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Long-tailed pair : ウィキペディア英語版
Differential amplifier

A differential amplifier is a type of electronic amplifier that amplifies the difference between two input voltages but suppresses any voltage common to the two inputs. It is an analog circuit with two inputs \scriptstyle V_\text^- and \scriptstyle V_\text^+ and one output \scriptstyle V_\text in which the output is ideally proportional to the difference between the two voltages
:V_\text = A(V_\text^+ - V_\text^-)
where \scriptstyle A is the gain of the amplifier.
== Theory ==

Many electronic devices use differential amplifiers internally.
The output of an ideal differential amplifier is given by:
:V_\text = A_\text(V_\text^+ - V_\text^-)
Where V_\text^+ and V_\text^- are the input voltages and A_\text is the differential gain.


In practice, however, the gain is not quite equal for the two inputs. This means, for instance, that if V_\text^+ and V_\text^- are equal, the output will not be zero, as it would be in the ideal case. A more realistic expression for the output of a differential amplifier thus includes a second term.
:V_\text = A_\text(V_\text^+ - V_\text^-) + A_\text\left(\frac^-}\right)
A_\text is called the common-mode gain of the amplifier.


As differential amplifiers are often used to null out noise or bias-voltages that appear at both inputs, a low common-mode gain is usually desired.
The common-mode rejection ratio (CMRR), usually defined as the ratio between differential-mode gain and common-mode gain, indicates the ability of the amplifier to accurately cancel voltages that are common to both inputs. The common-mode rejection ratio is defined as:
:\mathrm = 10\log_ \left (\frac} \right)^2 = 20\log_ \left (\frac|} \right)
In a perfectly symmetrical differential amplifier, A_\text is zero and the CMRR is infinite. 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.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
ウィキペディアで「Differential amplifier」の詳細全文を読む



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