The rapid growth data streaming demand in modern communication systems makes unprecedented challenges for wireless service providers. Along with the growth data streaming demand, the steady development of wireless application causes an issue of wireless coexistence with limited frequency spectrum. Therefore, the energy and spectrum efficient transmitters with higher data rate, small die area, and low integration cost are demanded for the modern communication systems.The fifth generation mobile network emerges as a promising revolution in mobile communications with higher data rates. In 5G mobile network, two kind of frequency bands, sub-6 GHz bands and millimeter-wave bands above 24 GHz, are classified. Due to the densely packed spectrum in sub-6 GHz bands, the transmitter designs have focused on improving spectral efficiency with frequency-localized waveforms such as Orthogonal Frequency Division Multiplexing. However, the OFDM signal has a major drawback of high peak-to-average-ratio, which results in degraded power efficiency of the transmitter. On the other hand, to support higher data rate, mm-wave bands can support bandwidths up to 2 GHz without aggregating bands together. However, at mm-wave bands, the maximum range to sustain reliable wireless links is decreased due the increasing pass loss. Fortunately, the phased array techniques, which have been used for defense and satellite applications for many years, enables the directive communications could be a promising solution to overcome the challenges.The objective of this dissertation is to present novel topologies for transmitter designs, which are suitable for modern communication systems in both sub-6 GHz and mm-wave bands. For sub-6 GHz bands, an efficient quadrature digital power amplifier as a standalone transmitter has been proposed. The proposed transmitter employs complex-domain Doherty and dual-supply Class-G to achieve up to four efficiency peaks and excellent system efficiency at power back-off. For mm-wave bands, an 18-50-GHz mm-wave transmitter has been proposed. The proposed mm-wave transmitter is designed for low power consumption, a small area, and supports emerging multibeam communications and directional sensing with an increased number of phased array elements from 18-50 GHz.
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