A differential amplifier is a type of electronic amplifier that amplifies the difference between two input voltages only. The differential amplifier two inputs are the inverting input and non-inverting input. Its output signal is 180° out of phase with inverting input signal and in phase with non-inverting input signal.

The different modes of operation of the differential amplifier are:

- Common mode input signal v
_{CM}. Both inputs are connected together to the input signal. - Large differential input signal.
- Small differential input signal v
_{i}<2v_{T}

The circuit below shows the differential amplifier with a large differential input signal. When an Operational Amplifier (OPAMP) is used in this configuration it is also called a comparator.

Notes:

- When both V1 and V2 are 2.5V, see the small signal input circuit DC analysis for calculating the voltages.
- When V2-2v
_{T}< V1 < V2+2v_{T}, the small signal input circuit AC analysis applies. - When V1>V2+2v
_{T}, Q1 is on and Q2 is off. \begin{equation} V_1 = V_{R1} + V_{BE} \end{equation} Ignoring I_{B}, thus I_{C}= I_{E}\begin{equation} V_{R2} = {(V_1 - V_{BE})R2 \over R1} \end{equation} - When V1<V2-2v
_{T}, Q1 is off and Q2 is on. \begin{equation} V_2 = V_{R1} + V_{BE} \end{equation} \begin{equation} V_{R3} = {(V_2 - V_{BE})R3 \over R1} \end{equation} - Move the knob to see the effects of V1 on the inverting and non-inverting outputs.