A current source providing an output current with a fixed current range includes a bias circuit, a resistor, a current mirror, and a controller. The bias circuit provides a first voltage weighted with a first tunable coefficient and a second voltage weighted with a second tunable coefficient. The resistor has a tunable resistance for determining a bias current according to a voltage difference between the first and the second voltages and the tunable resistance. The current mirror generates the output current according to the bias current. The controller adjusts the tunable resistance and one of the first and the second tunable coefficients to achieve a voltage-current coefficient with different values, while the bias current and the output current are kept within a fixed current range.