A 500-MHz Bandwidth 7.5-mV_pp Ripple Power-Amplifier Supply Modulator for RF Polar Transmitters

The parallel combination of a switching and a linear amplifier in the supply modulator for RF power amplifiers (PAs) has the potential to enhance energy efficiency while achieving wider bandwidth and lower ripple output voltage. In this paper, a linear amplifier that features a buffered-switching Class-AB bias scheme is presented for the supply modulator in polartransmitter structures achieving 500 MHz of small-signal 3-dB bandwidth at a 1.2-V supply. The linear amplifier absorbs and cancels up to 60 mA of ripple current from the switching amplifier. As such, the ripple in the output voltage of the hybrid linear-switching supply modulator is less than 7.5 mVpp. The switching amplifier provides most of the signal current for greatest efficiency owing to a proposed rail-to-rail currentsensing circuit. Current feedback in the switching amplifier achieves 1.68-MHz unity-gain bandwidth at 6-MHz switching frequency. Harmonic distortion in the output voltage of the supply modulator is below 40 dBc at 0.8 Vpp sinusoidal input up to 9 MHz. The peak efficiency is 87.7% for a 8.25-Ω load, while the maximum output power is 23.6 dBm for a 4.99-Ω load. The chip measures 1.35 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> in a 65-nm standard bulk CMOS process.