NationMaster - Encyclopedia: Differential amplifier

A differential amplifier is a type of 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

: Image File history File links Op-amp_symbol. ... An operational amplifier or op-amp is a very high-gain amplifier which has two inputs, one inverting (âˆ’) and one non-inverting (+). The output voltage is the difference between the + and âˆ’ inputs, multiplied by the open-loop gain: The amplifiers output can be single-ended or, less often, differential. ... The term amplifier as used in this article can mean either a circuit (or stage) using a single active device or a complete system such as a packaged audio hi-fi amplifier. ... In electronics, gain is usually taken as the mean ratio of the signal output of a system to the signal input of the system. ... Op-amp ICs (some single, some dual) in 8-pin dual in-line packages (DIPs) An operational amplifier, usually referred to as an op-amp for brevity, is a DC-coupled high-gain electronic voltage amplifier with differential inputs[1] and, usually, a single output. ... In electronics, emitter coupled logic, or ECL, is a logic family in which current is steered through bipolar transistors to compute logical functions. ...

$V_mathrm{out} = A_mathrm{d}(V_mathrm{in}^{+} - V_mathrm{in}^{-}) + A_mathrm{c}left(frac{V_mathrm{in}^{+} + V_mathrm{in}^{-}}{2}right)$

The common-mode rejection ratio is usually defined as the ratio between differential-mode gain and common-mode gain: Common mode rejection ratio (CMRR) is a measure of the capability of an instrument, ie. ...

In the above equation, 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:

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. This article does not cite any references or sources. ... For other kinds of motors, see motor. ... Look up servo in Wiktionary, the free dictionary. ... Surface mount electronic components Electronics is the study of the flow of charge through various materials and devices such as semiconductors, resistors, inductors, capacitors, nano-structures and vacuum tubes. ... Simple form of a differential amplifier A differential amplifier is a type of an electronic amplifier that multiplies the difference between two inputs by some constant factor (the differential gain). ... Integrated circuit of Atmel Diopsis 740 System on Chip showing memory blocks, logic and input/output pads around the periphery Microchips (EPROM memory) with a transparent window, showing the integrated circuit inside. ...

Examples

Long-tailed pair

A long-tailed pair or LTP is a common design in electronics for implementing a differential amplifier. It amplifies the current with very little voltage gain. 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. Image File history File links Long-tailed-pair. ... Image File history File links Long-tailed-pair. ... Surface mount electronic components Electronics is the study of the flow of charge through various materials and devices such as semiconductors, resistors, inductors, capacitors, nano-structures and vacuum tubes. ... A bipolar junction transistor (BJT) is a type of transistor. ... Large power N-channel field effect transistor The field-effect transistor (FET) is a transistor that relies on an electric field to control the shape and hence the conductivity of a channel in a semiconductor material. ... Structure of a vacuum tube diode Structure of a vacuum tube triode In electronics, a vacuum tube, electron tube, or (outside North America) thermionic valve or just valve, is a device used to amplify, switch or modify a signal by controlling the movement of electrons in an evacuated space. ... International safety symbol Caution, risk of electric shock (ISO 3864), colloquially known as high voltage symbol. ... Resistor symbols (American) Resistor symbols (Europe, IEC) Axial-lead resistors on tape. ... It has been suggested that this article or section be merged with Voltage source. ...

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. Common emitter amplifier, voltage divider bias (CEVDB) circuit configuration A common emitter is a type of electronic amplifier stage based on a bipolar transistor in series with a load element such as a resistor. ... In electronics, a common collector circuit, also known as an emitter follower circuit, refers to one type of circuit arrangement in which a bipolar transistor drives a load circuit such as a resistor or the next stage in an electronic amplifier. ...

The output from a differential amplifier is itself often differential. If this is not desired, then only one output can be used, disregarding the other output. Or to avoid sacrificing gain, a differential to single-ended converter can be utilized. This is often implemented as a current source.

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. For other uses, see Feedback (disambiguation). ... Op-amp ICs (some single, some dual) in 8-pin dual in-line packages (DIPs) An operational amplifier, usually referred to as an op-amp for brevity, is a DC-coupled high-gain electronic voltage amplifier with differential inputs[1] and, usually, a single output. ...

When used as a switch, the "left" base/grid is used as signal input and the "right" base/grid is grounded; output is taken from the right collector/plate. When the input is zero or negative, the output is close to zero; when the input is positive, the output is most-positive, dynamic operation being the same as the amplifier use described above.

Bias stability and independence from variations in device parameters can be improved by negative feedback introduced via cathode/emitter resistors.

Historical background

The long-tailed pair was originally a pair of vacuum tubes, about 20 years before transistors would be practically available. The circuit works the same way for all three-terminal devices with current gain. Structure of a vacuum tube diode Structure of a vacuum tube triode In electronics, a vacuum tube, electron tube, or (outside North America) thermionic valve or just valve, is a device used to amplify, switch or modify a signal by controlling the movement of electrons in an evacuated space. ...

The long-tailed pair circuit was designed and patented by Alan Blumlein in 1936 as an amplifier for small signals, and later applied to switching functions in radar and television. Today, its main feature is mostly vestigial, by virtue of the fact that long-tail resistor circuit bias points are largely determined by Ohm's Law and less so by active component characteristics. Alan Dower Blumlein was an electronics engineer who made a great many inventions in telecommunications, sound recording, stereo, television and radar. ...

The long-tailed pair was very successfully used in early British computing, most notably the Pilot ACE Model and descendants, Wilkes' EDSAC, and probably others designed by people who worked with Blumlein or his peers. The long-tailed pair has many attributes as a switch: largely immune to tube (transistor) variations (of great importance when machines contained 1,000 or more tubes), high gain, gain stability, high input impedance, medium/low output impedance, good clipper (with not-too-long tail), non-inverting (EDSAC contained no inverters!) and large output voltage swings. One disadvantage is that the output voltage swing (typically ± 10-20 V) was imposed upon a high DC voltage (200 V or so), requiring care in signal coupling, usually some form of wide-band DC coupling. Many computers of this time tried to avoid this problem by using only AC-coupled pulse logic, which made them very large and overly complex (ENIAC: 18,000 tubes for a 20 digit calculator) or unreliable. DC-coupled circuitry became the norm after the first generation of vacuum tube computers. EDSAC EDSAC (Electronic Delay Storage Automatic Calculator) was an early British computer (one of the first computers to be created). ... ENIAC ENIAC, short for Electronic Numerical Integrator And Computer,[1] was the first large-scale, electronic, digital computer capable of being reprogrammed to solve a full range of computing problems,[2] although earlier computers had been built with some of these properties. ...

Details of the long-tailed pair circuitry used in early computing can be found in "Alan Turing's Automatic Computing Engine" (Oxford University Press, 2005, ISBN 0 19 856593 3) in Part IV, 'ELECTRONICS'.

Contents

Examples

Long-tailed pair

Historical background

See also

External links