This paper presents the first 28-/37-/39-GHz linear Doherty power amplifier (PA) in silicon for broadband fifth-generation (5G) applications. We introduce a new transformer-based on-chip Doherty power combiner that can reduce the impedance transformation ratio (ITR) in power back-off (PBO) and, thus, improve the bandwidth and power-combining efficiency. We also devise a “driver-PA co-design” method that creates power-dependent uneven feeding in the Doherty PA and enhances the Doherty operation without any hardware overhead or bandwidth compromise. For the proof of concept, we implement a 28-/37-/39-GHz PA fully integrated in a standard 130-nm SiGe BiCMOS process, which occupies 1.8 mm $^{\mathbf {2}}$ . The PA achieves a 52% −3-dB small-signal $\text{S}_{\mathbf {21}}$ bandwidth and a 40% −1-dB large-signal saturated output power ( $\text{P}_{\mathbf {sat}}$ ) bandwidth. At 28/37/39 GHz, the PA achieves +16.8−/+17.1−/+17-dBm $\text{P}_{\mathbf {sat}}$ , +15.2−/+15.5−/+15.4-dBm $\text{P}_{\mathbf {1\,dB}}$ , and superior 1.72/1.92/1.62 times efficiency enhancement over class-B operation at 5.9-/6-/6.7-dB PBO. Moreover, the PA demonstrates multi-gigabit-per-second data rates with excellent efficiency and linearity for 64-quadrature amplitude modulation (64-QAM) in three millimeter-wave (mm-wave) 5G bands. This PA advances the state of the art for Doherty, wideband, and 5G silicon PAs in mm-wave bands. It supports drop-in upgrade for current PAs in existing mm-wave systems and opens doors to compact system solutions for future multiband 5G massive multiple-input multiple-output (MIMO) and phased-array platforms.

A 28-/37-/39-GHz Linear Doherty Power Amplifier in Silicon for 5G Applications

William H. Doherty was an American electrical engineer, best known for his invention of the Doherty amplifier. Born in Cambridge, Massachusetts, in 1907, Doherty attended Harvard University, where he received his bachelor's degree in communication engineering (1927) and his master's degree in engineering (1928). He then joined the American Telegraph and Telephone Company Long Lines Department in Boston. He remained there for a few months before joining the National Bureau of Standards, where he researched radio phenomena. Doherty joined Bell Telephone Laboratories (now Bell Labs) in 1929 and, while there, worked on the development of high power radio transmitters that were used for transoceanic radio telephones and broadcasting.

Doherty's Legacy: A History of the Doherty Power Amplifier from 1936 to the Present Day

The effect of the output impedance of peaking power amplifier (PA) on Doherty PAs (DPAs) is analyzed in this paper. In the design procedure of DPAs, the ideal case is that the output impedance of auxiliary PA is infinite at output power back-off (OPBO) level. However, it is almost impossible to realize this perfect condition in broadband DPAs. Therefore, when the output impedance of peaking path deviates from infinity, some potential effects on DPAs must be produced. In this paper, these effects are explained at the internal plane of transistors. The conclusion is that, at different normalized frequencies, there are different optimal impedance regions for the output impedance of peaking stage. This means that the noninfinite output impedances of peaking stage can enhance the performances of broadband DPAs so long as they are elaborately processed. A 1.65–2.7-GHz (48% bandwidth) broadband DPA is designed considering the effects of peaking PA. The experimental results show that this DPA obtains a drain efficiency of 41%–59.6% at 6-dB OPBO levels and a drain efficiency of 55.8%–72.2% at saturation power levels. The maximum output power across the entire operating band is 43.1–45.2 dBm with a gain of 9.0–10.2 dB. Furthermore, the designed DPA achieves an adjacent channel leakage ratio of −45.8 dBc with an output power of 36.1 dBm at 2.0 GHz after digital predistortion when it is excited by 5-MHz WCDMA signal with a peaking-to-average power ratio of 8.6 dB.

The Influence of the Output Impedances of Peaking Power Amplifier on Broadband Doherty Amplifiers

The Doherty power amplifier (DPA) has been extensively explored in the past and has become one of the most widely used power amplifier (PA) architectures in cellular base stations. The classical DPA suffers intrinsic bandwidth constrains which limit its application in future 5G wireless transmitters. In this paper, we present a comprehensive review of the DPA bandwidth enhancement techniques proposed in literature in order to provide a thorough understanding of the DPA's broadband design for high-efficiency 5G wireless transmitters. We elaborate on the main bandwidth limitation sources and provide circuit design insights. We then follow with an overview of bandwidth enhancement techniques developed for the DPA, including modified load-modulation networks, frequency response optimization, parasitic compensation, post-matching, as well as distributed DPA, dual-input digital DPA, transformer-based power-combining PA, and transformer-less load modulated PA architectures. Furthermore, challenges and design techniques for integrated circuit (IC) implementation of broadband DPAs are discussed, including a review of circuits developed in CMOS, SiGe, and GaN processes, and operating in RF and mm-Wave frequencies.

Breaking Bandwidth Limit: A Review of Broadband Doherty Power Amplifier Design for 5G

The next generation wireless network, 5G, is expected to provide ubiquitous connections to billions of devices as well as to unlock many new services through multigigabit-per-second data transmission. To meet the ever-increasing demands for higher data rates and larger capacities, new modulation schemes have been developed, and wider frequency bands, such as those at millimeter wave (mm-wave), have been designated for 5G [1], [2]. Massive multiple input/multiple output (MIMO), which uses a large number of antennas at the transmitter and receiver, has been considered one of the key 5G technologies to improve data throughput and spectrum efficiency [3]. These new application scenarios impose stringent requirements on wireless transceiver front ends and call for special considerations at the circuit and system design levels. In the transmitter, power amplifiers (PAs) should accommodate complex modulated signals, featuring a high peak-to-average-power ratio (PAPR) and a wide modulation bandwidth. Moreover, in massive MIMO arrays, PAs should maintain a high average efficiency to mitigate thermal heating issues.

Breaking the Bandwidth Limit: A Review of Broadband Doherty Power Amplifier Design for 5G

A push-pull Doherty Power Amplifier (DPA) is proposed that uses a wideband power combiner BALUN, which is capable to provide the optimum fundamental and 2nd harmonic loading conditions over a large operating bandwidth to the active devices. A 700W DPA demonstrator using these techniques provides high average efficiency (> 40%) for a 7.5dB PAR DVB-T signal over a 37% relative RF-bandwidth.

J. He

Papers

A wideband 700W push-pull Doherty amplifier