A differential amplifier is a type of electronic amplifier that amplifies the difference between two input voltages but suppresses any voltage common to the two inputs. It is an analog circuit with two inputs ${\displaystyle V_{\text{in}}^{-}}$ and ${\displaystyle V_{\text{in}}^{+}}$ and one output ${\displaystyle V_{\text{out}}}$, in which the output is ideally proportional to the difference between the two voltages:

where ${\displaystyle A}$ is the gain of the amplifier.

Single amplifiers are usually implemented by either adding the appropriate feedback resistors to a standard op-amp, or with a dedicated integrated circuit containing internal feedback resistors. It is also a common sub-component of larger integrated circuits handling analog signals.

where ${\displaystyle V_{\text{in}}^{+}}$ and ${\displaystyle V_{\text{in}}^{-}}$ are the input voltages, and ${\displaystyle A_{\text{d}}}$ is the differential gain.

In practice, however, the gain is not quite equal for the two inputs. This means, for instance, that if ${\displaystyle V_{\text{in}}^{+}}$ and ${\displaystyle V_{\text{in}}^{-}}$ are equal, the output will not be zero, as it would be in the ideal case. A more realistic expression for the output of a differential amplifier thus includes a second term:

where ${\displaystyle A_{\text{c}}}$ is called the common-mode gain of the amplifier.

As differential amplifiers are often used to null out noise or bias voltages that appear at both inputs, a low common-mode gain is usually desired.

The common-mode rejection ratio (CMRR), usually defined as the ratio between differential-mode gain and common-mode gain, indicates the ability of the amplifier to accurately cancel voltages that are common to both inputs. The common-mode rejection ratio is defined as