Buffer amplifier
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A buffer amplifier (sometimes simply called a buffer) is one that provides impedance transformation from high to low between one circuit and another.
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; also known as a voltage follower.
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 using the non-inverting input for the signal input.
Image:Unity gain buffer amp.svg
The circuit makes a copy of the input voltage at the output without drawing any current from the 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. However, it is commonplace to say that it has a gain of 0 dB, referring to the voltage gain.
Frequency response of buffer amplifier: normally it allows input signal from DC to AC of few kHz. But at very low frequencies gain (dB) becomes slighty negative.
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.
