Design of immittance inverters and phase inverters with non-foster elements

Non-Foster negative capacitances and inductances are attractive elements that allow designing broadband microwave devices and providing extra design flexibility due to their specific frequency dependence of reactance. Since this frequency dependence cannot be obtained from passive circuits according to Foster's theorem, the active circuits known as negative impedance converters (NICs) are used to realize non-Foster elements. Characteristics of NIC circuits depend strongly on parameters of active components used. We investigated how characteristics of a NIC to realize a negative capacitance for the GHz frequency range depend on the transistor critical frequency. It is demonstrated in the paper that a proper selection of transistors makes it possible to realize negative capacitances with a low variation over a wide frequency band. Possibilities of using the realized negative capacitance to design reactance networks of immittance inverters and broadband phase inverters were then investigated. The im-mittance inverters with non-Foster elements can be employed for filter design with an advantage of no need for subtraction of negative capacitances. The broadband phase inverter which is a cascade of two identical immittance inverters can be used to design a broadband 180° switchable-channel phase shifter with a low phase shift error.