Editing Buffer amplifier (section)

=== 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.