An L-band SiGe HBT differential amplifier with frequency-tunable and multiple stopbands

An L-band SiGe HBT differential amplifier with frequency-tunable and multiple stopbands is presented. To achieve frequency-tunable and multiple bandstop performance, multiple varactor-tuned LC-tank circuits are cascaded in a stacked form between emitters of the differential transistor-pair. The stacked multiple LC-tanks have an outstanding feature in that bandstop frequencies can be varied independently. The implemented 0.35 mum SiGe HBT amplifier with dual stopbands demonstrates an averaged bandstop performance of 13.3 dB over 0.5 to 0.8 GHz and 14.3 dB across 0.6 to 1.1 GHz. The input and output return losses are better than 17.5 dB and 11 dB over 0.2 to 1.5 GHz, respectively. The measured P_1dB is +3 dBm and IIP₃ is 0 dBm with V_cc = 6 V and I_c = 8 mA.