Using generalised circuits consisting of three basic elements (electrochemical resistors, electrical and chemical capacitors), the small signal response of mixed conductors is considered. Simple approximate circuits for polycrystals with grain boundaries that act as obstacles for ions, electrons or both are derived. It is shown that Warburg-type impedance can be induced also by grain boundaries, not only by electrodes. As shown exemplarily, such models are appropriate also for analysis of impedance spectra obtained in studies of mixed conductive cathodes in Li-batteries.