Research and Development of Doherty Power Amplifier based Broadband Highly Efficient and Linear Transmitter Front-end for 5G and millimeter wave Applications

Faculty Development Competitive Research Grant Program 2019-2021

The next generation wireless communication, such as 5G, relies on the premise of increased data rate with efficient spectrum usage. However, these requirements put a stringent constraint on the bandwidth, linearity, and efficiency of transmitter front-ends. RF power amplifier (RFPA) is one of the most important components in the front-end and it effectively controls the performance metrics of the transmitter. The RFPAs also affect the signal reception at the receiver part. In a designer’s perspective, the RFPA has direct effect on the transmitting range, signal fidelity, linearity, and robustness of the transmitter. Being the last stage of the transmitter chain, just prior to the antenna, the prime function of RFPAs is to boost the power level of the signal to be transmitted so that it doesn’t fade away or get lost in the wireless channel. To perform this operation successfully in an efficient manner, four key points to be kept in perspectives are a) the maximum limit up to which the signal can be boosted, b) the maximum gain that can be obtained, c) the efficiency of amplification (i.e. power loss as heat and dissipation etc.), d) the amount of distortion at the receiver end due to this amplification process.
Basically, the amplification process occurs via conversion of DC power into RF power. One important consideration is to determine how efficiently this conversion takes place, i.e. how much of the DC power gets converted into RF power. This is usually assessed through metrics called drain efficiency and power added efficiency (PAE). The drain efficiency is simply defined as the ratio of the amount of output RF power at the drain to the amount of input DC power. However, this definition is somewhat incomplete as it does not take into account the amount of input RF power, a very important parameter, which can affect the overall performance of the transmitter front-end. The PAE, on the other hand, gives a complete picture as it keeps into consideration the amount of input RF power as well. This is defined as the ratio of the effective amount of power added by the system to the amount of input DC power. Next, it is important to understand that the linearity is also a very important parameter for RFPAs. It determines how accurately the output signal resembles the input signal. This basically indicates the amount of distortion in the amplification process. Generally, the design process is a compromise between the linearity and efficiency. It is well established that high efficiency RFPAs have more non-linear behavior and vice versa. Increasing the balance point between high efficiency and high linearity is an essential part of current research.