Performance Improvement in Larger RF LDMOSFET Power Amplifiers

While larger transistors are used to deliver more power in RF transmitter power amplifiers, it has been observed that the performance per unit gate width decreases with increasing width. In this work, a major cause of this performance degradation in RF LDMOSFET power amplifiers is identified as the mutual inductance in the system, and a field solver and circuit simulator are used to quantify the mutual inductance. Circuit simulations show that the characteristics of devices with larger widths may be mimicked by models of smaller devices with increasingly larger inductances added to the terminals. In order to prevent the performance degradation caused by the mutual inductance, the use of baluns is proposed to apply phase-inverted signals to adjacent cells within the power amplifier device. Simulations comparing systems with and without baluns show that using baluns allows recovery of performance that is otherwise lost due to source degeneration caused by mutual inductance, as seen by the 7.8W output power with baluns as compared to 5.2W without for a W=64mm device at 900MHz.