Distributed Active Transformer (DAT): A New Power Combining and Impedance Transformation Technique
Ichiro Aoki (aoki@caltech.edu)

Joint project with Caltech MMIC Group

One of the most competitive and challenging areas in RF circuit design today is providing a way to substitute Si circuits for conventional GaAs ones. The GaAs based MMICs (monolithic microwave integrated circuit) have achieved good performances for more than a decade, becoming the main mature technology for many applications, from cellular phones to satellite communications. GaAs, however, is not cost competitive when compared to Si technology which is driven by mass-produced digital circuits. To be able to successfully substitute CMOS compatible Si for conventional GaAs technology, several technical challenges must still to be overcome. The most significant of these is the conductive substrate, which causes cross-talk, causes on-chip inductors and transmission lines to have extremely high loss. Also Si processes lack a low-loss, low-inductance ground plane. Since CMOS transistor breakdown voltages are very low, these factors make the needed power-combining and/or impedance matching very difficult to achieve. 

The novel distributed active transformer (DAT) power amplifier addresses these issues. By its circular-symmetric geometry and push-pull topology, it generates virtual ac grounds for every Vdd and GND connection, uses low loss slab inductors for drain and gate matching, and provides harmonic control using cross-connected capacitors. The active-transformer configuration is used for a series power combining while taking advantage of the circular push-pull geometry. This single-stage 0.35 µm CMOS class-E/F design successfully achieves 2.2-W output power at 2.4GHz with 31% PAE (41% with balanced output) and 9dB of gain. The supply voltage is 2V, and the circuit utilizes a die area of 2mm x 1.3mm with a total gate length of 73mm. Input and output impedances are 50 Ohms and the measured bandwidth is 540Mhz. This power amplifier is the first reported truly fully integrated power amplifier fabricated using CMOS technology and operating at GHz frequency range with watt-level output power.