Differential amplifier

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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 $V out$ :

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

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.

1 Examples
- 1.1 Long-tailed pair
2 See also
3 External links

[edit] Examples

[edit] Long-tailed pair

A long-tailed pair with current-mirror load and constant-current drive.

A long-tailed pair or LTP is a common design in electronics for implementing a differential amplifier. It consists of two bipolar junction transistors (BJTs), FETs, or vacuum tubes (valves), connected so that the BJT emitters (or FET sources or vacuum tube cathodes) are connected together. The common electrodes are then connected to a large voltage source through a large resistor, forming the "long tail" of the name, the long tail providing an approximate constant current source. In more sophisticated designs, a true (active) constant current source may be substituted for the long tail.

Connected in this fashion, this gives the circuit two inputs which are differentially amplified (subtracted and multiplied) by the pair. The output may be single-ended or differential depending on the needs of the subsequent circuitry.

In a long-tailed pair formed using BJTs, the emitters are connected together, and then through the current source to ground or to a negative supply (for an LTP using NPN transistors). In this form, one of the transistors can be thought of as an amplifier operating in common emitter configuration, and the other as an emitter follower, feeding the other input signal into the emitter of the first stage. Since a transistor will amplify the current flowing between base and emitter, it follows that the current flowing in the collector circuit of the first transistor is proportional to the difference between the two inputs. However since the circuit is totally symmetrical, either element can be viewed as an amplifier or as an emitter follower, understanding does not depend on which role you assign to which device.

Long-tailed pairs are frequently used in circuits that implement linear amplifiers with feedback, in operational amplifiers, and in other circuits that require a differential amplifier.