How you can amplify ac current?

Called a transformer! Not called amplification if a transformer is used. (it is being done with solid state now so Transformers are not being used in Power supplies much anymore) Look at the power pole that is a transformer and it steps the high voltage down to house hold voltages. Works both ways. The highest wires on a power pole can but in the hens of thousands of volts. The transmission lines are in the hundreds of thousands of volts.

If you are talking about a weak signal or radio signal then it is a amplifier, made from electronics, transistors, tubes, resistors, capacitors and coils. Integrated circuits (IC's), just have those in them much smaller. The circuits are the same. Look it up. There are probably thousands of ways to wire an amplifier.