Passive components

Resistors



                  One of the most common types of passive electrical components is the resistor. Resistors come in many types, from very simple resistors that fit comfortably in a toddler’s hand and are used in high school electronics experiments, to massive resistor boxes that are used in power stations and for other industrial purposes. Resistors are useful for adjusting the current flowing through a wire. As resistance increases relative to voltage, the current in a circuit will decrease. For this reason, when electricians and engineers need to reduce the current in a circuit, they will add resistors. When designing a circuit that can pass varying currents, professionals will add a switch that allows the current to pass through multiple branches. Some of these branches will have more resistance, and some will have less. The current can be increased by pointing the switch to a branch with less resistance, and decreased by pointing it toward a branch with more resistance.

Capacitor




Another common passive circuit component is the capacitor. Capacitors store energy inside themselves when a current is passed through the circuit. Capacitors have two sides, one that is positive and one that is negative, and the two sides are separated by an insulator. This creates a voltage difference, and the resulting electrostatic field holds energy that can be harnessed by the circuit. Because capacitors are able to store energy, they are useful for moderating circuits that experience variable currents. Capacitors can store or release their energy, as necessary, to increase or decrease the current flowing through the circuit.

Inductors



These are the last of the purely passive components.  An inductor is most commonly a coil, but in reality, even a straight piece of wire has inductance.  Winding it into a coil simply concentrates the magnetic field, and increases the inductance considerably for a given length of wire.  Although there are some very common inductive components (such as transformers, which are a special case), they are not often used in audio.  Small inductors are sometimes used in the output of power amplifiers to prevent instability with capacitive loads.

Active components

Semiconductors



There are many different types of semiconductors, but just a few are outlined below. The basic principle of a semiconductor is that it is only “semi” conductive. It doesn’t allow electricity to flow through it as easily as a conductor like copper, but it permits enough electrical flow to be part of a circuit. In consequence, semiconductors are very useful for altering the properties of an electrical current, and either amplifying it, changing its voltage, or (in modern electronics) altering the signal it communicates. As noted above, there are many different types of semiconductors. Some of the most important are noted and explained below.

Diodes



Most people are familiar with “light emitting diodes,” the tiny light bulbs present on many control panels and other displays. For this reason, many think that emission of light is the key characteristic of diodes, but in fact, not all diodes emit light. Instead, the defining characteristic of a diode is that it only permits electricity to flow in one direction, but not the other. This is because the ends of a diode are polarized, and electrons will only flow across positive-negative poles in one direction, from the negative to the positive. This is useful for controlling the current in a circuit.
Transistors



Transistors use a very small current passed into one of their terminals to control the (typically larger) current flowing through their other terminals. One very useful aspect of transistors is that the current they output can be substantially larger than the current that is passed to them; this is known as amplification. Because transistors use small currents on one terminal to control larger currents on other terminals, they make great switches for machines. Buttons on electrical equipment almost always lead to transistors which then control some option within a device or control chip. Transistors are critical to modern electronics, and have been called the most important invention of the 20th century, because microchips and microcontrollers are, at their essence, just billions of transistors switching in sequence.

Integrated circuits



Integrated circuits are better known today as “chips” or “microprocessors.” They consist of one circuit substrate, with numerous electronic circuits connected to it and operating in tandem. All modern electronics use integrated circuits, or have integrated circuits somewhere within them. Integrated circuits are made up of transistors, discussed above. In general, a single integrated circuit substrate will have billions of transistors, all flicking at once to control the countless options within an electronic device.

Optoelectronic devices



Optoelectronic devices are a huge category of circuit components. The components that fall within this category do one or more of these three things: emit light, sense light, or manage components that emit/sense lights. Fiber optic devices, for one, fall into the category of optoelectronic devices. The same goes for various diodes that do have the property of emitting light. But not only things that laypeople would recognize as “light bulbs” fall into this category. Just as importantly, there are lots of electronic components that are designed to detect light without ever emitting it themselves. Through complex physical reactions, light detectors can actually introduce electricity into a circuit, which is why they qualify as active circuit components.