Differential amplifier
From Free net encyclopedia
Image:Difference amplifier.png 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 amplifiers, or op-amps, and emitter coupled logic gates. Given two inputs <math>V_\mathrm{in}^{+}</math> and <math>V_\mathrm{in}^{-}</math>, a practical differential amplifier gives an output <math>V_\mathrm{out}</math>:
<math>V_\mathrm{out} = A_\mathrm{d}(V_\mathrm{in}^{+} - V_\mathrm{in}^{-}) + A_\mathrm{c}\frac{V_\mathrm{in}^{+} + V_\mathrm{in}^{-}}{2}</math>
where <math>A_\mathrm{d}</math> is the differential-mode gain and <math>A_\mathrm{c}</math> is the common-mode gain.
The common-mode rejection ratio is usually defined as the ratio between differential-mode gain and common-mode gain:
<math>\mathrm{CMRR} = \frac{A_\mathrm{d}}{A_\mathrm{c}}</math>
From the above equation, we can see that as <math>A_\mathrm{c}</math> approaches zero, CMRR approaches infinity. Thus, for a perfectly symmetrical differential amplifier <math>A_\mathrm{c} = 0</math>, and the output voltage is given by,
<math>V_\mathrm{out} = A_\mathrm{d}(V_\mathrm{in}^{+} - V_\mathrm{in}^{-})</math>
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 motors or servos, 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 circuits.
See also
External links
- BJT Differential Amplifier — Circuit and explanationde:Differenzverstärker
pt:Amplificador diferencial lt:Diferencialinis stiprintuvas el:Διαφορικός Ενισχυτής