Analysis and reduction of phase variations of variable gain amplifiers verified by CMOS implementation at C-band

Phase variations of variable gain cascode amplifiers are analysed and a method, which significantly reduces the phase variations versus gain is presented. The operation point current and the load of the variable gain transistors are stabilised by current weighting using dummy paths. The approach is verified by a fully integrated CMOS variable gain amplifier at C-band. A maximum gain of 18 dB with a 1 dB bandwidth of 5.2-5.7 GHz is measured. At 5.7 GHz and within a gain control range of 23 dB the absolute phase difference is reduced to a minimum of +/-48 degrees yielding an improvement by a factor of 5 compared with simple cascode topologies. A total supply power of 9 mW is consumed. A very low phase variation has been achieved at comparable gain control range and power consumption for a variable gain amplifier in CMOS.