An impedance circuit includes a first impedance terminal, a second impedance terminal, a first transistor, a second transistor, a low frequency signal blocking element, and a current-voltage transform circuit. The first transistor is coupled to the first impedance terminal, and controlled by a first voltage. The second transistor is coupled to the first impedance terminal, and controlled by a second voltage. The low frequency signal blocking element is coupled to the first transistor and the second impedance terminal. The current-voltage transform circuit is coupled to the first impedance terminal. The current-voltage transform circuit adjusts a terminal voltage at the first terminal of the current-voltage transform circuit according to a current flowing through the current-voltage transform circuit. The impedance circuit provides impedance between the first and the second impedance terminals according to the terminal voltage and the first voltage.