So far we have analyzed electrical circuits: The source signal has more power than the output variable, be it a voltage or a current. Power has not been explicitly defined, but no matter. Resistors, inductors, and capacitors as individual elements certainly provide no power gain, and circuits built of them will not magically do so either. Such circuits are termed electrical in distinction to those that do provide power gain: electronic circuits. Providing power gain, such as your stereo reading a CD and producing sound, is accomplished by semiconductor circuits that contain transistors. The basic idea of the transistor is to let the weak input signal modulate a strong current provided by a source of electrical power the power supply to produce a more powerful signal. A physical analogy is a water faucet: By turning the faucet back and forth, the water flow varies accordingly, and has much more power than expended in turning the handle. The waterpower results from the static pressure of the water in your plumbing created by the water utility pumping the water up to your local water tower. The power supply is like the water tower, and the faucet is the transistor, with the turning achieved by the input signal. Just as in this analogy, a power supply is a source of constant voltage as the water tower is supposed to provide a constant water pressure.
A device that is much more convenient for providing gain (and other useful features as well) than the transistor is the operational amplifier, also known as the op-amp. An op-amp is an integrated circuit (a complicated circuit involving several transistors constructed on a chip) that provides a large voltage gain if you attach the power supply. We can model the op-amp with a new circuit element: the dependent source.