Research Of Back-off Efficiency Enhancement Of The Power Amplifier Based On Reactance Compensation

To achieve high data rate and spectrum efficiency,modern communication systems generally adopt a high-order modulation scheme,which results in signals with large peak-to-average power ratio.To amplify these signals efficiently,the power amplifiers require maintaining high efficiency at back-off powers.Conventional power amplifiers have a significant drop in efficiency at the large back-off power,resulting in a series of problems such as energy consumption and heat dissipation.Thus,the Doherty power amplifier(DPA)has been widely used due to its simple implementation and significant efficiency enhancement.This paper aims to further enhance the efficiency of the Doherty power amplifier at back-off powers.The dissertation is organized as follows:1.For efficiency enhancement at back-off powers,the basic theory of Doherty power amplifier is analyzed in detail,and a DPA based on impedance buffer technique is proposed.The output matching network consists of a harmonic matching network and a fundamental matching network,respectively,for satisfying the harmonic and fundamental impedances.The harmonic matching network is designed by impedance buffer technology.The measurement results show that drain efficiency of the proposed DPA can achieve higher than 70% and 55% at saturation power and 6dB back-off power at the operation frequency of 3.45 GHz,respectively,while the saturation power is higher than 49.5 dBm.These results validate the enhanced capability of the proposed method in improving efficiency at back-off power.2.In order to extend the high efficiency range of the symmetric Doherty power amplifier,a method for enhancing back-off efficiency based on reactance compensation is proposed.Unlike the conventional DPA,the output impendence of the peaking amplifier at the low power is nearly a reactance which is used to increase the effective impedance of the carrier amplifier and enchance the efficiency of the power amplifier.The relationship between the compensation reactance and the back-off power range of the amplifier is theoretically analyzed,and a method for designing a symmetric Doherty amplifier with large back-off power range is proposed.3.To realize higher efficiency at large back-off power,the basic theory of continuous class F power amplifier is analyzed systematically.A highly efficient Doherty power amplifier with extended back-off power range is designed by combining the continuous inverse class F mode and the reactance compensation.The output matching network consists of a harmonic control network and a two-impedance matching network.The harmonic control network which is designed by using impedance buffer technique can meet the requirement of the second and third harmonic terminations in the continuous class-F mode.And the two-impedance matching network can elide the ?/4 transmission line thus to expand the bandwidth.The output impedance of the peaking amplifier can reach the expected reactance value by designing the output matching network appropriately.Thus can compensate the effective impedance of the carrier amplifier and expand the back-off power range of the Doherty power amplifier.For verification,a 3.3–3.8 GHz symmetric DPA was designed and measured.The designed DPA can deliver an efficiency of 45%–48% at 9 dB back-off power over the whole frequency band with a maximum output power larger than 44 dBm.When driven by a 40-MHz modulated signal,the DPA exhibits an adjacent channel leakage ratio of better than –50 dBc after linearization at an average output power of 36.5 dBm.Compared with the conventional Doherty amplifier,the proposed DPA can realize larger back-off power range.