Broadband, High Efficient Doherty Power Amplifier for Wireless Communications


Student thesis: Doctoral Thesis

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Award date5 Sept 2018


In this thesis, several broadband, high efficient Doherty Power Amplifiers (DPA) for wireless communication system were investigated. For different application scenarios, such as 3G, 4G and upcoming 5G wireless communication systems, the Doherty power amplifier have their own required performance due to advances in communication protocols. There are three major parts to this thesis.

In the first part, second harmonic short-circuit network (SHSNs) with mutual coupling is proposed, which gives higher efficiency without sacrificing bandwidth. A broadband Doherty amplifier was designed and fabricated based on commercially available gallium nitride HEMT (Cree CGH 40006P) devices to validate the proposed technique. Under continuous wave excitation, measured results of the proposed DPA demonstrated that a wide bandwidth of 40% (1.8 - 2.7 GHz) can be maintained, with the 6-dB back-off drain efficiency maintaining between 47.5% - 54%. Using a wideband code-division multiple access (WCDMA) 3GPP test signal with peak-to-average power ratio of 6.6-dB, the adjacent channel power ratio (ACPR) over the entire frequency band is below -30 dBc and -25 dBc at 6-dB back-off and saturation, respectively.

In the second part, a mixed topology Doherty power amplifier was proposed. With the aid of specific gate bias configuration of two active devices, the high efficiency at back-off can be achieved. Meanwhile, a harmonic injection network allows the second harmonic components can be mutually injected into the drain of other device. This improves the disadvantage caused by the specific gate bias. Hence, the broadband high efficiency Doherty power amplifier can be achieved. For demonstration purpose, a high efficiency Doherty prototype is devised based on two identical 10W GaN HEMTs (Cree CGH 40010F). Measurement results show that at least 60% drain efficiency is achieved at 6 dB back-off power from 1.4 to 2.1 GHz.

In the third and last part, an asymmetric drain biased post-matching DPA using harmonic injection is proposed that gives further back-off extension. The injected harmonic components are generated by the two active devices. Aided by the proposed harmonic injection network, drain waveform amplitude modulation for both devices can be achieved at saturation, which results in enhanced saturated power for both carrier and peaking devices, while carrier back-off power remains unchanged. As a consequence, the back-off region is extended. Moreover, the power utilization factor of the asymmetric drain biased Doherty power amplifier is also improved. A Doherty power amplifier for WCDMA systems was designed and fabricated based on commercially available gallium nitride HEMT (Cree CGH 40010F) devices to validate the proposed technique. Measured results of the proposed Doherty power amplifier demonstrated that operation at 10dB back-off is possible between 1.6 - 1.9 GHz, with efficiency better than 46%.