Study on Symmetrical and Asymmetrical Filters and Their Use in High-Efficiency Power Amplifiers


Student thesis: Doctoral Thesis

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Award date26 May 2021


Filters are ubiquitous and are indispensable building blocks of RF transceivers, as seemingly simple structures with the ability to suppress out-of-band interference. They have been well documented in the literature which has now been reignited with new design concepts and methodologies. In this thesis, two novel bandpass filters with symmetrical terminal ports are first developed. They combine the merits of dielectric and strip-line structures, with low insertion loss and high selectivity. As a module, these filters are normally placed after the power amplifiers (PAs) to realize filtering PAs. This inevitably increases circuit size, mismatch and loss. An integrated design would be more desirable since the filter will be matched directly to the transistor, thus reducing the size due to interconnecting cables. An asymmetrical bandpass structure can be designed as a part of the PA for an integrated design, which also acts as the PA output matching network (OMN) to obtain much better performance for both small- and large-signal responses. Three novel asymmetrical bandpass structures are integrated for Class F/F-1 PAs and Doherty power amplifiers (DPAs). 

First of all, a metal strip is attached to the surface of a high-permittivity dielectric rod to form a novel dielectric-loaded strip resonator (DLSR). The resonant mode comes from the strip which is no longer just a perturbation in a traditional strip-loaded dielectric resonator. For demonstration, a single-ended inline filter is exploited by using the single-strip DLSRs, while the generation mechanism of transmission zeros (TZs) is investigated. This filter is cascaded at the output of a Class-B PA to realize a bandpass response although its efficiency and output power is slightly degraded. Furthermore, an electrically-small differential filter is presented based on a modified twin strips, highlighting the versatile design of the DLSR. 

Secondly, a triple-mode filter based on a hollow strip-loaded ring-shaped dielectric resonator is designed by combining a dominant TE01δ mode with a high-order degenerate HE11δ pair. The frequencies of the HE11δ pair can be pulled down and split using a loaded silver segment on top of the dielectric. Moreover, the strip segment has a negligible effect on TE01δ and higher-order TM01δ modes, implying a separation between the passband and adjacent interferers. This filter is placed after the 50-Ω output port of a Doherty PA for an integrated design.

Thirdly, a wideband Class-F/F-1 PA based on a microstrip multimode bandpass impedance transformer (BPIT) is proposed. The modified quad-mode BPIT operation enables the creation of multiple transmission poles using only a single resonator, thus simultaneously achieving a flat wideband response in a small size. In addition, the drain bias and input feed line of the BPIT are arranged to satisfy the harmonic-impedance requirement of a Class-F/F-1 amplifier. Two PA prototypes exhibit desirable filtering characteristics centered at around 3.5 GHz for 5G applications. The maximum gains at saturation are larger than 12 dB with output power Pout>38.5 dBm. Maximum power added efficiencies (PAEs) are greater than 60%, and with both effective bandwidths of ~400 MHz for PAEs > 50%.

Fourthly, a novel dual-mode T-type impedance transformer (IT) is proposed to realize a complex-to-real conversion at output power back-off (OBO) and saturation simultaneously and at two frequencies. The lengths of drain bias and OMN in the carrier path are modified to reduce the conversion fluctuation and ensure that the input impedance of the proposed IT can satisfy the variation of broadband load-pull simulation. Saturated efficiency and output power at the upper band edge are improved by introducing an out-of-phase 2nd harmonic injection. A broadband DPA is fabricated with a wide bandwidth operating from 2.1 to 3.1 GHz. Measured results demonstrate a 10-11.1 dB gain and 42.2-43.3 dBm saturated output power. The 6-dB OBO and saturated efficiencies are 42.3-52% and 60.4-67.7%, respectively. 

Finally, a DPA with two passbands is proposed by using asymmetric T-type structures based on short- or open-circuit shunt stub (ATSS or ATOS). Two functions are integrated into one circuit with the ATSS and ATOS acting as the output matching networks in the carrier and peaking paths, respectively. They can realize a complex-to-real conversion at both OBO and saturation, and simultaneously at multiple frequencies, implying a wide load-modulation bandwidth. A TZ at center frequency is created by the ATSS, while the ATOS simultaneously generates multiple TZs at OBO and saturation, thereby forming two passbands. A novel and compact wideband stub-loaded power divider is used to split the input power. A prototype dual-passband DPA is implemented based on two 10-W GaN transistors to validate the methodology. The measured operating lower and upper bands are from 1.35 to 1.75 GHz and 3.05 to 3.45 GHz, respectively, with both having 400 MHz operating bandwidths. Measured results demonstrate a maximum saturated gain of 9.2 dB and an output power of 43.8 dBm. Maximum 6-dB OBO and saturated efficiencies are 55% and 67.5%, respectively. The circuit occupies a very compact size with an area of 0.5λg× 0.43λg.