Offset cancellation with subthreshold-operated feedback circuit for fully differential amplifiers

An offset cancellation technique for fully differential amplifiers is proposed. This technique uses subthreshold-operated operational amplifiers (subth-OP amps) for feedback biasing. Two subth-OP amps sense the two outputs of the differential amplifier and accordingly adjust the load currents in the amplifier to fix the outputs to a given reference voltage. The feedback operation is established only at dc and low frequencies because the subth-OP amps operate very slowly. The differential amplifier consequently operates as a high-pass filter and therefore shows no dc offset in its output, while it can normally amplify ac input signals. The results for the simulation and fabrication of the device are described.