LTE için geniş bantlı ve yüksek verimlilikli doherty güç yükselteç tasarımı /
Yazar: KAAN KOCA
Danışman: PROF. DR. SEDEF KENT PINAR ; DR. ÖĞR. ÜYESİ HÜSEYİN ŞERİF SAVCI
İstanbul Teknik Üniversitesi / Bilişim Enstitüsü / İletişim Sistemleri Ana Bilim Dalı / Uydu Haberleşmesi ve Uzaktan Algılama Bilim Dalı
Derece: Yüksek Lisans, 2023
Konu: Elektrik ve Elektronik Mühendisliği = Electrical and Electronics Engineering

Özet:
Bu tezde, LTE (Uzun Dönem Evrim) Band-7 ve Wi-Fi uygulamaları için uygun olan AB sınıfı ve C sınıfı GY ile tasarlanmıs ̧ genis ̧ bantlı bir Doherty güç yükselteç tasarlanmıs ̧tır. Çıkıs ̧uyumlandırmaag ̆ı,yardımcıyükselteçidealolmayansonsuz çıkıs ̧ empedansının parazitik cihazlar üzerindeki etkisine vurgu yapılarak teorik olarak analiz edilmis ̧tir. Yeni bir Doherty Güç Yükselteç (DGY) 25 W GaN HEMT (Yüksek Elektron Mobiliteli Transistör) ile tasarlanmıs ̧tır. DGY, temel tasarımlar ve açıklamalar için tipik olan 6 dB'lik bir OPBO (Güç Geri Çekme) deg ̆erini varsayar, çünkü ilgili voltaj seviyesi 1:4'tür ve tepe güç yükselteç giris ̧ voltajının dinamik aralıg ̆ının yarısında etkinles ̧tirilir. Ancak OPBO (Güç Geri Çekme) deg ̆erini arttırmak için öncelikle sinyalin PAPR (Tepe Etkin Güç Oranı) deg ̆eri ile uyumlu olması gerekmektedir. Asimetrik bir Doherty güç yükselteç tasarımı, tam çıkıs ̧ gücünde dog ̆rusallıg ̆ı korurken yüksek kazanç dag ̆ıtımını koruyarak verimlilig ̆i en üst düzeye çıkarmaya yardımcı olabilir. DGY, 3G (3. Nesil Mobil  ̇Iletis ̧im) /LTE (Uzun Dönem Evrim) modülasyon hızı ayarlarında yüksek RF GY verimlilig ̆i sag ̆lamayı amaçlamaktadır. Bu, Doherty GY'nın yüksek bir ortalama çıkıs ̧ gücünde yüksek PAPR (Tepe Etkin Güç Oranı) için DE (drenaj verimlilik) artırmasına ve PAPR (Tepe Etkin Güç Oranı) zayıf oldug ̆u yerlerde GY ısınmasını önemli ölçüde azaltmasına olanak tanır. OPBO (Güç Geri Çekme) aralığı asimetrik DGY teknig ̆i kullanılarak genis ̧letilmis ̧tir. Daha önce DGY topolojisinde kullanılan çeyrek dalga dönüs ̧türücüsü, ilgili Klopfenstein tapper ag ̆ı ile deg ̆is ̧tirildi. Gerçek dünyadaki prototip uygulamalar, bu deg ̆is ̧iklig ̆in verimlilik deg ̆erlerini korurken geleneksel topolojilere kıyasla elde edilen DGY bant genis ̧lig ̆ini (BW) artırdıg ̆ını göstermis ̧tir (Kesirli bant genis ̧lig ̆i %24'e es ̧ittir). Çıkıs ̧ birles ̧tirici, optimum karakteristik empedans ve faz ofset deg ̆eri kombinasyonlarına sahip konik empedans transformatörleri ve yük empedanslarından olus ̧ur. Bu, yüksek çıkıs ̧ gücü seviyesinde yük modülasyonu ve yüksek geri tepme sag ̆lamak için yapılır. Simülasyonların bir sonucu olarak, DGY'nın CG2H40025 transistörlerle uygulanması, %69'dan daha yüksek bir doymus ̧ verimlilikle 79 W'tan daha yüksek bir çıkıs ̧ gücü sag ̆lar. Tüm frekans bandı boyunca, maksimum çıkıs ̧ gücü 47 dBm'den fazladır ve bu da bu transistörün maksimum güç is ̧leme faktörüne karşılık gelir. Verimlilik açısından, doygunlukta %69 ile %79 arasında ve 6-dB geri çekmede %50 ile %72 arasında deg ̆is ̧mektedir.

Abstract:
This thesis describes a Wi-Fi and LTE Band-7 compatible broadband Doherty power amplifier with class AB and class C PA s. Theoretically, the output matching network is examined with a focus on how parasitic devices are affected by the auxiliary amplifier's non-ideal infinite output impedance. Using 25 W GaN HEMT transistors operating at 2.2 – 2.8 GHz, a novel Doherty-like power amplifier (DPA) is created. The DPA makes the assumption that the OBO value will be 6 dB because the voltage level involved is 1:4 and the peak power amplifier is turned on at half of the input voltage's dynamic range. However, the signal must first be compatible with the PAPR value in order to raise the OBO value. The design of an asymmetrical Doherty power amplifier can help maximize efficiency by maintaining high gain delivery while maintaining linearity at full output power. The DPA aims to provide high RF PA efficiencies at 3G/LTE modulation rate settings. By increasing drain efficiency for high PAPR modulation at a high average output power, the Doherty PA is able to significantly lessen PA heating in regions with low PAPR. The OPBO range was increased by using the asymmetric DPA technique. In the DPA topology, the corresponding Klopfenstein tapper network took the place of the quarter-wave transformer. When compared to conventional topologies, real-world prototype implementations have demonstrated that this modification increases the achieved DPA bandwidth (BW) while maintaining efficiency values.(Fractional bandwidth equals 24%) The output combiner is made up of tapered impedance transformers and load impedances that have optimal characteristic impedance and phase offset value combinations. This is done to ensure load modulation and high back-off at a high output power level. As a result of simulations, the application of the DPA with CG2H40025 transistors yields an output power greater than 79 W, with a saturated efficiency greater than 69%. Across the entire frequency band, the maximum output power is greater than 47 dBm, which corresponds to the maximum power handling factor of this transistor. In terms of efficiency, it ranges between 69% and 79% at saturation and 50% and 72% at 6-dB back-off. Recent multiband communication schemes have enlarged the BW needs, there are two main approaches regarding the power amplifier (PA): to employ resonant narrowband structures along the whole application band or to employ of a wideband PA. Thus, the the limited BW of the Doherty PAs (DPAs) has become a drawback for such multi-band applications. By modifying the reflection coefficient in this thesis, we hope to address output matching networks' (OMNs') flaws by broadening the BW of DPA. By utilizing two different transistors, the asymmetric Doherty PA can achieve a wider OPBO range. The power ratio of the carrier and peak amplifiers, however, determines the back-off range, making it impossible to achieve an arbitrary power back-off range. Therefore, it is of great importance to increase the OPBO range of asymmetric DPA to a desirable value for higher PAPR applications. Although a DPA with a complex coupling load is used for extended OPBO range, the complex load complicates the design of the main input and output matching network and may affect the matching in saturation. The impedance inverter that connects the main and peaking amplifiers is the most noticeable and traditional cause of the Doherty amplifier's limited bandwidth. To achieve a proper load modulation of the higher impedance (2Ropt) into a lower impedance of Ropt in the upper 6 dB regime, the amplifier structure needs to convert the low common impedance of Ropt/2 into a higher impedance of 2Ropt, which the main device perceives as load in the lowpower region. Only at the center frequency where the formulas apply can the impedance inverter achieve this load modulation mechanism. The impedance inverter can only achieve this load modulation mechanism at the center frequency where the formulas apply. The impedance inverter will no longer have λ/4 length as a result of the frequency deviation, which changes the purely resistive load (2Ropt) that the main device sees into a complex load. This causes a non-optimal load modulation that gets worse as the frequency deviation increases.
The situation becomes considerably more convoluted in the practical implementation of the Doherty amplifier, where an additional impedance inverter (transformer) is frequently required to transform the common load impedance into the final system impedance. The impact of these impedance inverters on the Doherty amplifier's possible bandwidth is thoroughly analyzed in the following section. Another component that is frequency-sensitive is the offset line at the peaking amplifier's input. In the upper 6 dB regime, this offset line is required to achieve in-phase combining of the output fundamental currents of the main and peaking amplifiers. It may be difficult to achieve phase compensation at all design frequencies using a fixed-length offset line, in the practical implementation of wideband Doherty amplifiers because the phase behaviors of the main and peaking amplifiers may not be similar over the design band due to their different bias and matching conditions. By using similar input/output matching network topologies for the main and peaking amplifiers, for example, a potential solution to this issue could be achieved by optimizing the paths of the main and peaking amplifiers during the design process in order to make it simpler to compensate for the phase difference using a fixed-length offset line. Another factor influencing the Doherty amplifier's wideband capability is its frequency-dependent turn-on of the peaking amplifier. The class-C amplifier exhibits different gain expansion behaviors at various frequencies, i.e., the drive power level at which the peaking device activates is frequency dependent, which affects the load modulation mechanism and lowers backoff efficiency. Such a situation can be avoided by carefully planning the input matching network of the peaking amplifier over the desired design band, which will cause the amplifier to operate at the same drive power level across the overall design spectrum. Load tuning in DPA using the Klopfenstein taper has been attempted, but not as a replacement for load modulation schemes. The Klopfenstein taper and the multi-stage transformer are combined in this thesis. Broadband matching of the main PA's output reactance is made possible by the use of this combiner, which aims to resolve the trade off between DPA gain, PAE, and BW. This tuning relies on relatively simple control of the reactance at the output of the Klopfenstein taper, allowing the relative group delay between the branches to be tuned appropriately. In this thesis, we propose a novel method for tuning the Klopfenstein taper and peak amplifier current by selecting the appropriate load impedance, as well as a simple method for achieving the appropriate Doherty load modulation across the entire bandwidth. A Doherty PA that provides both wideband and high backoff is proposed by using these methods as a hybrid.It is well known that achieving Doherty that can operate over a wide frequency range is a difficult task. Furthermore, the methods used to increase the backoff are known to reduce bandwidth. The goal is to realize these two critical capabilities in the same study and design a broadband Doherty PA at high OPBOs. The design proposed in this paper incorporates symmetric load matching networks for broadband Doherty power amplifiers (DPAs). The output combiner consists of a tapered impedance transformer with optimized characteristic impedance and phase offset values to provide load modulation at a high output power level and high backoff. The chosen bandwidth covers LTE Band-7 and Wi-Fi as well as part of the band that could be adopted for multiband systems in the future. The peak-to-average power ratio (PAPR) of LTE signals is extremely high. For uplink communication, LTE employs singlecarrier frequency division multiple access (SC-FDMA). Although the PAPR of SC-FDMA modulation (6 – 7dB) is higher than that of W-CDMA (3 – 4dB) and GSM (0 dB), it is worth noting that HSPA (High Speed Packet Access) can have high PAPR in some cases, posing similar PA efficiency challenges. A commercially packaged GaN HEMT on SiC (CG2H40025F from Cree Inc.) with a typical in-band output power of 25 W at a 28-V drain bias was used as the active device.