Design and Analysis of Broadband Doherty Power Amplifiers Open Access
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Modern communications systems utilize complex modulation schemes with high Peak to Average Power Ratio (PAPR). Power Amplifiers that can handle high PAPR with improved efficiency at Output Power Back Off (OPBO) and good linearity over the output power range are needed for LTE 3G, 4G and 5G systems. Furthermore, extended bandwidth is needed to handle reconfigurable systems with agile carrier frequencies. The Doherty Power Amplifier (DPA) is an efficiency enhancement scheme which addresses efficiency at OPBO and linearity simultaneously. Doherty Power Amplifiers utilize active loadpull to increase efficiency at OPBO. The standard DPA consists of two amplifiers and an Impedance Inverting Network (IIN). This implementation is narrowband by nature due to the fact that IIN is usually implemented with a quarter wave transmission line. Some of the broad band methods include minimizing the frequency sensitivity of the Impedance Inverting Network (IIN) by reducing the transformation ratio for the quarter wave transmission line. Other methods use the lumped element equivalent of the quarter wave IIN to absorb the output capacitance of the amplifiers. A Transformer-Less Load Modulated scheme has been proposed. This method uses a combination of Tee and Pi three element networks to construct the output power combining network. Since the Tee and Pi networks are inherently narrow band, the increase in bandwidth is modest. The design methodology used in this paper treats the output power combiner as a lossless two port which functions as an IIN and filter network. This thesis presents a methodical approach to designing broadband Doherty Power Amplifiers (DPA). A method of identifying a range of lower Q load impedances which simultaneously satisfies output power, PAE and bandwidth requirements is presented. A systematic method of defining output matching circuits that satisfy the impedance requirements at each discrete OPBO level. The Real Frequency Technique is used to define lossless networks which function as impedance inverting networks while absorbing device output capacitance. Finally, a broadband DPA is designed to verify this proposed methodology.