Broadband Sequential Power Amplifier Design with a Tunable Coplanar Waveguide Structures Ken Mcknight, Dr, Mona Zaghloul, Dr Ali Darwish Army Research Lab, 2800 Powder Mill Road, Adelphi MD, 20783, US Kenneth.M.Mcknight10.civ@us.army.mil Abstract Modern wireless communications systems use spectrally efficient modulation schemes that produce signals with high peak to average power ratios (PAPR). This paper presents the design of an innovative sequential power amplifier (SPA) which utilizes a programmable branch line coupler as the output combiner. The combiner uses a variable coplanar waveguide to tune to tune the branch elements for improved amplifier efficiency. By using two and three stage couplers a relative bandwidth exceeding 50% can be achieved with better than 40% efficiency at X-band. I. Introduction Modern wireless communications systems use spectrally efficient modulation schemes that produce signals with high peak to average power ratios (PAPR). As a result, power amplifiers with high linearity and good efficiency are needed for these systems. Class A or class A-B amplifiers are generally chosen for the superior linearity however, their peak efficiency occurs near output power saturation levels. Generally, these classes of power amplifiers operate at 6dB output power back off power levels (OPBO) in systems which require high PAPR. As a result, the power amplifiers operate at efficiency levels well below their maximum levels. The Doherty Amplifier was originally proposed as a solution to this problem. The drawback of the Doherty configuration is that it is inherently narrow band due to the network used to combine the main and peaking amplifier outputs. The concept of Sequential Amplifiers was introduced as an alternative to the Doherty configuration [1]. Like the Doherty, Sequential Amplifiers consists of a main and peaking amplifier. The combining network is a Branch Line Coupler which has a broader band response. The coupling coefficient for the Branch Line Coupler can be tailored to provide good efficiency up to the 6dB OPBO level. This is accomplished by choosing the coupling ratio such that minimal power is dissipated in the resistively load isolated port. Above the 6dB OPBO level, more power is dissipated in the resister connected to the isolated port. As a result, the efficiency of the combiner decreases and the overall efficiency of the Sequential Amplifier deteriorates. This paper proposes a Sequential Amplifier scheme which utilizes a programmable Branch Line Coupler to adjust the coupling coefficient as the output power of the Peaking Amplifier increases. The programmable coupler is based variable characteristic impedance transmission lines as presented in [2]. II. Theory of Operation The Sequential Amplifier (SPA) proposed is based on the digital implementation presented in [1]. The baseband signal and the controls for the coupler are generated from a FPGA. The baseband signal is up-converted using a Quadrature LO for proper phase alignment. The outputs of the main and peaking amplifiers are then feed into a branch line coupler with the isolated port terminated using a 50 Ohm load. For output power levels at 6dB OPBO and below, the efficiency of the SPA can be given as: ߟ ௌ௉஺ ൌ ௉ ಾ೔೙ ௉ ವ಴ ಾ೔೙ ߟ ஼௢௠௕௜௡௘௥ (1) The total SPA efficiency is a combination of the amplifier efficiency and the combiner efficiency.