Broadband distributed type Doherty amplifier


Student thesis: Master's Thesis

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  • Tik Shun LEUNG

Related Research Unit(s)


Awarding Institution
Award date2 Oct 2008


Distributed amplifiers (DAs) provide broadband amplification due to its transmission line type characteristics, formed by the active device’s capacitance and interconnected inductances. Although DAs has flat gain over its operating frequencies band, the power added efficiency (PAE) of DAs is low particular at higher frequencies. The low-PAE disadvantage of DAs is mainly caused by the presence of a dummy load, which absorbs the backward traveling wave, at the output artificial transmission line. The dummy load consumes significant power and hence reduces the PAE. While the Doherty amplifiers, which consist of a carrier amplifier and a peaking amplifier, offers higher PAE over a wide dynamic range using load modulation, it however, can only operate in narrow bandwidths due to the presence of a power splitter and quarter-wave transmission line. Therefore, the work presented here aims to combine the benefit of DAs and Doherty amplifiers for use in current wireless communication systems offering wide bandwidth, small size and low cost benefits. An unequal bias distributed amplifier (UBDA) is proposed for the improvement in the PAE of the conventional distributed amplifier. A higher efficiency bias technique is applied by biasing the first gain cell with the highest current while the other two gain cells are biased at a smaller current. By applying the proposed bias technique, the inefficient use of a dummy load at the output artificial transmission line can be eliminated without affecting the bandwidth. The proposed UBDA provides a maximum power added efficiency of 27.8% which is 16.8% higher than that of the CDA. Finally, a broadband distributed type Doherty amplifier (DDA) is proposed. The proposed DDA offers an operating frequency band from 0.8GHz to 2.6GHz. While having the same broadband characteristic as that of the DAs, the proposed DDA also has the high efficiency characteristic found in Doherty amplifiers. The proposed DDA provides a significant PAE improvement over the optimized conventional distributed amplifiers. Maximum PAE improvement occurs at 2.2GHz with a measured output power of 19.4dBm and a PAE of 41.3%, which is 32% higher than that of the optimized conventional distributed amplifier.

    Research areas

  • Broadband amplifiers