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A buffer amplifier (sometimes simply called a buffer) is one that provides electrical impedance transformation from one circuit to another.

Typically a 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.

If the voltage is transferred unchanged (the voltage gain is 1), the amplifier is a unity gain buffer; also known as a voltage follower.

Although the voltage gain of a 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 dB), referring to the voltage gain.

Diagram of an op-amp–based unity gain buffer amplifier

A unity gain buffer amplifier may be constructed very simply by connecting the output of an operational amplifier to its inverting input (negative feedback), and connecting a signal source to the non-inverting input. For this circuit, V_out is simply equal to V_in.

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 (MΩ to 10 TΩ), meaning that the input of the op-amp does not load down the source or draw any 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 bridging connections, which reduce power consumption in the source, distortion from overloading, crosstalk and other electromagnetic interference.

Other unity gain buffer amplifiers include the bipolar junction transistor in common-collector configuration (called an emitter follower because the emitter voltage follows the base voltage); a pair of such transistors (Darlington pair); or similar configurations using field effect transistors (a source follower), vacuum tubes (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.

The frequency response of the buffer amplifier is normally flat from from DC (0 Hz) to a few kHz. But at very low frequencies gain (dB) becomes slightly negative.^{[dubious — see talk page]}

It is common for a single 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.

• Preamplifier

• Unity Gain Buffer Amplifier — Bucknell