Editing Buffer amplifier

{{Short description|Electronic amplifier, a circuit component}}
[[Image:Ideal Buffers.svg|thumbnail|200px|Figure 1: Ideal voltage buffer (top) and current buffer (bottom)]]{{Not to be confused with|Data buffer}}
In [[electronics]], a '''buffer amplifier''' is a [[unity gain]] [[amplifier]] that copies a [[signal]] from one [[Electrical network|circuit]] to another while transforming its [[electrical impedance]] to provide a more ideal source (with a ''lower'' [[output impedance]] for a '''voltage buffer''' or a ''higher'' output impedance for a '''current buffer'''). This "buffers" the signal source in the first circuit against being affected by currents from the [[electrical load]] of the second circuit and may simply be called a '''buffer''' or '''follower''' when context is clear.

==Voltage buffer==
A voltage buffer amplifier is used to transform a voltage signal with high [[output impedance]] from a first circuit into an identical voltage with low impedance for a second circuit. The interposed buffer amplifier prevents the second circuit from loading the first circuit unacceptably and interfering with its desired operation, since without the voltage buffer, the voltage of the second circuit is influenced by output impedance of the first circuit (as it is larger than the input impedance of the second circuit). In the ideal voltage buffer (Figure 1 top), the input impedance is infinite and the output impedance 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.

If the voltage is transferred unchanged (the voltage [[gain (electronics)|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&nbsp;[[decibel|dB]]), referring to the voltage gain.

As an example, consider a [[Thévenin'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 ''{{Sfrac|V{{sub|A}} R{{sub|L}}|R{{sub|L}} + R{{sub|A}}}}''. 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.

==Current buffer==
Typically a current buffer amplifier is used to transform a current signal with a low [[output impedance]] from a first circuit into an identical current with high impedance for a second circuit.<ref>{{Cite web|url = http://web.mit.edu/6.012/www/SP07-L20.pdf|title = Lecture 20 - Transistor Amplifiers (II) - Other Amplifier Stages|quote = A current buffer takes the input current which may have a relatively small Norton resistance and replicates the current at the output port, which has a high output resistance ... Input resistance is low ... Output resistance is high ... transform a current source with medium source resistance to an equal current with high source resistance}}</ref> The interposed buffer amplifier prevents the second circuit from loading the first circuit's current unacceptably and interfering with its desired operation. In the ideal current buffer (Figure 1 bottom), the output impedance is infinite (an ideal current source) and the input impedance is zero (a short circuit). 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.

For a current buffer, if the current is transferred unchanged (the current [[gain (electronics)|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.

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 ''{{sfrac|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.

==Voltage buffer examples==

=== Op-amp implementation ===
[[Image:Block Diagram for Feedback.svg|thumb|Figure 2: A negative feedback amplifier|290px|left]] [[Image:Op-Amp Unity-Gain Buffer.svg|thumb|Figure 3. An [[op-amp]]–based unity gain buffer amplifier]] [[Image:Voltage follwer boosted 4clamp II.svg|thumb|A voltage follower boosted by a [[transistor]]; also can be seen as the "ideal transistor" without a base-emitter [[Forward bias|forward bias voltage drop]] on the input signal. This is the basic circuit of [[linear voltage regulator]]s]]
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). '''Unity gain''' here implies a ''voltage gain'' of one (i.e. 0 dB), but significant ''current gain'' is expected. In this configuration, the entire output voltage (β = 1 in Fig. 2) is fed back into the inverting input. The difference between the non-inverting input voltage and the inverting input voltage is amplified by the op-amp. This connection forces the op-amp to adjust its output voltage to simply equal the input voltage (V<sub>out</sub> follows V<sub>in</sub> so the circuit is named op-amp voltage follower).

The impedance 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]].

=== Simple transistor circuits ===
[[Image:Bipolar Voltage Follower.png|thumb|250px|Figure 4: Top: BJT voltage follower Bottom: Small-signal, low-frequency [[equivalent circuit]] using [[hybrid-pi model]]]]
[[Image:MOSFET Voltage Follower.png|thumb|250px|Figure 5: Top: MOSFET voltage follower Bottom: Small-signal, low-frequency [[equivalent circuit]] using [[hybrid-pi model]]]]
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 (though the loss may be small and unimportant) and add a [[DC offset]]. Only one transistor is shown as the active device in these schematics (however, the current source in these circuits may require transistors too).

====Impedance transformation using the bipolar voltage follower====
Using the small-signal circuit in Figure 4, the impedance seen looking into the circuit is
::<math> R_{\rm in} = \frac {v_x} {i_x} = r_{\pi} + (\beta + 1) ({r_{\rm O}} || {R_{\rm L}}) </math>

(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

====Impedance transformation using the MOSFET voltage follower====
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.

As frequency is increased, the parasitic capacitances of the transistors come into play and the transformed input impedance drops with frequency.

====Chart of single-transistor amplifiers====
Some configurations of 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.{{Dubious|date=August 2015}}  These amplifiers have high input impedance, which means that the digital system will not need to supply a large current.
{| class="wikitable" border="1"
|-
! Amplifier type
! <small>MOSFET</small> (NMOS)
! BJT &nbsp;&nbsp;&nbsp;(npn)&nbsp;&nbsp;&nbsp;
! Notes
|-
| [[Common gate]]/[[Common base|base]]
| [[File:N-channel JFET common gate.svg|frameless]]
| [[File:NPN common base.svg|frameless]]
| Typically used for current buffering
|-
| [[Common drain|Common drain/collector]]
| [[File:N-channel JFET source follower.svg|frameless]]
| [[File:NPN emitter follower.svg|frameless]]
| Voltage gain is close to unity, used for voltage buffering.
|}

==== Logic buffer amplifiers ====
{{Main article|Digital buffer}}
A non-linear buffer amplifier is sometimes used in digital circuits where a high current is required, perhaps for driving more gates than the normal [[fan-out]] of the logic family used, or for driving displays, or long wires, or other difficult loads. 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 buffer amplifiers, and an '''octal buffer''' is a single package containing 8 buffer amplifiers. The terms '''inverting buffer''' and '''non-inverting buffer''' effectively correspond with high-current capability single-input NOR or OR gates respectively.

==== Speaker array amplifiers ====

The majority of amplifiers used to drive large speaker arrays, such as those used for rock concerts, are amplifiers with 26-36dB voltage gain capable of high amounts of current into low impedance speaker arrays where the speakers are wired in parallel.

==== Driven guards ====
A [[driven guard]] utilizes a voltage buffer to protect a very high impedance signal line by surrounding the line with a shield driven by a buffer to the same voltage as the line, the close voltage matching of the buffer prevents the shield from leaking significant current into the high impedance line while the low impedance of the shield can absorb any stray currents that could affect the signal line.

==Current buffer examples==
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.

=== Simple transistor circuits ===
[[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>'']]
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.

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]].

== See also ==
* [[Preamplifier]]
* [[Common base]]
* [[Common gate]]
* [[Common collector]]
* [[Common drain]]
* [[Current differencing buffered amplifier]]
* [[Negative feedback amplifier]]
* [[Driven shield]]
* [[VCVS filter|Voltage controlled voltage source filter]]

==References==
{{Reflist}}

[[Category:Electronic amplifiers]]