In electronics, an amplifier is a device or circuit that increases or magnifies the strength of an electrical signal. This ability finds them use in audio and video devices as well as in other technological applications, such as computing.
The term gain refers to the factor by which an amplifier multiplies a signal. Most amplifier circuits are voltage amplifiers, and their gain is the proportional increase in voltage from the input signal to the output signal. In some circuits, however, gain is the increase in current or power. Some amplifiers have negative gain values, indicating that a positive input voltage is amplified to a negative output voltage, and vice versa.
An amplifier can be thought of as a variable switch. Instead of simply switching a current on or off, it responds to a low-amplitude signal by producing high-amplitude variations in output. All amplifiers rely on a power source, such as a dry cell or other direct-current (DC) supply.
Vacuum tubes
The devices at the cores of the earliest electronic amplifiers were vacuum-tube triodes, sometimes called valves for their ability to control the flow of current. A vacuum-tube triode contains a pair of solid parallel-plate electrodes-the emitter and collector-separated by a gap occupied by low-pressure gas that is essentially a vacuum. A third electrode-the base-is a metal grid between the other two electrodes and parallel to them.
The emitter is connected to the negative terminal of a DC power supply, and a heating filament causes its electrons to form an invisible "cloud" around the emitter. This reaction is a manifestation of an effect called thermionic emission, whereby thermal energy causes electrons to "boil" from hot metal surfaces.
The collector is connected to the positive terminal of the DC supply. Depending on the voltage of the base relative to the emitter, the electric field between the emitter and collector can pull electrons through the vacuum, causing a current to flow. A positive voltage of the base relative to the emitter encourages electrons to flow to the collector, whereas a slight negative voltage impedes that flow. The input signal causes a current to flow between the base and the emitter, but variations in that current are much smaller than variations in the flow of current between the emitter and the collector-this differential is the basis of the amplification effect. When the amplifier output is connected to a load, the current amplification results in a voltage amplification.
Because vacuum-tube triodes can be built to withstand heavy current flows, they continue to be used for some high-power applications-the amplification of signals for radio and television transmission, for example. In most applications, however, semiconductor transistors take the place of triodes. Semiconductor amplifiers are more compact than vacuum tube amplifiers and more resistant to physical shocks and vibrations.
Bipolar transistors
The transistors in solid-state semiconductor amplifiers have three electrodes, just as the triodes in tube amplifiers do. If the transistor is of the bipolar-junction type, it consists of an n-p-n or p-n-p trilayer of n-type and p-type silicon. The emitter and collector electrodes connect to the outer layers of semiconductor, and the base electrode connects to the middle layer. The amplified current that flows through the emitter-collector circuit increases in proportion to the current through the base-emitter circuit.
