A circuit includes a first transistor, a second transistor, a third transistor and a fourth transistor. The first transistor has a first end and a second end. The second transistor has a first end and a second end, wherein the first end of the second transistor is coupled to the first end of the first transistor. The third transistor has a first end and a second end, wherein the second end of the third transistor is coupled to the second end of the second transistor. The fourth transistor has a first end coupled to the second end of the first transistor. The fourth transistor has a bottom gate and an oxide semiconductor layer, and the second transistor has a top gate and a silicon semiconductor layer.