An L-band frequency and rejection-level tunable SiGe HBT differential amplifier with dual stopband is presented. To achieve frequency and rejection-level tunable performance, dual LCR-tank circuit with an active load is incorporated into the design of the series feedback loops of the differential amplifier. The active load consists of a varactor diode represented as a variable C and a common-emitter transistor represented as a variable R. The frequency and rejection level can be tuned independently by controlling a cathode bias voltage of the varactor diode or a base bias voltage of the transistor. The implemented 0.35 μm SiGe HBT amplifier with dual stopband demonstrates a frequency tuning of 0.53–1.16 GHz and a rejection-level variation up to 9.5 dB. The input and output return losses are better than 17.5 and 11 dB over 0.2–1.5 GHz, respectively. The measured P1dB is+3 dBm and IIP3 is 0 dBm with Vcc = 6 V and Ic = 8 mA.
