CMOS op-amps for biomedical applications

A new family of two stage op-amps with broadband open loop response is introduced. These have a significant reduction of the DC open loop gain and maintain a high gain at frequencies as low as 1 Hz. This provides them with low DC offset. The proposed circuits are based on the Quasi-Floating Gate (QFG) technique, using a novel implementation for active loads that provides a very low effective resistance at DC and very-low frequencies without penalizing the high resistance at the pass band gain significantly. Moreover, a wide-band AC coupling structure also based on QFG transistors has been combined with the proposed technique, rejecting both the differential input DC voltage and the offset introduced by the circuit. These features make the proposed op-amps suitable for biological signal processing, where small amplitude signals are commonly affected by large DC offsets. Cadence simulation results employing a 0.5 micrometers technology are provided for validating the proposed circuits.