Gain & Attenuation

Quick Answer

Gain is the ratio of a stage's output to its input. Above one it amplifies; below one it attenuates, and the loss is usually quoted as a positive figure with the direction understood. Engineers state both in decibels because the steps of a signal chain then add instead of multiplying.

The ratio has to say what it is a ratio of. Voltage gain, current gain and power gain are three different numbers for the same stage, and in decibels they are not even converted the same way: a voltage ratio of two comes to about 6 dB, a power ratio of two to about 3 dB. A gain figure with no quantity named is unfinished.

A negative decibel figure means only that the output sits below the input. Zero decibels means the two are equal, not that nothing came through, which is the commonest misreading of a frequency response plot. Attenuators are the deliberate case, built to a specified loss so that a signal fits the input range of whatever follows.

Gain is also a function of frequency, and often of load and signal level as well. A single number on a datasheet is a value at one operating point, which is why a response is drawn as a curve rather than quoted as a figure — a Bode plot shows gain and phase across the range where the circuit is actually used. Quoting a gain without saying where it was measured hides more than it tells.