Voltage generators may generate a level of a high target voltage with respect to a low external power supply voltage. A reference voltage generator includes a clamp regulator which is driven by a first power supply voltage supplied from an external source and receives a first voltage to generate a clamp voltage, and a level amplifier which is driven by a second power supply voltage that is higher than the first power supply voltage and receives the clamp voltage to generate a reference voltage. The clamp voltage may be set to have a voltage level which results in a successful restore operation with respect to a memory cell array in a dynamic random access memory (DRAM).