Differential amplifier

The circuit diagram of the differential amplifier based on the operational amplifier is shown in Fig. 1.

Fig. 1. Differential amplifier based on the operational amplifier

A differential amplifier based on the operational amplifier is an electronic amplifier with an output voltage V_out equal to the difference between V₁ and V₂, multiplied by a constant:

To suppress the common-mode component, i.e. amplification of only the difference between V₁ and V₂, it is necessary to observe the ratio of the resistors' resistances:

R₁ / R_f = R₂ / R_g

With this ratio, the gain of the common-mode component will be equal to zero, and V_out will depend only on the difference between V₁ and V₂:

V_out = R_f / R₁ × (V₂ – V₁)

A differential amplifier is used in signal circuits in which useful information is carried not by a specific voltage value relative to a common conductor, but by the voltage difference between two signal lines.

The simplest example is a current sensor in multimeters or power supplies. In these circuits, the potential difference between the terminals of a low-resistance resistor connected in series with the load and used as a current sensor.

Digital technology interfaces (Ethernet, CAN, USB, HDMI, and others) are implemented using a differential circuit. In it, information is transmitted over pairs of wires. Common mode noise, perceived at the output of the communication line, affects both conductors in a pair equally and is easily suppressed in the input differential amplifier.