A 105db-gain 500mhz-bandwidth 0.1Ω-output-impedance amplifier for an amplitude modulator in 65nm cmos

A 500 MHz -3 dB bandwidth linear amplifier in 65 nm CMOS at 1.2 V supply is presented for an amplitude modulator in a polar transmitter. The design uses a gain-boosting scheme for 105 dB DC gain at an 8 ¿ load and a buffered switching Class-AB bias scheme. It has 0.1 ¿ output impedance at 5 MHz and can drive 60 mA peak current. The chip efficiency is 83.5% and area is 1.35 mm².

Polar modulation, where a constant-amplitude phase signal is amplified by an efficient switched-mode power amplifier (PA) and an envelope signal is modulated at the drain of the PA, has been proven to improve efficiency and linearity of the PA system. However, the amplitude modulator (AM) used in such modulation systems must have wide bandwidth, low switching ripple and high efficiency. Although several AM structures have been reported the hybrid switching amplitude modulator (HSAM) shows excellent performance [1]–[3]. The HSAM consists of a linear amplifier as a voltage source and a switching amplifier as a dependant current source. The linear amplifier using voltage feedback defines output voltage with high frequency information in a bandwidth-expanded envelope signal and the switching amplifier using current feedback drives almost all the current to develop output voltage with the help of a current sensing circuit. If the current sensing circuit does not work well, it makes an offset to the switching amplifier and results in additional power consumption of the linear amplifier [2]. To meet a stringent spectral mask, low output impedance of the linear amplifier is needed [1]. In conclusion, wide bandwidth and low output impedance of the linear amplifier, and accurate current sensing are critical points in the design of the HSAM.