L-band SiGe HBT frequency-tunable differential amplifiers with dual-bandpass or dual-bandstop responses have been developed for the next generation adaptive and/or reconfigurable wireless radios. Varactor-loaded dual-band resonators comprised of series and parallel LC circuits are employed in the output circuit of differential amplifiers for realizing dual-bandpass responses as well as the series feedback circuit for dual-bandstop responses. The varactor-loaded series and parallel LC resonator can provide a wider frequency separation between dual-band frequencies than the stacked LC resonator. With the use of the varactor-loaded dual-band resonator in the design of the low-noise SiGe HBT differential amplifier with dual-bandpass responses, the lower-band frequency can be varied from 0.58 to 0.77 GHz with a fixed upper-band frequency of 1.54 GHz. Meanwhile, the upper-band frequency can be varied from 1.1 to 1.5 GHz for a fixed lower-band frequency of 0.57 GHz. The dual-band gain was 6.4 to 13.3 dB over the whole frequency band. In addition, with the use of the varactor-loaded dual-band resonator in the design of the low-noise differential amplifier with dual-bandstop responses, the lower bandstop frequency can be varied from 0.38 to 0.68 GHz with an upper bandstop frequency from 1.05 to 1.12 GHz. Meanwhile, the upper bandstop frequency can be varied from 0.69 to 1.02 GHz for a lower bandstop frequency of 0.38 GHz. The maximal dual-band rejection of gain was 14.4 dB. The varactor-loaded dual-band resonator presented in this paper is expected to greatly contribute to realizing the next generation adaptive and/or reconfigurable wireless transceivers.