Analytic methods of analysis in analogue circuits, using or not computer methods, provide solutions that, depending on the system order and number of circuit elements, present a considerable complexity, being generally very hard to interpret. When the system order increases, both the symbolic representation of the transfer function coefficients and its analysis become more and more complex. The time constant theorem (TCT) is a traditional and useful tool for approximate analytic studies of cut-off frequencies. It is applicable directly (considering only some circuit capacitors) to low-pass and high-pass systems and adapted to wide-band ones. This paper proposes a new and more accurate approach for determining cut-off frequencies of wide-band systems, taking into account all existing capacitors in the circuit.