Novel design approaches for microwave active circuits using composite right-/left-handed (CRLH) transmission line (TL) stubs are presented. We show that, by modifying the dispersion diagram of the CRLH TL stub, the frequency band or the harmonic tuning capability can be enhanced in such a way that it would have been difficult or impractical if done using conventional micro-strip line stubs. The frequency response of the CRLH TL stub can be controlled almost arbitrarily while at the same time reducing the stub length significantly, because the dispersion curve in the left-handed region and in the right-handed region is controlled independently. As a proof of concept, a triple-band rectifier, single-band and dual-band harmonic tuning circuits for class-F amplifiers are demonstrated.