In a nutshell, electronic systems deal with electric currents that change value rapidly. Various components that can exist in a circuit act on those changes in different ways.
Every circuit will contain components that are used to either control the flow of electric energy or respond to it. Active elements direct the flow of current, while passive components will set power around active components as well as store or dissipate electric energy.
Passive components mostly include two-terminal components such as resistors, capacitors, inductors, and transformers. They are components incapable of controlling (directing) current by means of another electrical signal.
Most electrical components are passive. No matter what they are doing in the circuit there is a net loss. They mainly focus on setting voltage and current around other components.
Active devices control the flow of current and contribute some sort of a gain.
Circuit of the LM4765-2×30 Watt operational amplifier
The difference between active and passive is that active components require a power source to work, whereas passive components do not. Think of active components as tiny black-boxes that need their own power to work. Active components include some sort of gain, such as op-amplifiers, transistors, triode vacuum tubes (valves), and tunnel diodes.
A good example of gain is a record player amplifier which takes tiny electrical wiggles from the needle moving across the record and amplifies them so they are big enough to move a speaker cone. If you accepted that statement maybe we should pause. How can a device give out more than it takes in?
The term amplifier is mostly a lie. Nothing gets amplified. The device uses a small input current to control a much larger output current from another source of power. So the output of the amplifier really consists of a larger image of the input signal which is not itself really made bigger. A small current is simply controlling a larger one. In a way, you can say this is not unlike hydraulic systems like a crane, a small force on a small lever can control a larger force to move the crane. In the case of the crane the hydraulic system uses a series of different sized pistons to actually amplify the force, while in an electronic amplifier, one current controls another separate larger one.
Vacuum tubes and transistors
Plato said necessity is the mother of invention. One problem that plagued Edison in the early days of the successful light bulb, was the filament would slop over and burn out. Edison had a simple idea to fix this by attaching a metal lead from the center of the filament to the top of the glass bulb to hold it up. It would extend out of the glass at the top of the bulb. He noted he would receive a shock if he touched this metal support. He discovered that if this plate had a positive voltage applied to it the light would work, but if it had a negative voltage the light would not work. This is essential in understanding why amplifiers work.
The Fleming valve was the first practical application of the “Edison effect” (thermionic emission) discovered in 1883 by Thomas Edison shortly after his invention of the incandescent light bulb, that is, the emission of electrons by a lamp’s heated filament to a nearby metal plate.
A vacuum tube is a light bulb with a mesh of wire between the filament and another electrode. When a small current is passed over the wire mesh, a larger current would flow from the filament through the mesh and out the other electrode. The British called them valves which makes a lot of sense.
In 1906, Lee DeForest added the grid element to the Fleming Valve and created the triode vacuum tube, which he called the “Audion”. It was the first purely electronic component that could amplify a signal and with it’s invention, radio, television, and a host of other technologies became possible. He was issued US patent number 879,532 on February 18, 1908 for this invention.
The earliest Audions had a single grid and plate (single wing); later ones had two grids and plates (double wing). In most bulbs, two filaments were provided so that the second could be used when the first one burned out. In some later bulbs intended for amplifier use, both filaments were used simultaneously for greater output.
Birth of a new era
Vacuum tubes help create so many advancements in culture and society from the Television, to radio and communications. It is the grandfather to the information age. That said, there are draw-backs to using vacuum tubes. They get very hot and because of this require a lot of power. They also burn out a lot and require constant expensive replacements. The heating element needs to also heat up before it will operate correctly.
Inside is a heated metal element that emits electrons and a short distance away is a positive metal plate. The flow of these electrons is a current. Between the two elements is a fine wire mesh. As the electrical potential is changed on the mesh the flow of electrons is influenced. When the mesh is made more negative the flow of electrons is reduced. A very small change on the mesh can influence a much larger current.
Depending on the way the initial currents are adjusted, the two can either create a larger image of the input signal. That’s called analog or linear operation, or you can make the switch totally on or off, called digital. The tube is very inefficient as it requires a lot of power to heat up.
Then something happened that changed the world again. Someone figured out how to do the same thing without a heating element.
The transistor can do the same job as the vacuum tube but do it much smaller, cheaper and better. They are made of classes of metals called semiconductors. A standard transistor is the junction of 3 pieces of semiconducting material. One of the most famous of them is silicon. It is turned into a semiconductor by the careful placing of impurities. This provides a measured amount of resistance throughout the material as well as conductance at specific places.
The transistor is an active device with three connections. It does not require heat to work so it is more efficient that tubes and are a lot smaller and cheaper. Vacuum tubes required several hundred volts for their operation. Transistors will work with just 2 volts.
From the site The Transistor – History & Developments
Out of the blue, the wet device created the largest amplification he’d seen so far. He and another scientist, Robert Gibney, stared at the experiment, stunned. They began fiddling with different knobs and buttons: by turning on a positive voltage they increased the effect even more; turning it to negative could get rid of it completely. It seemed that whatever those electrons had been doing on the surface to block amplification had somehow been canceled out by the water — the greatest obstacle to building an amplifier had been overcome.