High linearity lead-lag style envelope modulator for RF power amplifiers

Abstract

A new split frequency envelope modulator for envelope tracking radio-frequency power amplifiers is proposed based on a lead-lag network. By mathematically deriving the transfer functions of the lead-lag modulator and the conventional split frequency type, the lead-lag is shown to have a significantly flatter phase response. The frequency response of the two modulators is verified by simulation, where the phase transient of the lead-lag is significantly less than the 360° of the conventional type. They are further simulated with a 3 MHz bandwidth 3GPP long-term evolution (LTE) signal and the lead-lag shown to reduce the modulator's normalized root-mean-square error (NRMSE) from −27.3 to −39.2 dB. A practical demonstrator was developed around an existing high-efficiency modulator architecture. To maintain system efficiency synthetic impedance was incorporated in the low-frequency switched mode power supply (SMPS) path. This was achieved with voltage and current feedback around the SMPS. The dynamic wideband signal response was investigated by applying a 3 MHz LTE envelope signal to the modulator and comparing the input and output signals. The measured NRMSE was improved from −27.5 to −30.0 dB by adopting the lead-lag structure and the dynamic frequency response verifies correct operation.