A passive amplifier structure capable of multiplying the voltage of an input signal is provided. In a first stage, a first and a second capacitance are connected between a first and a second input port of a balanced input port in response to a first oscillator signal. In a second stage, in response to a second oscillator signal having a phase different from that of the first oscillator signal, the first capacitance is connected between the first input port and a third capacitance and the second capacitance is connected between the second input port and the third capacitance. An output voltage over the third capacitance is obtained from terminals of the third capacitance. Presented embodiments also describe an automatic gain control feature.