Concurrent dual-band switch-mode power amplifiers require high common-mode impedance at their intermodulation frequencies. Baluns utilizing quarter-wave effects only present perfect open common-mode impedance at their design frequency. Attempting to use a balun in a dual-band push-pull power amplifier without taking the new dual-band requirements for common-mode impedance into account will result in efficiency loss. This thesis gives a complete derivation of the maximum theoretical drain efficiency in a class-D amplifier, and compares it with the dual-band case using an ideal balun at the output. This ideal balun is explored and the mechanics behind this efficiency loss is revealed. A solution is provided showing that the addition of some transmission lines can move impedance to those specific frequencies by rotation of the common-mode impedance without affecting the differential mode match.