I. Introduction
High-Speed broadband telecommunication technologies largely progress in the next-generation mobile communication system. High-efficiency and low-distortion characteristics with a wide frequency range are strongly demanded in power amplifiers, which are the key devices of the system. In the present base-station amplifiers handling modulated signals with a high peak factor such as wideband code division multiple access (W-CDMA) signals, the linearization techniques such as a feed-forward and a digital pre-distortion have been adopted [1], [2]. The deterioration and asymmetries of intermodulation distortion (IMD), which are related to the memory effect, are obstacles to the distortion compensation. It has been reported that IMD asymmetries are caused by the termination impedance characteristics of the difference frequency and second harmonics [3]–[8]. Hitherto, difference frequency has been terminated by a bias circuit of an amplifier [9]–[11]. In these approaches, the baseband impedance was reduced by optimizing the bypass capacitor and the linewidth of the drain-bias circuits. However, the carrier spacing of 15 MHz at the maximum is necessary on the W-CDMA system. In a high power amplifier, the bias circuit is not sufficient to terminate the baseband impedance of 15 MHz in a short-circuit condition. Thus far, an improvement methodology of IMD asymmetries has not yet been clearly reported for wide carrier-spacing signals in high power-amplifier applications.