The

S

parameter expression of high-frequency models of the high electron mobility transistors (HEMTs) with basic feedback structure

especially the transmission gain

S

21

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

S

21

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.