Voltage Followers, can also be referred to as Unity Gain Amplifiers or Buffer Amplifiers. Refer to the non-inverting amplifier configuration below, we may simply make R1 = 0 and R2 = ∞ to obtain the unity gain amplifier. This circuit is commonly referred to as a voltage follower, since the output “follows” the input.
According to the closed-loop gain formula of non-inverting amplifier, we have G = 1+R1/R2. Let R1 = 0 and R2 = ∞, we easily get G ≈ 1. This calculation is not accurate and only for estimation.
Although we call it unity gain amplifier, VOUT is not exactly equal to VIN . Actually it has a gain of slightly less than unity due to voltage drop from the wires, as well as op-amp's offset voltage, but the difference is usually small.
In the ideal case, VOUT = VIN , RIN = ∞, and ROUT = 0.