I've gone through a lot of articles to find the correct meaning of buffer in the electronics sense.
From what I've understood, a buffer is a device or part of a big circuit that provides the output as present at the input.
So, in a buffer, the output will be equal to input. What about the current gain in a buffer? I understand that a buffer will have unity voltage gain.
Can someone throw some additional light on this?
An ideal voltage buffer has infinite current gain, its output is an ideal voltage-controlled voltage source with zero impedance.
Real voltage buffers of course only have finite (but very high) current gain. With FET inputs, the practically achievable current gain might be in the billions or more.
You use them whenever you have a weak voltage source with high internal impedance that you want to turn into a better voltage source (with low impedance) to drive downstream circuitry without overloading the weak source.
Note that "infinite current gain" doesn't mean that a buffer will always output a very large current. It just means that in order to drive a very large output current, it only needs a very small (or ideally zero) input current. As an example, let's build a voltage follower (buffer) with a TL072 and drive 1mA with its output. Its input current is at most 0.1nA, which gives us a current gain in excess of 10 million (1mA / 0.1µA).
One issue with your question is you assume that the output and input are voltage. They do not need to be. Though much rarer, it is possible to have current be your signal. You can just think of a buffer as something that shields the driver from the effects of the load, and preserves the intended signal into the load.
That means that it presents itself to the driver such as the input signal is not distorted (i.e. loaded down) and has the capability to reproduce that input signal at the output in a way where it is preserved despite the characteristics of the load (i.e. is able to drive the load without getting loaded down and getting distorted).
So a voltage buffer can reproduce the input voltage even if the current required to do so is high, whereas a current buffer can reproduce the input current even if the voltage required to do so is high.
To accurately read the intended input signal, the buffer would ideally present an infinite input impedance for a voltage signal since it takes minimal current to produce a given voltage across an infinite impedance, and zero input impedance for a current signal since it takes minimal voltage to produce a given current across zero impedance.
To accurately reproduce the input signal at the output into the load, it would ideally have an series output impedance of zero ohms for a voltage signal (to minimize the voltage drop caused current into the load which would result in lost output voltage) or an infinite parallel output impedance for a current signal (to minimize current leaking away before it gets to the load).
buffer
/ˈbʌfə/
noun
1.
a person or thing that reduces a shock or that forms a barrier between incompatible or antagonistic people or things.
In electronics it usually means a device inserted between a weak signal source and a device or devices which might overload the source.
From what I've understood, buffer means a device or part of a big circuit that provides the output as present in the input.
Yes, but it doesn't have to be complicated. It could be just one or two transistors providing a high output current while stealing little power from the source.
So, in a buffer, the output will be equal to input.
Maybe, but it could also improve the original signal as in the case of a Schmitt trigger so that noisy input signals are squared up and driven hard to 0 or 1. A buffer may also convert a logic signal from, say, 3.3 V logic to 5 V or 24 V logic.
What about the current gain in a buffer? I understand that a buffer will have unity voltage gain.
Not always unity voltage gain as explained above. Yes, generally there will be more current available from the buffer - but it depends what the buffer has been designed to do - drive more devices or chenge the voltage levels, for example.
Whenever you connect something (a "load") across a voltage source, obviously some current will be pushed through that load.
By "voltage source", I mean anything that produces a potential difference, whether it be a battery, a solar panel, a microphone, an audio amplifier output, or the output from an op-amp. Literally anything that produces a potential difference. Usually, in the context of a buffer, the source is some kind of time-varying potential difference, produced by an amplifier stage of some kind, such as an op-amp, or by a transducer such as a microphone.
All voltage sources have some inherent output resistance, via which this current must pass in order to reach the next stage. Output current will cause a voltage "drop" across this resistance, according to Ohm's law \$V=I\times R\$. Consequently, the next stage (the "load") never sees the source's actual potential difference, only what's left over after the source's own internal resistance has "consumed" some fraction of it.
Obviously this is inconvenient, because it's a cause of signal distortion. To mitigate this distortion, you have three options:
Design a better source, with lower output resistance
Design a better load, that draws as close to zero current as possible
Put something between the two stages, a "buffer", that draws almost zero current from the source, and reproduces source's potential at its own output, having a low enough resistance that the load can draw whatever it needs without significantly affecting signal potential.
Option 3, the buffer, has two important properties:
Very high input resistance, so that it draws as little current from the source as possible.
Very low output resistance, so that the next stage can draw whatever current it requires without compromising signal potential.
In terms of your question, which refers to "current gain", these two properties could be paraphrased as a buffer having a large current gain. That is, drawing insignificant current itself from the source, and providing any amount of current required by the next stage.
Since the intended purpose of a buffer stage is usually to exactly reproduce some potential, in that context voltage gain is typically 1. However, there's technically no reason why you couldn't build a buffer with any voltage gain you desire. It might not be a "buffer" in the truest sense, but it can still perform the intended function of a buffer, which is to prevent subsequent stages from over-loading some weak voltage source.
Most usually buffer's purpose to exclude load influence to signal source. As example, if signal is voltage, the buffer has high impedance on input and low in output. Buffer may have a gain, but most cases it is 1.
