Study of "tanh" ideal and lossy ELIN integrators

The "tanh" ideal and lossy ELIN (externally linear, internally nonlinear) integrators as basic building blocks for active filter design are studied. The circuits have been obtained using state-space synthesis and are composed of "tanh" and "sinh/cosh" blocks which can be implemented in bipolar technology. It is shown that a "tanh" ideal ELIN integrator is difficult to use in practice because an appropriate dc equilibrium does not exist. Thus, for the working of such an ELIN system, which realizes the ideal integrator\´s transfer characteristic, the input dc level must be zero. The problems of dc and low frequency operation can be eliminated by using the "tanh" lossy ELIN integrator. The linearity range of the proposed structure depends on the accuracy in which the range of the "tanh" transfer characteristics is valid, this being limited (from the existing condition of the input inverse function, "arctanh") to the bias current value. Also studied is the unbalance effect of the two bias currents of the nonlinear "tanh" transconductance on the "tanh" lossy ELIN integrator in dc and ac operation. Finally, the core layout of the "tanh" lossy ELIN integrator in 0.8 μm BiCMOS technology is presented. The proposed circuit has been simulated in 0.8 μm BiCMOS technology showing remarkably good performance, and proving the practical applicability of the proposed structure.