In electronics, amplification is a measure of the ability of a two-port circuit (often an amplifier) to increase the power or amplitude of an input signal to the output port by adding the converted energy from a power source to the signal. It is often expressed using the logarithmic decibel units (dB) (“dB gain”).
Decibel (symbol: dB) is a logical unit used to express the ratio of one physical property value to another and can be used to express a value change (e.g. +1 dB or -1 dB) value. In the latter case, it expresses the ratio between a value and a reference value; when used in this way, the decibel symbol must be attached with a suffix indicating the reference value or other property. For example, if the reference value is 1 volt, then the suffix is “V” (ie “20 dBV”) and if the reference value is 1 milliwatt, the suffix is “m” (ie “20 dBm”). However, the acoustic pressure level refers to the “hearing threshold” (generally given at 20 micropascals at 1 kHz), and the suffix is “SPL” (ie “60 dB SPL”).
There are two different scales used to express a ratio in decibels, depending on the nature of the quantity: the field quantity ratio or the quantitative power ratio. (The quantitative field ratio is also referred to as the root power ratio or amplitude ratio.) When expressing amounts of power, the decibel count is ten times the logarithm at the base 10 of the ratio between two power quantities.
This means that a power factor change of 10 corresponds to a change of 10 dB. When field quantities are expressed, a change in amplitude by a factor of 10 corresponds to a change in the level of 20 dB. The additional factor of two is due to the logarithm of the square relation between power and amplitude. The decibel scales differ, so direct comparisons can be made between the power and the amount of associated field when expressed in decibels.
The definition of decibel is based on telephony power measurement at the beginning of the twentieth century in the Bell system in the United States. A decibel is a tenth (so) of a bel, named in honor of Alexander Graham Bell; however, bel is rarely used.
Today, the decibel is used for a wide variety of measurements in science and engineering, most prominent in acoustic theory, electronics and control. In electronics, gains of amplifiers, signal attenuation, and signal-to-noise ratios are often expressed in decibels.