Noise degradation of cascodes in broadband power amplifiers

This paper compares the noise performance of the common-source and the cascode topology. Although the cascode topology has several advantages over the common-source stage, the noise performance degrades due to the channel noise and the induced gate noise of the common-gate stage. To underline the theory two multi-decade GaN feedback power amplifiers were designed in common-source and cascode topology, using a submicron AlGaN/GaN MMIC technology on SiC substrate. Both designs achieve 13 dB gain and simultaneously good input and output matching. The 3 dB cutoff frequency of the common-source design is 12 GHz and the cascode feedback amplifier achieves even 17 GHz. At mid-band the common-source and the cascode design achieve a moderate noise figure of 3 dB and 4 dB while maintaining an output power over the complete frequency range of 28 dBm and 29 dBm respectively.