Abstract: A filter network that exhibits a bandstop response comprises a network of basically low pass or high pass structure. The network is developed by choosing a bandstop transfer function, a fourth-orderelliptic is disclosed, which is converted to a low pass function. This function describes a passive low pass LC ladder network including a negative impedance element. A Bruton transformation is applied to the passive LC network to give an active version including an ungrounded negative resistance network in a series arm and frequency-dependent negative resistances in shunt arms without requiring inductance coils for its implementation.