There's only one more thing to consider about the transistor, and that is the fact that its amplification of currents is less than ideal. Oh, you can be sure that as more and more current is applied to it, more and more will appear at the other end. But along the way, a nasty side-effect crops up, which is nonlinearity. This four-bit word actually describes a fairly simple problem, whereby the ratio between the controlling input and the output doesn't always stay fixed. When a small amount of voltage is put into a transistor, it will have more gain than for a larger voltage. With an audio amplifier, nonlinearity in the circuit creates distortion.
The underlying reason for this is quite complex, however the solution is very simple: negative feedback. Some examples of negative feedback in the home can be found inside a toilet tank, as well as in your oven. Suppose you're about to cook a frozen dinner. You'd go to the oven and set the thermostat for, say, 400 degrees. The heating coils begin to glow and remain on until the correct temperature is reached, at which point they will turn off. After a few minutes, as the oven begins to cool, the coils are again activated thus maintaining a constant temperature.
Earlier we saw how a power supply is designed to sense its
own output, so as to regulate the voltage that's produced. And as a circuit drew
more and more current (or if the air conditioner came on lowering the AC input
voltage) the supply could then adjust itself to produce more output. The same
thing happens with an oven, only in this case the thermostat does the sensing,
and it controls the heating coils.
More sophisticated ovens use a proportional controller rather than simply turning the coils on and off. As the temperature drops, the voltage is increased; once it approaches the right amount, the voltage is decreased. But the whole point of all this is that it is possible for a circuit to control itself, by merely sampling some of its own output. The power supply needs to do this because the line voltage may drop, or the circuit being powered may demand more current. A transistor amplifier uses negative feedback to overcome its own inherent nonlinearity, though the principle is identical.
Now do you see why I mentioned a toilet? As the tank becomes full the float inside rises. And since the float is attached to the very valve that is filling the tank, a closed loop is formed. Same thing for those gadgets that control the speed of your car on a highway. Even constructive criticism could be considered a form of negative feedback -- if enough of you wrote to complain about these ridiculous analogies, I'd probably stop using them! But seriously, all of these are legitimate examples of negative feedback, even if they appear a bit far-fetched at times.