Optical communications is expanding into new applications such as infrared wireless communications; therefore, designing high performance circuits has gained considerable importance. In this dissertation a wide dynamic-range variable-gain transimpedance amplifier (TIA) is introduced. It adopts a regulated cascode (RGC) amplifier and an operational transconductance amplifier (OTA) as the feed forward gain element to control gain and improve the overload of the optical receiver. A fully-differential variable-gain TIA in a 0.35µm CMOS technology is realized. It provides a bit error rate (BER) less than 10-12 for an input current from 6µA-3mA at 3.3V power supply. For the transimpedance gain variation, from 0.1kΩ to 3kΩ, -3dB bandwidth is higher than 1.7GHz for a 0.6pF photodiode capacitance. The power dissipations for the highest and the lowest gains are 8.2mW and 24.9mW respectively. A new technique for designing uniform multistage amplifiers (MA) for high frequency applications is introduced. The proposed method uses the multi-peak bandwidth enhancement technique while it employs identical, simple and inductorless stages. It has several advantages, such as tunability of bandwidth and decreased sensitivity of amplifier stages, to process variations. While all stages of the proposed MA topology are identical, the gain-bandwidth product can be extended several times. Two six-stage amplifiers in a TSMC 0.35µm CMOS process were designed using the proposed topology. Measurements show that the gain can be varied for the first one between 16dB and 44dB within the 0.7-3.2GHz bandwidth and for the second one between 13dB and 44dB within a 1.9-3.7GHz bandwidth with less than 5.2nV/√Hz noise. Although the second amplifier has a higher gain bandwidth product, it consumes more power and occupies a wider area. A technique for capacitance multiplication is utilized to design a tunable loop filter. Current and voltage mode techniques are combined to increase the multiplication factor (M). At a high input dynamic range, M is adjustable and the capacitance multiplier performs linearly at high frequencies. Drain-source voltages of paired transistors are equalized to improve matching in the current mirrors. Measurement of a prototype loop filter IC in a 0.5µm CMOS technology shows 50µA current consumption for M=50. Where 80pF capacitance is employed, the capacitance multiplier realizes an effective capacitance varying from 1.22nF up to 8.5nF.

Emerging wireless standards are designed to be spectrally efficient to address the high cost of licensing wireless spectra. Unfortunately, the resulting signals have a high peak-to-average ratio that reduces the base station power amplifier efficiency at the back-off power level. The wasted energy is converted to heat that degrades the device reliability and increases the base-station’s carbon footprint and cooling requirements. In addition, these new standards place stringent re- quirements on the amplifier output power, linearity, efficiency, and bandwidth. To improve the back-off efficiency, a Doherty amplifier, which uses two device in parallel for back-off efficiency enhancement, is deployed in a typical base station. Unfortunately, the conventional Doherty amplifier is narrowband and thus cannot satisfy the bandwidth requirement of the modern base station that needs to support multiple standards and backward compatibility. In this thesis, we begin by studying the class F/F−1 high efficiency mode of operation. To this end, we designed a narrowband, harmonically-tuned 3.3 GHz, 10 W GaN high efficiency amplifier. Next, we investigate how to simultaneously achieve high efficiency and broad bandwidth by harnessing the simplified real frequency technique for the broadband matching network design. A 2 to 3 GHz, 45 W GaN amplifier and a 650 to 1050 MHz, 45 W LDMOS amplifier were designed. Finally, we analyze the conventional Doherty amplifier to determine the cause of its narrow bandwidth. We find that the narrow bandwidth can be attributed to the band-limited quarter-wave transformer as well as the widely adopted traditional design technique. As an original contribution to knowledge, we propose a novel Doherty amplifier configuration with intrinsically broadband characteristics by analyzing the load modulation concept and the conventional Doherty amplifier. The proposed amplifier uses asymmetrical drain voltage biases and symmetrical devices and it does not require a complex mixed-signal setup. To demonstrate the proposed concept in practice, we designed a 700 to 1000 MHz, 90 W GaN broadband Doherty amplifier. Moreover, to show that the proposed concept is applicable to high power designs, we designed a 200 W GaN broadband Doherty amplifier in the same band. In addition, to show that the technique is independent of the device technology, we designed a 700 to 900 MHz, 60 W LDMOS broadband Doherty amplifier. Using digital pre-distortion, the three prototypes were shown to be highly linearizable when driven with wideband 20 MHz LTE and WCDMA modulated signals and achieved excellent back-off efficiency. Lastly, using the insights from the previous analyses, we propose a novel mixed-technology Doherty amplifier with an extended and reconfigurable back-off level as well as an improved power utilization factor. The reconfigurability of the proposed amplifier makes it possible to customize the back-off level to achieve the highest average efficiency for a given modulated signal without redesigning the matching networks. A 790 to 960 MHz, 180 W LDMOS/GaN Doherty amplifier demonstrated the extended bandwidth and reconfigurability of the back-off level. The proposed amplifier addresses the shortcomings of the conventional Doherty amplifier and satisfies the many requirements of a modern base station power amplifier.

Emerging wireless standards are designed to be spectrally efficient to address the high cost of licensing wireless spectra. Unfortunately, the resulting signals have a high peak-to-average ratio that reduces the base station power amplifier efficiency at the back-off power level. The wasted energy is converted to heat that degrades the device reliability and increases the base-station’s carbon footprint and cooling requirements. In addition, these new standards place stringent re- quirements on the amplifier output power, linearity, efficiency, and bandwidth. To improve the back-off efficiency, a Doherty amplifier, which uses two device in parallel for back-off efficiency enhancement, is deployed in a typical base station. Unfortunately, the conventional Doherty amplifier is narrowband and thus cannot satisfy the bandwidth requirement of the modern base station that needs to support multiple standards and backward compatibility. In this thesis, we begin by studying the class F/F−1 high efficiency mode of operation. To this end, we designed a narrowband, harmonically-tuned 3.3 GHz, 10 W GaN high efficiency amplifier. Next, we investigate how to simultaneously achieve high efficiency and broad bandwidth by harnessing the simplified real frequency technique for the broadband matching network design. A 2 to 3 GHz, 45 W GaN amplifier and a 650 to 1050 MHz, 45 W LDMOS amplifier were designed. Finally, we analyze the conventional Doherty amplifier to determine the cause of its narrow bandwidth. We find that the narrow bandwidth can be attributed to the band-limited quarter-wave transformer as well as the widely adopted traditional design technique. As an original contribution to knowledge, we propose a novel Doherty amplifier configuration with intrinsically broadband characteristics by analyzing the load modulation concept and the conventional Doherty amplifier. The proposed amplifier uses asymmetrical drain voltage biases and symmetrical devices and it does not require a complex mixed-signal setup. To demonstrate the proposed concept in practice, we designed a 700 to 1000 MHz, 90 W GaN broadband Doherty amplifier. Moreover, to show that the proposed concept is applicable to high power designs, we designed a 200 W GaN broadband Doherty amplifier in the same band. In addition, to show that the technique is independent of the device technology, we designed a 700 to 900 MHz, 60 W LDMOS broadband Doherty amplifier. Using digital pre-distortion, the three prototypes were shown to be highly linearizable when driven with wideband 20 MHz LTE and WCDMA modulated signals and achieved excellent back-off efficiency. Lastly, using the insights from the previous analyses, we propose a novel mixed-technology Doherty amplifier with an extended and reconfigurable back-off level as well as an improved power utilization factor. The reconfigurability of the proposed amplifier makes it possible to customize the back-off level to achieve the highest average efficiency for a given modulated signal without redesigning the matching networks. A 790 to 960 MHz, 180 W LDMOS/GaN Doherty amplifier demonstrated the extended bandwidth and reconfigurability of the back-off level. The proposed amplifier addresses the shortcomings of the conventional Doherty amplifier and satisfies the many requirements of a modern base station power amplifier.