power the base, since the emitter doesn't need to be connected for electrons to start flowing only the base does. Can someone help me understand why you would power both when powering the base seemingly does the same?

Answer 1

For conventional current flow going from (+) to (-)

NPN transistor

1. inject current into base causes small current to flow from base to emitter
2. when the base is accepting current it will be about a diode drop more positive than the emitter
3. with the base current there can be current flowing from collector to emitter which can be larger than the base current
4. collector will normally be setup to be more positive than the emitter

PNP transistor

1. extract current from base causes small current to flow from emitter to base
2. when taking current from the base it will be about a diode drop more negative than the emitter
3. with the base current there can be current flowing from emitter to the collector which can be larger than the base current
4. emitter will normally be setup to be more positive than the collector

Keeping the above in mind, which are the proper biasing and current directions for operation of the transistor, you get to select just what represents the power supply to a given circuit. There are many ways to place circuitry around a transistor that abide by the above "rules" and provide a functional circuit.