Editing Buffer amplifier (section)

==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.