Multiband and multimode digital polar CMOS RF power amplifiers

Abstract

Digitally modulated CMOS power amplifiers (DPAs) for multiband and multimode RF polar transmitters are researched.

Firstly, a digital polar CMOS power amplifier (DPA) with a 102-dB power dynamic range using a digitally controlled bias generator is proposed. The proposed DPA achieves a wide output power range and digital transmit-power control (TPC) range of a DPA to sup-port multimode operation without additional components such as a variable gain amplifier (VGA) and a variable attenuator (VATT). To improve the local oscillator (LO) leakage, which limits the minimum output power of the DPA, the unit amplifier cell of the DPA em-ploys a balanced mixer-type LO canceller at the power stage and a virtual ground is intro-duced in the layout of the output-power combining networks. A high dynamic range of the output power is simply achieved by adjusting gate-bias voltage of a cascode transistor in each power amplifier cell using a digitally controlled bias generator. This architecture allows a few-mA drain current at a low Tx power level without degrading the linearity. The peak output power is 24.4 dBm with an overall efficiency of 43% at 800 MHz. The output power range is 102.8 dB. The DPA has linear output power of 22 dBm with an average efficiency of 35% for WCDMA using a simple static pre-distortion scheme. The Tx power control range is 49.7 dB with only digital TPC methods.

Secondly, a 0.7 GHz - 1.9 GHz dual band digital polar CMOS power amplifier with over 100-dB power dynamic range using an on-chip reconfigurable matching network is proposed. The proposed DPA for a dual band transmitter is implemented integrating the high dynamic range DPA core with a transmission line transformer (TLT)-based reconfigu-rable output matching network in 65nm CMOS. This DPA supports the frequency range of 0.75-1.9GHz by purely digital controls, ACW and BCW, without extra components such as a wideband VGA and a wideband VATT, while it provides power dynamic range of over 100dB at each frequency band. It delivers the power dynamic ranges of 105.5 and 100.5 dB, providing the 69.8 and 68.7 dB digital transmit power control ranges at 0.9 and 1.7GHz, respectively for WCDMA. As a result, the proposed DPA includes the functions of the conventional Tx blocks including a wideband PA with a driver-stage, baseband DACs, mixers, a wideband RF VGA for the wideband Tx system. Therefore, the complexity, area, and power consumption of the wideband Tx system can be substantially reduced by using this simple DPA architecture.

Finally, a triple power mode high dynamic range DPA for back-off efficiency enhance-ment is proposed. The proposed DPA for back-off efficiency enhancement is implemented integrating the high dynamic range DPA core with the switched transformer. Using the switched transformer with additional reduced duty ratio of square wave LO input signal at back-off power, triple-power-mode for efficiency improvement is accomplished. The PAE improvement of 14.2 % is achieved from 14.5 % to 28.7 % at 6.5 dB back-off output power from the peak output power of the DPA using the low power mode (LPM). The LPM of the DPA offers a 97% relative improvement in efficiency. The digital TPC range of 68.2 dB for WCDMA is accomplished with the sufficiently large margin.