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In communication systems, gain quantifies the extent of signal amplification or attenuation.

Power gain denotes the common logarithm of the ratio of output power to input power and is expressed in bels.

For practical measurements, a decibel is commonly used. It is a subunit equivalent to one-tenth of a bel.

Consider a network with input and output voltages and resistances. If the output power equals the input power in this network, the gain is zero decibels.

When the output power becomes twice the input power, the gain is of three decibels. Conversely, if the output power falls to half the input power, the gain is of minus three decibels.

Alternatively, power can be represented using voltage and resistance.

The rules of logarithms are applied to obtain an expression for power gain in terms of the voltage and resistance ratios.

When the input resistance equals the load resistance, the gain becomes proportional to the output and input voltage ratio.

Similarly, the gain can be represented in terms of the current and resistance ratios.