In large-signal operation, to ensure the outputs from the main and auxiliary PAs are combined in phase, a 90 degree phase shift is added to the input of the auxiliary amplifier.Ĭompared to sub-6 GHz Doherty amplifiers, mmWave PAs add several design challenges. The impedance inverter at the output of the main amplifier provides a 90 degree phase shift between the main and auxiliary amplifier paths. Together, they present optimal impedances to the amplifiers in both compressed and backed-off operation. The output matching network of the Doherty PA consists of two impedance transformers: an impedance inverter and a common matching network. With high level input signals, the auxiliary amplifier begins to amplify, boosting the output power capability of the PA. The improvement in PA efficiency stems from the complementary operation of the main and auxiliary amplifiers: when the PA is operating at moderate power levels, only the main amplifier is active, which reduces DC power consumption. The Doherty amplifier configuration (see Figure 1) comprises a main or carrier amplifier, biased in class AB, and a parallel auxiliary or peaking amplifier, biased in class C. 1 This, combined with the company’s experience with mmWave PAs and front-end modules in various GaAs technologies, has been used to extend the Doherty amplifier to mmWave. PRFI has designed GaN PAs using both discrete devices and as custom MMICs, including a GaN on SiC Doherty PA for the 3.5 GHz 5G band. This means the advantages of GaN on SiC - high power density, higher output impedance and lower I 2R losses - can be realized in the 28 GHz 5G band. However, GaN on SiC processes with gate lengths of 0.15 μm are now available through foundries, with attractive performance at mmWave. Competing technologies such as GaAs and SiGe achieve higher operating frequencies with similar geometries, and these have become preferred high performance semiconductor technologies at mmWave. GaN on SiC has been successfully used at sub-6 GHz and to Ku-Band frequencies, but the source-coupled field plates often used to enhance breakdown voltage and increase power density have limited the maximum operating frequency of GaN transistors. Details of the design, simulation, layout and packaging will be discussed.Īs GaN on SiC technology advances, new possibilities for PAs emerge. The MMIC was assembled in a cost-effective, compact 4 mm x 4 mm QFN package. First-pass design success was achieved using an asymmetric topology fabricated on the commercial 0.15 μm G28v5 GaN on SiC foundry process from Wolfspeed. This article will describe the design of a Doherty PA MMIC for the 5G frequency band at 28 GHz. More recently, short gate length GaN on SiC MMIC processes have become commercially available, offering the possibility of designing high efficiency Doherty PAs at mmWave frequencies.
GaN technology offers significant performance advantages for realizing RF/microwave PAs. LDMOS, which is commonly used in discrete form below 6 GHz, has limited performance and more integrated approaches are needed to minimize parasitic inductances and capacitances. Although the benefits of the Doherty architecture are compelling, the challenges of designing Doherty PAs increase as the frequency of operation moves toward mmWave. This new solution gives the possibility to tune the combined assembly for maximum performances, compensating any possible spreading in the electrical parameters of Main/Peak stages.Doherty power amplifiers (PA) are widely used below 6 GHz to improve power-added efficiency (PAE) for communications applications. GreenWaves shall start with the design of a new adaptive phase-amplitude changer for fine adjustments of Main and Peak signals at the inputs of RF Doherty amplifier. In this context, GreenWaves is also in charge of studying technical solutions for wideband Doherty amplifiers capable to cover with one single HW version multiple adjacent LTE bands.
Both versions are currently being tested with Digital PreDistorter (DPD) in Infineon labs.Īdditional eight Doherty designs in different bands from 700MHz to 2.5GHz are planned to be done by GreenWaves starting from June. Symmetrical Doherty Amplifier in 1.9GHz band with two Infineon LDMOS transistors PXFC192207Īs first step of the design, RF simulations have led to the definition of PCB layout including input and output matching networks.īy means of tuning activities in GW lab, two hardware versions of the board have been generated, one for optimum drain efficiency and one for minimum phase distortion.