A new method to reduce the size of amplifiers and reject harmonics using spiral-defected ground structure (Spiral-DGS) is presented. A microstrip transmission line having Spiral-DGS provides increased slow-wave factor (SWF) and excellent rejection characteristics for a specified harmonic frequency band as if it is a band rejection filter. Due to the increased SWF, the physical lengths of matching networks are shortened while the original matching and performances are preserved. The reduced lengths by Spiral-DGS are 39% and 44% of the original lengths in input and output matching networks, respectively. It is shown that the measured S-parameters of the reduced amplifier agree well with those of the original amplifier. The measured second harmonic of the reduced amplifier is much less than that of me original amplifier by at least 10dB.
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