A Digitally Controlled Analog Predistortion Linearizer for the Correction of Deep and Shallow Class-AB RF Power Amplifiers

This article presents a digitally controlled dual-branch dual-transistor analog predistortion (APD) for the linearization of RF power amplifiers (PAs). This APD is capable of complementing the gain expansion and compression of deep class-AB PAs (also known as “gain peaking”). This APD is based on a dual-branch architecture with a main and peaking amplifier connected to an isolated combiner. Bipolar transistors and emitter degeneration techniques are adopted to match the APD profile to the PA gain and phase trajectory. Amplifier stability is analyzed over the APD tuning range, and the design parameters are optimized to achieve optimum fit with PA trajectories over the bandwidth. The APD is first evaluated in dual-input mode for the augmented testing range. Using Gaussian pulses, the APD optimal settings are found for a variety of frequencies and powers. Measured results demonstrate the ability of the APD to compensate for a wide range of biases with an independent amplitude and phase control. When combined with a PA with low bias current, a gain ripple of ±0.3 dB and phase variation of ±3° are observed. The APD is then converted to a single-input/single-output block for testing in a realistic base-station environment. Using the standard-compliant waveform analysis software, the APD is demonstrated with 5G downlink 100-MHz signals between 1.8 and 2.2 GHz, attaining an adjacent channel leakage ratio (ACLR) of less than −46.1/−46.6 dBc, an error vector magnitude (EVM) of 0.8%, and a remarkably low power consumption of only 18 mW.