A Wideband Supply Modulator for 20 MHz RF Bandwidth Polar PAs in 65 nm CMOS

Polar modulated RF amplifiers have the potential to enhance efficiency while achieving sufficient linearity for a signal having non-constant envelope. However, switching modulators used in such architectures to generate the envelope signal are difficult to implement because of the high bandwidth and low switching ripple requirement, while achieving high efficiency even at large power back-off. This paper presents a wideband supply modulator for a 20 MHz RF bandwidth polar modulated PA. Realized in 65 nm CMOS, it consists of a cascoded nested Miller compensated linear amplifier and a class D switching amplifier. The class-AB linear amplifier with cascoded Miller compensation in the modulator reduces the output switching ripple to 4.3 mVrms at a switching frequency of 118 MHz. With an optimized design the modulator achieves a maximum efficiency of 87.5% and a small signal $-$ 3 dB bandwidth of 285 MHz. At 10 dB back-off the efficiency is more than 40%. The modulator can deliver 22.7 dBm output power to a 5.3 $\Omega$ load. It satisfies the linearity requirement of IEEE 802.11g WLAN signal when tested in a real polar modulated amplifier.