RF Power Performance of an LDMOSFET on FM Broadcast Band II (88-108 MHz) Power Amplifier Applications

The demand for high-efficiency and thermally stable RF power amplifiers (PAs) within the FM Broadcast Band II (88–108 MHz) has led to the adoption of Laterally Diffused Metal Oxide Semiconductor Field-Effect Transistors (LDMOSFETs) due to their superior gain, ruggedness, and linearity. This study investigates the RF power performance of an NXP BLF188XR LDMOSFET device configured in a Class AB amplifier topology, optimized for 50 Ω load impedance. The design employed a two-stage matching network using microstrip lines and biasing circuits tuned for the FM broadcast frequency range. Experimental and simulated analyses were performed using Advanced Design System (ADS 2022) and Vector Network Analyzer (VNA) measurements. Results show that at a supply voltage of 50 V and drain current of 1.5 A, the amplifier achieved an output power (P_out) of 280 W at 98 MHz, corresponding to a drain efficiency (η_d) of 72.4 % and power gain (G_p) of 18.7 dB. Across the band, the amplifier maintained a gain flatness of ±0.4 dB and a return loss (S₁₁) below –18 dB, indicating excellent input matching. The power-added efficiency (PAE) averaged 68.9 %, while thermal analysis using ANSYS Fluent revealed a junction temperature rise of 24 °C above ambient under continuous-wave operation at 100 MHz. The measured and simulated data exhibited strong correlation with an average deviation of less than 3 %, validating the accuracy of the model. Compared to conventional bipolar and MOSFET RF devices, the LDMOSFET demonstrated higher efficiency, improved linearity, and enhanced thermal reliability, making it highly suitable for next-generation FM broadcast transmitters and digital audio broadcasting (DAB) infrastructures.