Buer amplier

A buer amplier (sometimes simply called a buer)is one that provides electrical impedance transformationfrom one circuit to another. Two main types of buerexist: the voltage buer and the current buer.

in

out

+

A in

inout

in

Figure 1: Top: Ideal voltage buer Bottom: Ideal current buer

1 Voltage buerA voltage buer amplier is used to transfer a voltagefrom a rst circuit, having a high output impedance level,to a second circuit with a low input impedance level. Theinterposed buer amplier prevents the second circuitfrom loading the rst circuit unacceptably and interfer-ing with its desired operation. In the ideal voltage buerin the diagram, the input resistance is innite, the outputresistance zero (impedance of an ideal voltage source iszero). Other properties of the ideal buer are: perfect lin-earity, regardless of signal amplitudes; and instant outputresponse, regardless of the speed of the input signal.If the voltage is transferred unchanged (the voltage gainAv is 1), the amplier is a unity gain buer; also knownas a voltage follower because the output voltage followsor tracks the input voltage. Although the voltage gain of avoltage buer amplier may be (approximately) unity, itusually provides considerable current gain and thus powergain. However, it is commonplace to say that it has a gainof 1 (or the equivalent 0 dB), referring to the voltage gain.As an example, consider a Thvenin source (voltage VA,series resistance RA) driving a resistor load RL. Becauseof voltage division (also referred to as loading) the volt-age across the load is onlyVA RL / ( RL + RA ). However,if the Thvenin source drives a unity gain buer such as

that in Figure 1 (top, with unity gain), the voltage input tothe amplier is VA, and with no voltage division becausethe amplier input resistance is innite. At the output thedependent voltage source delivers voltage Av VA = VA tothe load, again without voltage division because the out-put resistance of the buer is zero. A Thvenin equivalentcircuit of the combined original Thvenin source and thebuer is an ideal voltage source VA with zero Thveninresistance.

2 Current buer

Typically a current buer amplier is used to trans-fer a current from a rst circuit, having a high outputimpedance level, to a second circuit with a low inputimpedance level. The interposed buer amplier pre-vents the second circuit from loading the rst circuits cur-rent unacceptably and interfering with its desired opera-tion. In the ideal current buer in the diagram, the out-put impedance is zero and the input impedance is innite.Again, other properties of the ideal buer are: perfect lin-earity, regardless of signal amplitudes; and instant outputresponse, regardless of the speed of the input signal.For a current buer, if the current is transferred un-changed (the current gain i is 1), the amplier is again aunity gain buer; this time known as a current followerbecause the output current follows or tracks the input cur-rent.As an example, consider a Norton source (current IA, par-allel resistance RA) driving a resistor load RL. Because ofcurrent division (also referred to as loading) the currentdelivered to the load is only IA RA / ( RL + RA ). How-ever, if the Norton source drives a unity gain buer suchas that in Figure 1 (bottom, with unity gain), the currentinput to the amplier is IA, with no current division be-cause the amplier input resistance is zero. At the out-put the dependent current source delivers current i IA= IA to the load, again without current division becausethe output resistance of the buer is innite. A Nortonequivalent circuit of the combined original Norton sourceand the buer is an ideal current source IA with inniteNorton resistance.

3 Voltage buer examples

1

2 3 VOLTAGE BUFFER EXAMPLES

3.1 Op-amp implementation

Figure 2: A negative feedback amplier

Vin Vout

Figure 3. An op-ampbased unity gain buer amplier

A unity gain buer amplier may be constructed by ap-plying a full series negative feedback (Fig. 2) to an op-amp simply by connecting its output to its inverting input,and connecting the signal source to the non-inverting in-put (Fig. 3). In this conguration, the entire output volt-age ( = 1 in Fig. 2) is placed contrary and in series withthe input voltage. Thus the two voltages are subtractedaccording to Kirchhos voltage law (KVL) and their dif-ference is applied to the op-amp dierential input. Thisconnection forces the op-amp to adjust its output voltagesimply equal to the input voltage (V follows V so thecircuit is named op-amp voltage follower).The importance of this circuit does not come fromany change in voltage, but from the input and outputimpedances of the op-amp. The input impedance of theop-amp is very high (1 M to 10 T), meaning that theinput of the op-amp does not load down the source anddraws only minimal current from it. Because the outputimpedance of the op-amp is very low, it drives the load asif it were a perfect voltage source. Both the connectionsto and from the buer are therefore bridging connections,which reduce power consumption in the source, distortionfrom overloading, crosstalk and other electromagnetic in-terference.

3.2 Single-transistor circuitsOther unity gain buer ampliers include the bipolarjunction transistor in common-collector conguration(called an emitter follower because the emitter voltagefollows the base voltage, or a voltage follower becausethe output voltage follows the input voltage); the eld ef-fect transistor in common-drain conguration (called a

Figure 4: Top: BJT voltage follower Bottom: Small-signal, low-frequency equivalent circuit using hybrid-pi model

Figure 5: Top: MOSFET voltage follower Bottom: Small-signal,low-frequency equivalent circuit using hybrid-pi model

source follower because the source voltage follows thegate voltage or, again, a voltage follower because the out-put voltage follows the input voltage); or similar cong-urations using vacuum tubes (cathode follower), or otheractive devices. All such ampliers actually have a gain ofslightly less than unity, but the dierence is usually smalland unimportant.

3.2.1 Impedance transformation using the bipolarvoltage follower

Using the small-signal circuit in Figure 4, the impedanceseen looking into the circuit is

3Rin =vxix

= r + ( + 1)(rOjjRL)

(The analysis uses the relation gmr= (IC /VT) (VT /IB)=, which follows from the evaluation of these parametersin terms of the bias currents.) Assuming the usual casewhere rO >> RL, the impedance looking into the bueris larger than the load RL without the buer by a factorof ( + 1), which is substantial because is large. Theimpedance is increased even more by the added r, butoften r

4 6 EXTERNAL LINKS

Common base Common gate Common collector Common drain Negative feedback amplier Driven shield

6 External links Voltage Follower Using OP AMP Unity Gain Buer Amplier Bucknell

57 Text and image sources, contributors, and licenses7.1 Text

Buer amplier Source: http://en.wikipedia.org/wiki/Buffer%20amplifier?oldid=652085279 Contributors: Booyabazooka, Omegatron,Fredrik, Dave Bass, Sam Hocevar, Cmdrjameson, Madhujc, Jshadias, Thunderbird, Nimur, Fresheneesz, Maser228, Light current, Riki-maru, Bggoldie, Gilliam, Bluebot, Simon G Best, Rogerbrent, JoeBot, Vanisaac, Circuit dreamer, Thijs!bot, Trevyn, Rehnn83, Twsx,Anaxial, Serenthia, Larryisgood, Inductiveload, PipepBot, Celestialmechanic, Mild Bill Hiccup, DragonBot, Brews ohare, Addbot, Jncra-ton, Couposanto, , Yobot, Rubinbot, Rchandna, AndyHe829, Abaddon1337, Mkratz, Magasjukur, Npnikhil, ClueBot NG, Snotbot,Sean.bailey00, UA31, Jwang3445, SpecMade, MasterTriangle12, Surface wall and Anonymous: 34

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Voltage bufferCurrent bufferVoltage buffer examplesOp-amp implementation Single-transistor circuits Impedance transformation using the bipolar voltage followerImpedance transformation using the MOSFET voltage followerChart of single-transistor amplifiers

Integrated buffer amplifiers Speaker array amplifiers

Current buffer examplesSingle-transistor circuits

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