Power Amplifier Design Case Study: 2.4 GHz 10W PA Design and Implementation

Project Overview and Specifications

This comprehensive case study documents the complete design process of a 2.4 GHz, 10W power amplifier for WiFi 6 applications. The project demonstrates real-world RF design challenges and solutions, from initial specifications through final production validation. Our target application requires high efficiency (>40% PAE), excellent linearity for OFDM signals, and robust thermal performance in a compact form factor. This case study provides practical insights into modern PA design methodology, component selection, simulation techniques, and measurement validation that engineers can apply to their own projects.

Transistor Selection and Analysis

Device selection is arguably the most critical decision in PA design, directly impacting performance, cost, and design complexity. For our 2.4 GHz, 10W application, we evaluated GaN HEMT, LDMOS, and GaAs pHEMT technologies. GaN HEMTs offer superior power density and efficiency but at higher cost. LDMOS provides excellent cost-effectiveness for lower frequency applications. After detailed analysis considering breakdown voltage, current capability, thermal resistance, and cost, we selected the Qorvo TGF2023-SM GaN on SiC HEMT device. This device offers 15W capability at 2.45 GHz with excellent thermal performance (Rth = 8°C/W) and proven reliability in similar applications.

Large-Signal Device Modeling

Accurate device modeling is essential for successful PA design, particularly for efficiency optimization and linearity prediction. We began with the vendor-supplied nonlinear model for the TGF2023-SM, which includes both small-signal S-parameters and large-signal IV characteristics. Load-pull simulations at the target frequency and power level revealed optimal load impedances for maximum efficiency and output power. The simulation results showed peak PAE of 58% with ZL = 15 + j8 Ω at the current generator reference plane. Source-pull analysis indicated optimal source impedance of ZS = 5 - j3 Ω for maximum gain and stability.

Input Matching Network Design

The input matching network transforms the 50Ω system impedance to the optimal source impedance while providing DC bias insertion and stability. Our design employs a two-section L-C network using high-Q inductors and capacitors suitable for the power levels involved. The first section provides coarse impedance transformation from 50Ω to an intermediate impedance, while the second section fine-tunes the match to the optimal source impedance. Simulation showed input return loss better than -15 dB across the entire band with excellent stability margins (K > 1.5). A series RC network provides additional stability at low frequencies where the device may exhibit potential instability.

Output Matching Network Design

The output matching network is more challenging than the input due to higher power levels, harmonic content, and the need for harmonic suppression. Our design uses a multi-section approach combining fundamental matching with harmonic termination. The fundamental matching network transforms the optimal load impedance to 50Ω while the harmonic termination networks present high impedance to the 2nd and 3rd harmonics, improving efficiency. Simulation results showed output return loss better than -12 dB with harmonic suppression exceeding -30 dBc for the 2nd harmonic and -35 dBc for the 3rd harmonic.

Bias Network and Thermal Design

The bias network provides stable gate and drain voltages while isolating the RF circuits from the power supplies. Gate bias is set at -2.8V for Class AB operation, providing a compromise between efficiency and linearity. The drain bias of 28V is filtered using multiple stages of LC filtering to prevent power supply modulation and oscillation. Thermal design is critical for the 10W power level - our design uses a multi-layer PCB with extensive thermal vias and a large ground plane acting as a heat spreader. Thermal simulation predicted a junction temperature rise of 65°C above ambient with proper heat sinking.

PCB Layout and Implementation

High-frequency PCB layout requires careful attention to transmission line design, via placement, and thermal management. Our design uses a 4-layer stackup with Rogers RO4350B substrate for the RF layers and FR-4 for the inner layers. Microstrip transmission lines are used throughout with 50Ω characteristic impedance. Critical RF paths are kept as short as possible with minimal via usage. The ground plane provides both RF return current paths and thermal spreading. Component placement optimizes both RF performance and thermal management, with the power transistor centrally located for heat spreading.

Simulation Results and Optimization

Full electromagnetic simulation using Keysight ADS Momentum validated the design before fabrication. The simulation included all parasitic effects, package models, and PCB layout. Initial results showed excellent agreement with hand calculations, with small adjustments needed for optimal performance. Harmonic balance simulation predicted PAE of 45% at 10W output power with gain of 28.5 dB. Stability analysis confirmed unconditional stability (K > 1) across the entire frequency band. Yield analysis using Monte Carlo simulation showed 95% yield assuming ±5% component tolerances, validating the design robustness.

Fabrication and Assembly

PCB fabrication used standard processes compatible with the chosen materials and stackup. Component selection prioritized high-Q elements suitable for the power levels and frequency range. The GaN device requires careful handling due to ESD sensitivity, with proper grounding procedures during assembly. Reflow soldering profiles were optimized for the mixed-technology assembly including both ceramic and plastic components. Visual inspection and basic continuity testing verified assembly quality before RF testing. Initial yield was 92%, primarily limited by inductor tolerance variations affecting the matching networks.

Measurement Results and Validation

Comprehensive measurements validated the design performance against specifications. S-parameter measurements using a vector network analyzer confirmed input/output matching and gain performance. Large-signal measurements using signal generators and power meters characterized output power, efficiency, and compression behavior. Harmonic measurements using a spectrum analyzer verified harmonic suppression performance. Linearity measurements using modulated signals validated EVM performance for WiFi applications. All measured results met or exceeded the design specifications, with PAE reaching 42% at 10W output power.

Design Optimization and Iterations

Initial prototype measurements revealed several optimization opportunities. The input matching could be improved by adjusting L1 from 3.9 nH to 4.2 nH, improving input return loss by 2 dB. Output power could be increased by 0.5 dB by reducing the output matching inductor by 10%, though this slightly reduced efficiency. A second iteration incorporated these changes along with improved thermal via placement, resulting in 3°C lower operating temperature. The optimized design became the production baseline, demonstrating the iterative nature of RF design and the importance of hardware validation.

Reliability and Production Considerations

Long-term reliability testing included thermal cycling, power cycling, and high-temperature life testing. The GaN device showed excellent reliability with no failures after 1000 hours at 125°C junction temperature. Production considerations included component sourcing, test requirements, and yield optimization. Automated test equipment was developed for production screening, measuring key parameters in under 30 seconds per unit. Statistical process control monitored production yield and identified component tolerance sensitivities. Final production yield reached 96% with proper incoming component screening.

Cost Analysis and Market Positioning

Detailed cost analysis revealed the GaN device represents 45% of the total BOM cost, with passive components and PCB contributing 35% and 20% respectively. Cost reduction opportunities include alternative device selection for lower-power variants and PCB optimization for higher volume production. Competitive analysis showed our design offers superior efficiency and size compared to existing solutions, justifying a modest price premium. The final production cost of $7.20 in 10K volume met the target cost structure while delivering market-leading performance.

Future Enhancements and Roadmap

Several enhancement opportunities have been identified for future versions. Digital pre-distortion (DPD) could improve linearity for advanced modulation schemes. Envelope tracking could further improve efficiency for applications with high peak-to-average power ratios. Package integration using flip-chip or QFN packaging could reduce size and cost. Next-generation GaN devices promise higher efficiency and lower cost, enabling further optimization. These enhancements will be evaluated based on market requirements and cost-benefit analysis.