Differential amplifier: Difference between revisions

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Differential amplifier schematic. *Biasing and other accompanying circuitry not shown.*

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 $V_{\mathrm {in} }^{+}$ and $V_{\mathrm {in} }^{-}$ , a practical differential amplifier gives an output $V_{\mathrm {out} }$ :

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

where A_d is the differential-mode gain and A_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:

$\mathrm {CMRR} ={\frac {A_{\mathrm {d} }}{A_{\mathrm {c} }}}$

From the above equation, we can see that as A_c approaches zero, CMRR approaches infinity. The higher the resistance of the current source, R_e, the lower A_c is, and the better the CMRR. Thus, for a perfectly symmetrical differential amplifier with A_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 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.

External links

BJT Differential Amplifier — Circuit and explanation

@@ Line 4: / Line 4: @@
 <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.
+where '''''A''<sub>d</sub>''' is the differential-mode gain and '''''A''<sub>c</sub>''' is the common-mode gain.
 The [[common-mode rejection ratio]] is usually defined as the ratio between differential-mode gain and common-mode gain:
@@ Line 10: / Line 10: @@
 <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.  The higher the resistance of the current source, <math>R_\mathrm{e}</math>, the lower <math>A_\mathrm{c}</math> is, and the better the CMRR. Thus, for a perfectly symmetrical differential amplifier with <math>A_\mathrm{c} = 0</math>, the output voltage is given by,
+From the above equation, we can see that as '''''A''<sub>c</sub>''' approaches zero, CMRR approaches infinity.  The higher the resistance of the current source, '''R<sub>e</sub>''', the lower '''''A''<sub>c</sub>''' is, and the better the CMRR. Thus, for a perfectly symmetrical differential amplifier with '''''A''<sub>c</sub>''' = 0, the output voltage is given by,
 <math>V_\mathrm{out} = A_\mathrm{d}(V_\mathrm{in}^{+} - V_\mathrm{in}^{-})</math>

Revision as of 15:05, 19 May 2006

See also

External links