[EricH] covered the basic version, here’s a little (or a lot :rolleyes:) more detail if you want it:
Note that there are many different types of transistors, based on the different types of semiconductor (different types of doping) that are in the region of the transistor, and what materials are actually used in its construction. The most common type of transistor, especially in intro EE classes, is a NPN Bipolar Junction Transistor. The Wikipedia article on BJTs is a little long, but I think it’s fairly readable if you have a basic knowledge of voltage and current and so forth.
Like [EricH] said, in NPN transistors, current passes from the base and collector to the emitter, and in forward-active mode when using DC, the relationship between these two currents can be approximated as linear, i.e. <Collector-Emmiter current> = B * <Base-Emiter Current>, where B is usually something around 100. AC signals (like audio signals) get more complicated, but if you understand the DC relationship, it’s somewhat similar, so if you have a small amount of current from an audio entering the base, then a larger current will be generated from the collector to the emitter. Again as [EricH] said, if you know the resistance through your circuit (more properly, the impedance since audio is an AC signal), you can determine the voltage generated by Ohm’s law. Voltage is usually fairly equivalent to volume in audio circuits.
A couple of extensions to cover some of your other questions:
In PNP transistors (made with a different set of dopings), the current basically flows in the opposite directions, but the concept of the relationship between emitter-base current and emitter-collector current is essentially the same.
Transistors have different regions of operation. The three basic regions are foward-active, cutoff, and saturation. Which one the transistor is in basically depends on how much voltage and current are applied to the transistor. For audio amplification, you usually run the transistor in forward-active mode, because this allows for the linear amplification of the input signal. Cutoff happens when the base-emitter voltage drops below a certain point, usually about .6V for BJTs, and results in very little current flow through the transistor. This is usually referred to as the transistor being “off.” In either of the other two modes, the transistor is usually considered to be “on.” If the current through the transistor gets too high, the transistor goes into saturation, which is basically like it sounds: the current through the transistor has maxed out.
Unlike in audio circuits, if you’re using a transistor in a digital circuit (like a computer), you try to minimize the amount of time spent in forward-active, because the output voltage can be somewhere between the voltages defined as “0” and “1.” So in a digital circuit, “off” is cutoff, and “on” would be saturation. This is probably what you saw referenced in some of the websites, because 0V is usually chosen for a digital 0, and 5V is a common value for a digital “1.” Again, note that this is a different usage case than what you’re talking about with audio circuits.
Did that cover all your questions? Feel free to post back if you’re still confused.
–Ryan
Thanks in advance.