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| A '''buffer amplifier''' (sometimes simply called a '''buffer''') is one that provides [[electrical impedance]] transformation from one circuit to another. Two main types of buffer exist: the '''voltage buffer''' and the '''current buffer'''.
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| [[Image:Ideal Buffers.svg|thumbnail|200px|Figure 1: Top: Ideal voltage buffer Bottom: Ideal current buffer]]
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| ==Voltage buffer==
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| A voltage buffer amplifier is used to transfer a voltage from a first circuit, having a high [[output impedance]] level, to a second circuit with a low [[input impedance]] level. The interposed buffer amplifier prevents the second circuit from loading the first circuit unacceptably and interfering with its desired operation. In the ideal voltage buffer in the diagram, the input resistance is infinite, the output resistance zero (impedance of an ideal voltage source is zero). Other properties of the ideal buffer are: perfect linearity, regardless of signal amplitudes; and instant output response, regardless of the speed of the input signal.
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| If the voltage is transferred unchanged (the voltage [[gain]] ''A<sub>v</sub>'' is 1), the amplifier is a '''unity gain buffer'''; also known as a '''voltage follower''' because the output voltage ''follows'' or tracks the input voltage. Although the voltage gain of a voltage buffer amplifier may be (approximately) unity, it usually provides considerable current gain and thus power gain. However, it is commonplace to say that it has a gain of 1 (or the equivalent 0 [[decibel|dB]]), referring to the voltage gain.
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| As an example, consider a [[Thevenin's theorem|Thévenin source]] (voltage ''V<sub>A</sub>'', series resistance ''R<sub>A</sub>'') driving a resistor load ''R<sub>L</sub>''. Because of [[voltage division]] (also referred to as "loading") the voltage across the load is only ''V<sub>A</sub> R<sub>L</sub> / ( R<sub>L</sub> + R<sub>A</sub> )''. However, if the Thévenin source drives a unity gain buffer such as that in Figure 1 (top, with unity gain), the voltage input to the amplifier is ''V<sub>A</sub>'', and with ''no voltage division'' because the amplifier input resistance is infinite. At the output the dependent voltage source delivers voltage ''A<sub>v</sub> V<sub>A</sub> = V<sub>A</sub>'' to the load, again without voltage division because the output resistance of the buffer is zero. A Thévenin equivalent circuit of the combined original Thévenin source ''and'' the buffer is an ideal voltage source ''V<sub>A</sub>'' with zero Thévenin resistance.
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| ==Current buffer==
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| Typically a current buffer amplifier is used to transfer a current from a first circuit, having a low [[output impedance]] level, to a second circuit with a high [[input impedance]] level. The interposed buffer amplifier prevents the second circuit from loading the first circuit unacceptably and interfering with its desired operation. In the ideal current buffer in the diagram, the input impedance is zero and the output impedance is infinite (impedance of an ideal current source is infinite). Again, other properties of the ideal buffer are: perfect linearity, regardless of signal amplitudes; and instant output response, regardless of the speed of the input signal.
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| For a current buffer, if the current is transferred unchanged (the current [[gain]] ''β<sub>i</sub>'' is 1), the amplifier is again a '''unity gain buffer'''; this time known as a '''current follower''' because the output current ''follows'' or tracks the input current.
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| As an example, consider a [[Norton's theorem|Norton source]] (current ''I<sub>A</sub>'', parallel resistance ''R<sub>A</sub>'') driving a resistor load ''R<sub>L</sub>''. Because of [[current division]] (also referred to as "loading") the current delivered to the load is only ''I<sub>A</sub> R<sub>A</sub> / ( R<sub>L</sub> + R<sub>A</sub> )''. However, if the Norton source drives a unity gain buffer such as that in Figure 1 (bottom, with unity gain), the current input to the amplifier is ''I<sub>A</sub>'', with ''no current division'' because the amplifier input resistance is zero. At the output the dependent current source delivers current ''β<sub>i</sub> I<sub>A</sub> = I<sub>A</sub>'' to the load, again without current division because the output resistance of the buffer is infinite. A Norton equivalent circuit of the combined original Norton source ''and'' the buffer is an ideal current source ''I<sub>A</sub>'' with infinite Norton resistance.
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| ==Voltage buffer examples==
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| === Op-amp implementation ===
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| [[Image:Block Diagram for Feedback.svg|thumb|Figure 2: A negative feedback amplifier|290px|left]][[Image:Op-Amp Unity-Gain Buffer.svg|230px|thumb|Figure 3. An [[op-amp]]–based unity gain buffer amplifier]]
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| A [[1 (number)|unity]] gain buffer amplifier may be constructed by applying a full series [[negative feedback amplifier#Feedback and amplifier type|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 input (Fig. 3). In this configuration, the entire output voltage (β = 1 in Fig. 2) is placed contrary and in series with the input voltage. Thus the two voltages are subtracted according to [[Kirchhoff's circuit laws#Kirchhoff's voltage law (KVL)|Kirchhoff's voltage law]] (KVL) and their difference is applied to the op-amp differential input. This connection forces the op-amp to adjust its output voltage simply equal to the input voltage (V<sub>out</sub> follows V<sub>in</sub> so the circuit is named op-amp voltage follower).
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| The importance of this circuit does not come from any change in voltage, but from the input and output impedances of the op-amp. The input impedance of the op-amp is very high (1 [[Ohm|MΩ]] to 10 [[teraohm|TΩ]]), meaning that the input of the op-amp does not load down the source and draws only minimal current from it. Because the output impedance of the op-amp is very low, it drives the load as if it were a perfect [[voltage source]]. Both the connections to and from the buffer are therefore [[impedance bridging|bridging]] connections, which reduce power consumption in the source, [[distortion]] from overloading, [[crosstalk]] and other [[electromagnetic interference]]. | |
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| === Single-transistor circuits ===
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| [[Image:Bipolar Voltage Follower.png|thumbnail|150px|Figure 4: Top: BJT voltage follower Bottom: Small-signal, low-frequency [[equivalent circuit]] using [[hybrid-pi model]]]]
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| [[Image:MOSFET Voltage Follower.png|thumbnail|150px|Figure 5: Top: MOSFET voltage follower Bottom: Small-signal, low-frequency [[equivalent circuit]] using [[hybrid-pi model]]]]
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| Other unity gain buffer amplifiers include the [[bipolar junction transistor]] in [[common collector|common-collector]] configuration (called an ''emitter follower'' because the emitter voltage follows the base voltage, or a ''voltage follower'' because the output voltage follows the input voltage); the [[field effect transistor]] in [[common drain|common-drain]] configuration (called a [[source follower]] because the source voltage follows the gate voltage or, again, a ''voltage follower'' because the output voltage follows the input voltage); or similar configurations using [[vacuum tube]]s ([[cathode follower]]), or other active devices. All such amplifiers actually have a gain of slightly less than unity, but the difference is usually small and unimportant.
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| ====Impedance transformation using the bipolar voltage follower====
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| Using the small-signal circuit in Figure 4, the impedance seen looking into the circuit is
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| ::<math> R_{in} = \frac {v_x} {i_x} = r_{\pi} + (\beta + 1) ({r_O} || {R_L}) </math>
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| (The analysis uses the relation ''g<sub>m</sub>r<sub>π</sub> = (I<sub>C</sub> /V<sub>T</sub>) (V<sub>T</sub> /I<sub>B</sub>)'' = β, which follows from the evaluation of these parameters in terms of the bias currents.) Assuming the usual case where ''r<sub>O</sub>'' >> ''R<sub>L</sub>'', the impedance looking into the buffer is larger than the load ''R<sub>L</sub>'' without the buffer by a factor of (β + 1), which is substantial because β is large. The impedance is increased even more by the added ''r<sub>π</sub>'', but often ''r<sub>π</sub>'' << (β + 1) R<sub>L</sub>, so the addition does not make much difference
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| ====Impedance transformation using the MOSFET voltage follower====
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| Using the small-signal circuit in Figure 5, the impedance seen looking into the circuit is no longer ''R<sub>L</sub>'' but instead is infinite (at low frequencies) because the MOSFET draws no current.
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| As frequency is increased, the parasitic capacitances of the transistors come into play and the transformed input impedance drops with frequency.
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| ====Chart of single-transistor amplifiers====
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| All configurations of a single-transistor amplifier can be used as a buffer to isolate the driver from the load. For most digital applications, an NMOS voltage follower (common drain) is the preferred configuration; or an inverter (common source), if necessary. These amplifiers have high input impedance, which means that the digital system will not need to supply a large current.
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| {| class="wikitable" border="1"
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| |-
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| ! Amplifier type
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| ! MOSFET (NMOS)
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| ! BJT (npn)
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| ! Notes
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| |-
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| | Common source / Common emitter
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| | [[File:N-channel JFET common source.svg|thumb|[[Common source]]]]
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| | [[File:NPN common emitter.svg|thumb|[[Common emitter]]]]
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| | [[Inverter (logic gate)|Inverts]] the input signal
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| |-
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| | Common gate / Common base
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| | [[File:N-channel JFET common gate.svg|thumb|[[Common gate]]]]
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| | [[File:NPN common base.svg|thumb|[[Common base]]]]
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| | Typically used for voltage buffering (not current buffering); generally unsuitable for [[Transistor–transistor logic|TTL]] voltage buffer
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| |-
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| | Common drain / Common collector
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| | [[File:N-channel JFET source follower.svg|thumb|[[Common drain]]]]
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| | [[File:NPN emitter follower.svg|thumb|[[Common collector]]]]
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| | Voltage gain is close to unity , used for current buffering, highly dependant on the beta of a transistor.
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| |}
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| === Integrated buffer amplifiers ===
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| It is common for a single [[Dual in-line package|package]] to contain several discrete buffer amplifiers. For example, a '''hex buffer''' is a single package containing 6 discrete buffer amplifiers, and an '''octal buffer''' is a single package containing 8 discrete buffer amplifiers.
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| === Speaker array amplifiers ===
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| The majority of amplifiers used to drive large speaker arrays, such as those used for rock concerts, are unity-gain, high-current amplifiers. Some current amplifiers take the voltage output from Class A/B, B, or tube (valve) amplifiers, while others contain built-in voltage amplifiers as a pre-amp stage. The result is a signal nearly identical to the input signal in terms of voltage, but capable of sending high amounts of current into low impedance speaker arrays where the speakers are wired in parallel.
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| ==Current buffer examples==
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| Simple unity gain buffer amplifiers include the [[bipolar junction transistor]] in [[common base|common-base]] configuration, or the [[MOSFET]] in [[common gate|common-gate]] configuration (called a ''current follower'' because the output current follows the input current). The current gain of a current buffer amplifier is (approximately) unity.
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| === Single-transistor circuits ===
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| [[Image:Bipolar current follower2.PNG|thumbnail|200px|Figure 6: Bipolar current follower biased by current source ''I<sub>E</sub>'' and with active load ''I<sub>C</sub>'' ]]
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| Figure 6 shows a bipolar current buffer biased with a current source (designated ''I<sub>E</sub>'' for DC emitter current) and driving another DC current source as active load (designated ''I<sub>C</sub>'' for DC collector current). The AC input signal current ''i<sub>in</sub>'' is applied to the emitter node of the transistor by an AC [[Norton's theorem|Norton current source]] with Norton resistance ''R<sub>S</sub>''. The AC output current ''i<sub>out</sub>'' is delivered by the buffer via a large coupling capacitor to load ''R<sub>L</sub>''. This coupling capacitor is large enough to be a short-circuit at frequencies of interest.
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| Because the transistor output resistance connects input and output sides of the circuit, there is a (very small) backward voltage feedback from the output to the input so this circuit is not unilateral. In addition, for the same reason, the input resistance depends (slightly) upon the output load resistance, and the output resistance depends significantly on the input driver resistance. For more detail see the article on [[common base|common base amplifier]].
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| == See also ==
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| * [[Preamplifier]]
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| * [[Common base]]
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| * [[Common gate]]
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| * [[Common collector]]
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| * [[Common drain]]
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| * [[Negative feedback amplifier]]
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| == External links ==
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| *[http://www.mycircuits9.com/2012/12/voltage-follower-using-op-amp.html Voltage Follower Using OP AMP]
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| * [http://www.facstaff.bucknell.edu/mastascu/elessonshtml/OpAmps/OpAmp3Note1Buffer.html Unity Gain Buffer Amplifier — Bucknell]
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| [[Category:Electronic amplifiers]]
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