The Common Source Amplifier is a commonly used amplifier circuit configuration.

It has a unit current gain because in this circuit, the input signal is amplified and output as current.

The variation of the input signal directly affects the variation of the drain current.

The Common Source Amplifier consists of a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor):

Where the source is connected to the load resistor, the gate is connected to the input signal, and the drain is connected to the output signal.

When the input signal is applied to the gate, the conduction of the transistor is controlled, thereby affecting the drain current.

In simple terms, when the input signal is in the positive half cycle, the gate voltage rises, causing the MOSFET to conduct, increasing the drain current; when the input signal is in the negative half cycle, the gate voltage decreases, causing the MOSFET to cut off, reducing the drain current.

Therefore, due to the linear relationship between the output current and the input signal, the Common Source Amplifier has a unit current gain.