Many radio-frequency circuits incorporate power resistors that dissipate energy during circuit operation. Such circuits include radio-frequency (RF) hybrids, power combiners and dividers, isolators, duplexers, outphasing RF power amplifiers, composite RF amplifier systems, termination networks and filters, among other devices. In such applications, resistors are often specified as isolation resistors, terminating resistors, dump resistors, etc. While the power dissipation associated with these resistors is undesirable, their terminal characteristics (and absorption of RF electrical power) are typically necessary for proper circuit function. For example, a terminating resistor of a specified resistance value is necessary in an isolating power combiner (such as, for example, a Wilkinson combiner) in order to preserve isolation between the combiner input ports. Likewise, a circulator having a terminating resistor coupled to a port thereof may be used as an RF isolator that passes signals flowing in one direction while terminating signals flowing in an opposite direction. The energy absorption of the resistor coupled to the circulator is required for the isolator to function as intended.
Because efficiency and energy draw are important in many RF applications, the loss associated with such resistors is undesirable. Moreover, the heat generated by such resistors is undesirable and can limit miniaturization of these items and place burdens on supporting devices (e.g., increasing cooling requirements, requiring fans, etc.). It would thus be desirable to realize the electrical function of these power resistors (e.g., providing a nearly constant equivalent resistive input impedance at a specified operating frequency and absorbing RF electrical power), while reducing the dissipation of the absorbed power.