Variable gain low noise amplifier and method for controlling gain of variable gain low noise amplifier

A variable gain low noise amplifier (LNA), comprising:

• a first transistor, wherein a gate terminal of the first transistor is configured to receive an input signal;
• a first degeneration inductor, coupled to a source terminal of the first transistor, wherein the first transistor and the first degeneration inductor belong to a first branch of the variable gain LNA;
• a second transistor, wherein a gate terminal of the second transistor is configured to receive the input signal;
• a second degeneration inductor, coupled to a source terminal of the second transistor, wherein the second transistor and the second degeneration inductor belong to a second branch of the variable gain LNA;
• a resistor, coupled to the second degeneration inductor;
• and
• a switch, coupled across the resistor;
• wherein a gain of the variable gain LNA is determined by controlling whether to turn off the second branch, and the switch is controlled according to whether to turn off the second branch.

A method for controlling a gain of a variable gain low noise amplifier (LNA), comprising:

• utilizing a gate terminal of a first transistor to receive an input signal, wherein a first degeneration inductor is coupled to a source terminal of the first transistor, and the first transistor and the first degeneration inductor belong to a first branch of the variable gain LNA;
• utilizing a gate terminal of a second transistor to receive the input signal, wherein a second degeneration inductor is coupled to a source terminal of the second transistor, and the second transistor and the second degeneration inductor belong to a second branch of the variable gain LNA, wherein a switch and the source degeneration inductor are coupled in series to the source terminal of the second transistor;
• controlling the switch according to whether to turn off the second branch;
• and
• controlling whether to turn off the second branch, to determine the gain of the variable gain LNA.