In 5G transmitters, high efficiency, high linearity, and compatibility with MIMO and beamforming techniques are of utmost importance. Typically, enhancement techniques like supply modulation or load modulation are used when dealing with envelope modulated communication signals, which increasingly have high peak-to-average-power ratios (PAPR). Supply modulation is most favored in handsets. Techniques like "Envelope Tracking (ET)" or "Envelope Elimination and Restoration (EER)" are employed to generate high efficiency over a large power back-off range. These techniques are robust to antenna impedance variations compared to load modulation techniques like Doherty. However, the downside of supply voltage modulation is the need for a dynamic supply modulator. Such supply modulators are very difficult to implement for systems with video bandwidths >20MHz. For modern communication systems like 5G, video bandwidths for both the base stations and handsets are drastically increasing (up to ~1GHz). 5G communication is also going to operate at very high frequencies, commonly known as mm-wave. This degrades the efficiency of the transmitter because of increased losses in the output conductance of the active device and from any power combining network. Furthermore, due to the use of (massive) MIMO/smart-antenna techniques in these 5G base stations, there will also be undesired time-fluctuating antenna impedances due to PA back-coupling through the antenna array, which are extremely difficult to handle in efficiency enhancement techniques based on load modulation. In this thesis, a new type of wideband "supply voltage modulation" has been investigated. The aimed concept uses multiple constant supply voltages rather than a single time variant dynamic voltage supply. Several "parallel" PA devices are connected to different DC supply voltages while their RF output terminals are AC connected. The PA connected to a lower DC supply is used in the low power region, while the PA with a higher DC supply is used in the high power region. This ensures that the DC power consumption is output power dependent. By gradually controlling the amplifier branches through their RF inputs, proper transitions can be made between the different amplifier branches (i.e., from low power to high power region and vice versa)