Comparison of class A and class D RF amplifier operation with focus on reverse intermodulation and efficiency performance

A Current Mode Class D (CMCD) power amplifier at 900 MHz is presented. After a theoretical analysis, a class D amplifier is designed, simulated and afterwards assembled and characterized. Thereby, a maximum drain efficiency of 80.6%, and a maximum power added efficiency (PAE) of 72.8% at a switching frequency of 864 MHz, and an output power of 28.2 dBm were obtained. A linear class A amplifier using the same transistor exhibits a drain efficiency of 48.9% and 48.0% PAE at a comparable output power. An analysis of the reverse intermodulation distortion (IMD) of the switched-mode class D amplifier in comparison to the class A amplifier shows a 15dB improvement of the third order intermodulation ratio. To the authors' knowledge, this is the first practical verification of the superior reverse intermodulation performance of switched-mode amplifiers.