A circuit comprises a first PMOS transistor, a second PMOS transistor, a first NMOS transistor, a second NMOS transistor, a third NMOS transistor, and a fourth NMOS transistor. The PMOS transistors and the NMOS transistors are configured to provide a first voltage reference node having a first reference voltage and a second voltage reference node having a second reference voltage. The first reference voltage and the second reference voltage serve as a first reference voltage and a second reference voltage for a memory cell, respectively.