What are the practical reasons to use a voltage-follower?
Could you give a particular example why is a voltage follower is used?


A voltage follower typically has a very high input impedance and a very low output impedance.  Why is this important or necessary?

Well, it is often the case that a voltage amplifier stage will have a moderately high output impedance.  This output impedance will form a voltage divider with the load impedance and reduce the voltage gain of the stage.  For example:







simulate this circuit &ndash; Schematic created using CircuitLab

Here, I model a voltage amplifier with an open circuit (unloaded) gain of 100 and an output resistance of \$1k \Omega\$.

But, with an \$8 \Omega\$ load connected, the loaded gain drops to 

$$A_v = 100 \dfrac{8}{8 + 1000} = 0.794$$

The voltage gain of the stage is reduced to less than 1!

However, by inserting an (ideal) voltage follower between the amplifier and the load:







simulate this circuit

The overall loaded voltage gain is now 100, the unloaded voltage gain.

This may seem paradoxical since the voltage follower has a voltage gain of 1 but remember, the voltage follower is still an amplifier.  That is, it increases the power of the signal.

Specifically, by presenting an open (or effectively open) circuit to the preceding voltage amplifier stage, no (or effectively no) signal power is required to drive the voltage follower and thus, no signal power is lost in the output resistance of the voltage amplifier

And, by presenting a zero (or effectively zero) output impedance to the load, there is no (or effectively no) power lost in the output resistance of the voltage follower.

We say then that the voltage follower is a buffer between the voltage amplifier and the load.