Question

how transformer act as amplifier?

Answer 1

consider it to be a step up transformer
if wrng plz correct

Answer 2

The relationship between Voltage (V), Current (I) and inductance of a transformer can be summarized as follows:
$$
\large{
\frac{V_P}{V_S} = \frac{I_S}{I_P}=\frac{N_\text{P}}{N_\text{S}}=\sqrt{\frac{L_P}{L_S}}=a
}
$$
Where P is primary, S is secondary and \(a\) is the gain.

Because of \(a\), we consider the transformer to be an amplifier. Note that the energy remains the same at the input and output (ideally), what changes is the current and voltage.