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A differential amplifier is a type of an Electronic Amplifier that multiplies the difference between two inputs by some constant factor (the differential Gain ). A differential amplifier is the input stage of Operational Amplifier s, or op-amps, and Emitter Coupled Logic gates. Given two inputs V_\mathrm{in}^{+} and V_\mathrm{in}^{-}, a practical differential amplifier gives an output V_\mathrm{out}:

V_\mathrm{out} = A_\mathrm{d}(V_\mathrm{in}^{+} - V_\mathrm{in}^{-}) + A_\mathrm{c} rac{V_\mathrm{in}^{+} + V_\mathrm{in}^{-}}{2}

where A_\mathrm{d} is the differential-mode gain and A_\mathrm{c} is the common-mode gain.

The Common-mode Rejection Ratio is usually defined as the ratio between differential-mode gain and common-mode gain:

\mathrm{CMRR} = rac{A_\mathrm{d}}{A_\mathrm{c}}

From the above equation, we can see that as A_\mathrm{c} approaches zero, CMRR approaches infinity. The higher the resistance of the current source, R_\mathrm{e}, the lower A_\mathrm{c} is, and the better the CMRR. Thus, for a perfectly symmetrical differential amplifier with A_\mathrm{c} = 0, the output voltage is given by,

V_\mathrm{out} = A_\mathrm{d}(V_\mathrm{in}^{+} - V_\mathrm{in}^{-})

Note that a differential amplifier is a more general form of amplifier than one with a single input; by grounding one input of a differential amplifier, a single-ended amplifier results.

Differential amplifiers are found in many systems that utilise Negative Feedback , where one input is used for the input signal, the other for the feedback signal. A common application is for the control of Motor s or Servo s, as well as for signal amplification applications. In discrete Electronics , a common arrangement for implementing a differential amplifier is the Long-tailed Pair , which is also usually found as the differential element in most op-amp Integrated Circuit s.


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