A CMOS IQ Digital Doherty Transmitter using modulated tuning capacitors

This paper presents a new approach to increase the output power and to enhance the drain efficiency of Direct Digital RF Modulators (DDRM). Two differential four-phase DDRMs are organized in a Doherty-like configuration using two different transformers. The modulated tuning capacitors concept is proposed to achieve a high efficiency at maximum output power and at back-off. To demonstrate this principle, a 2 GHz IQ Digital Doherty Transmitter with on-chip transformers has been integrated in 90 nm CMOS Technology. The digital IQ transmitter achieves a maximum output power of 24.8 dBm with 26% drain efficiency and 26% drain efficiency at 6 dB back-off. With a 10 MHz RFBW multi-tone OFDM signal, the transmitter consumes 176 mA from a 2.4 V supply. It achieves 18.8 dBm RMS output power with 18% average drain efficiency.