Typically a buffer amplifier is used to transfer a voltage from a first circuit, having a high impedance level, to a second circuit with a lower 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 amplifier is a unity gain buffer.
A unity gain buffer amplifier may be constructed very simply by connecting the output of an operational amplifier to its inverting input, and using the non-inverting input.
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, vacuum tubes, or other active devices.
All such amplifiers actually have a gain of slightly less than unity, but the difference is usually small and unimportant.
Although the voltage gain of a buffer amplifier may be (approximately) unity, it usually provides considerable current gain and thus power gain. Therefore it is erroneous (though commonplace) to say that it has a gain of 0 dB.