Abstract: The S parameter expression of high-frequency models of the high electron mobility transistors (HEMTs) with basic feedback structure, especially the transmission gain S₂₁, is presented and analyzed. In addition, an improved feedback structure and its theory are proposed and demonstrated, in order to obtain a better gain-flatness through the mutual interaction between the series inductor and the parallel capacitor in the feedback loop. The optimization solution for the feedback amplifier can eliminate the negative impacts on transmission gain S₂₁ caused by things such as resonance peaks. Furthermore, our theory covers the shortage of conventional feedback amplifiers, to some extent. A wideband low-noise amplifier (LNA) with the improved feedback technology is designed based on HEMT. The transmission gain is about 20 dB with the gain variation of 1.2 dB from 100 MHz to 6 GHz. The noise figure is lower than 2.8 dB in the whole band and the amplifier is unconditionally stable.

Key words: Low-noise amplifier (LNA), Ultra-wideband, HEMT, Feedback, Gain flatness

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