DC-to-5-GHz variable gain amplifier for high speed DSO

This paper presents the design of a variable gain amplifier for the frontend circuitry of 5-GHz high-speed digital storage oscilloscopes. The design challenge includes wide bandwidth and low distortion over the frequency range from dc to 5 GHz. The Cherry-Hooper amplifier configuration is used to tackle the issue. The designed variable gain covers the specified range from -2 to 38 dB. The gain flatness carries out ripples less than 1 dB. To verify our approach, two versions of the amplifier circuits are fabricated. With four cascaded stages, the maximum gain achieves larger than 40 dB. The circuits are designed in 0.35 μm SiGe technology.