Patent ID: 7564295

Claim:
A bias circuit for a sense amplifier having a device under sensing, the bias circuit comprising: a set of MOS transistors coupled between power supply terminals; the device under sensing having an un-programmed resistance value and a programmed resistance value, and coupled to the set of MOS transistors; and an automatic bias circuit coupled to the set of MOS transistors, comprising: at least one first device formed substantially the same as the device under sensing and having the un-programmed resistance value; and at least one second device formed substantially the same as the device under sensing and having the programmed resistance value, wherein the at least one first device comprises two of the first device connected in parallel and the at least one second device comprises two of the second device connected in parallel, wherein the bias circuit produces a reference resistance value for the device under sensing, the reference resistance value substantially equals to a middle level between the un-programmed and the programmed resistance values, and wherein the at least one first device and the at least one second device are serially connected.