Conditions for realization of an active filter in a multiloop structure with identical biquadratic sections (MIB) are considered in this paper. It is shown that poles (zeros) of a transfer function of a MIB network with biquadratic section

can be grouped in pairs such that for each pair

and

, the relation

holds where

. A new concept of a characteristic circle of a rational function is introduced. It is shown that its existence is necessary and sufficient for MIB network realization.