Transitory recovery time of bio-potential amplifiers that include CMOS pseudo-resistors

In this paper a study of the recovery time after a high-amplitude input transitory voltage is evaluated, for a low-frequency bio-potential amplifier. The importance of this parameter for very small low frequency cut-off circuits relies on the expectation, given by the linear model, that the amplifier may take hundreds of seconds to come out of the saturation state, what could be unacceptable for real applications. Actually, the nonlinearity of some components that are used in the amplifiers feedback circuit, particularly the CMOS pseudo-resistors, contributes to the reduction of the recovery time. The bio-amplifier evaluation was carried out using a typical two-stage circuit topology. The circuit behavior is evaluated for several CMOS pseudo-resistor values, with nMOS and pMOS solutions, using a new pseudo-resistor non-linear model. The frequency response target is a 10KHz bandwidth with low frequency cutoff of 1Hz (-3dB). The behavior was evaluated using a MOSIS SCN05 technology (0.5μm) and BSIM3V3.1 models on Eldo™ SPICE analog simulator.