AM-PM and AM-AM Distortion Reduction Techniques for Fully-integrated Silicon-Germanium 5G Power Amplifiers

The fifth generation cellular network technology (5G) is expected to ramp up during 2020. To accommodate ever increasing bandwidth requirements, the 5G employs high spectral efficiency modulation schemes and carrier frequencies in the millimeter-wave frequency (mmWave), where large portions of unused spectrum are available. Since the path loss increases with the frequency, massive multiple-input multiple-output (MIMO) technologies will focus the RF energy onto the receiver using hundreds of antenna elements. Each antenna element is driven by a power amplifier (PA) which, being the last element in the transmit (TX) chain, dominates the efficiency and linearity performance of the whole transmitter. Moreover, higher order modulation schemes are more taxing in terms of linearity requirements both as to amplitude-to-amplitude (AM-AM) and to amplitude-to-phase (AM-PM) distortion. This thesis focuses on circuit solutions to obtain high linearity PAs suitable for high data rate mmWave applications. Several fully-integrated silicon- germanium (SiGe) PAs targeting 20 dBm output power are proposed. Starting from a class J design used as a reference, the main source of AM-PM distortion is demonstrated to be the BJT input impedance variation with the input signal amplitude. As a first solution to address the issue, a ladder filter based input matching network (IMN) is used to offer a 5-fold improvement on AM-PM distortion over a simple resonant IMN with minimum PAE penalty. To further reduce the impact of nonlinearities in the transmitted signal, another PA is proposed, combining the linearity of a CMOS predriver with the high power handling capability of a SiGe output stage. This PA features 1.5◦ AM-PM distortion at 1-dB compression point (P1dB ) with 20 % maximum PAE. Those solutions seem lead to a reduction in efficiency when continuous-wave measurements are considered. However, compared to the reference designs, less backoff is needed for the same amount of error vector magnitude (EVM) and thus, higher average efficiency can be achieved. Other techniques such as changing the bias point of the PA with the signal envelope or the Doherty amplifier can also be used to recover the lost efficiency.