An output buffer includes an input/output end, a voltage source, a first transistor and a second transistor. The first transistor includes a first end coupled to the input/output end, a second end coupled to the voltage source, and a control end coupled to the voltage source. The second transistor includes a first end coupled to the input/output end, a second end coupled to the voltage source, and a control end coupled to the voltage source. The control end of the first transistor and the control end of the second transistor are substantially perpendicular to each other, and the punch through voltage of the first transistor is higher than the punch through voltage of the second transistor.