Analysis and Design of Efficiency-Enhancement Microwave Power Amplifiers Using the Doherty Technique

Modern wireless communications systems standardized for the third generation (3G) and fourth generation (4G) have introduced challenging requirements on the design of base station and handset transmitters. These communications systems are utilizing high data rate, spectrally-efficient digital modulation schemes (e.g. QAM, OFDMA). The resulting signals to be communicated have rapidly varying envelope over a wide dynamic range, which results in high levels of peak-to-average-power ratio (PAPR). To avoid signal clipping and spectral spreading, these signals need linear amplification. Therefore, the power amplifier unit has to be operated at considerable amounts of back-off, which in turn results in very low efficiency. Low efficiency operation in turn lowers device lifetime and increases the infrastructural and operational costs of the base station. With this regard, the power amplifier becomes the most critical part of the transmitter. Power amplifiers that provide a good compromise between efficiency and linearity are crucial for modern communications systems. Among others, GaN HEMT technology has proven itself to be a competent alternative to the conventional Si and GaAs technologies. The superior material properties of GaN together with the benefits of HEMT technology led to the design of devices with high power density, high efficiency, wideband capability and high frequency operation. The Doherty amplifier has been widely adopted in the last decade being a relatively simple efficiency-enhancement amplifier architecture. This amplifier can provide significant efficiency enhancement over single-ended amplifiers as well as inherent linearity. There has been a lot of work on Doherty amplifiers in the recent years. Most of the published amplifiers were narrowband and do not fulfill the wideband/multiband requirements of modern base stations. Wideband Doherty amplifiers or other efficiency-enhancement techniques are crucial for these base stations to reduce their overall size and cost. With regard to the Doherty amplifier, it was a convention that xxviii this technique is unsuitable for wideband applications due to the frequency sensitive impedance inverter in its structure. In this thesis, main focus was given to deeply study the bandwidth limiting factors of the Doherty amplifier, and possibly, to provide solutions that can help in extending the bandwidth of this amplifier technique. Being the clearest source of limited bandwidth, a complete frequency response analysis of the output combining network has been performed. From the analysis it was found that the Doherty amplifier can still provide back-off efficiency enhancement over class B amplifier up to a relative bandwidth of 56% bandwidth. A modification on the output combining network was also proposed by using impedance inverters with less transformation ratio, which resulted in efficiency improvement by 9.2% from that of the conventional Doherty amplifier up to 45% bandwidth. The derived analysis was then verified by the design and implementation of multiple wideband GaN Doherty amplifiers with state-of-the-art bandwidths ranging from 28% to 42% and output power levels from 20W to 200W. Superior performances were obtained with, for example, a minimum 6dB back-off power-added efficiency (PAE) of 50%, measured over 28% bandwidth of an 80W Doherty amplifier. Very good linearity was also characterized with the designed amplifiers using two-tone, UMTS and LTE signals. Additional linearization using memory polynomial digital predistortion (DPD) resulted in ACLR levels that fulfill the requirements of the corresponding communications systems. In order to extend the power range of the Doherty amplifier, a generalized frequency response analysis that applies for any Doherty amplifier configuration with arbitrary breaking point was derived. Based on the analytical results, the 80W wideband Doherty amplifier was modified to operate at 9 dB back-off instead of 6 dB back-off power. High PAE of at least 45% was then measured at around 9 dB output back-off, and over 28% bandwidth.