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. Our target application requires high efficiency (>40% PAE), excellent linearity for OFDM signals, and robust thermal performance in a compact form factor.
Design Specifications
- • Frequency: 2.4-2.5 GHz
- • Output Power: 10W (40 dBm)
- • Gain: 28 ± 1 dB
- • PAE: >40% @ P1dB
- • P1dB: >39 dBm
- • Supply Voltage: 28V
- • EVM: <-25 dB (64-QAM)
- • Operating Temp: -40°C to +85°C
- • Size: 15 × 10 mm maximum
- • Cost Target: <$8 in 10K volume
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.
Device Comparison Matrix
| Technology | Power Density | Efficiency | Cost |
|---|---|---|---|
| GaN HEMT | 5-8 W/mm | 50-65% | $$$$ |
| LDMOS | 1-2 W/mm | 40-55% | $$ |
| GaAs pHEMT | 0.5-1 W/mm | 35-50% | $$$ |
Selected: Qorvo TGF2023-SM GaN on SiC HEMT - 15W capability with excellent thermal performance (Rth = 8°C/W)
Large-Signal Device Modeling
Accurate device modeling is essential for successful PA design, particularly for efficiency optimization and linearity prediction. Load-pull simulations at the target frequency and power level revealed optimal load impedances.
Optimal Impedances (Current Generator Reference)
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.
Input Network Component Values
Primary Matching:
- • L1: 3.9 nH (series)
- • C1: 1.8 pF (shunt)
- • L2: 2.2 nH (series)
- • C2: 0.8 pF (shunt)
Stability & Bias:
- • R_stab: 10 Ω (series)
- • C_stab: 100 pF (series)
- • L_bias: 100 nH (RF choke)
- • C_bias: 1000 pF (bypass)
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.
- Output return loss better than -12 dB across the band
- 2nd harmonic suppression exceeding -30 dBc
- 3rd harmonic suppression exceeding -35 dBc
Bias Network and Thermal Design
Thermal design is critical for the 10W power level. Gate bias is set at -2.8V for Class AB operation, providing a compromise between efficiency and linearity.
Thermal Management
- Multi-layer PCB with extensive thermal vias
- Large ground plane as heat spreader
- Predicted junction temperature rise: 65°C above ambient
PCB Layout and Implementation
High-frequency PCB layout requires careful attention to transmission line design, via placement, and thermal management.
PCB Design Specifications
- • Layer 1: RO4350B (0.1mm)
- • Layer 2: FR-4 Ground (0.1mm)
- • Layer 3: FR-4 Power (0.1mm)
- • Layer 4: RO4350B (0.1mm)
- • Total thickness: 0.8mm
- • 2oz copper on all layers
- • 0.2mm thermal vias (48 total)
- • Large ground plane heat spreader
- • Thermal pad 5×5mm
- • Rth(pcb): 15°C/W
Measurement Results and Validation
Comprehensive measurements validated the design performance against specifications. All measured results met or exceeded the design specifications.
Measured vs. Simulated Results
| Parameter | Spec | Simulated | Measured |
|---|---|---|---|
| Gain @ 2.45 GHz | 28 ± 1 dB | 28.5 dB | 28.2 dB ✓ |
| PAE @ 10W | >40% | 45% | 42% ✓ |
| P1dB | >39 dBm | 39.8 dBm | 39.5 dBm ✓ |
| 2nd Harmonic | <-30 dBc | -32 dBc | -31 dBc ✓ |
Design Lessons Learned
Key Challenges & Solutions
Challenge: Thermal management
- • Solution: Multi-layer thermal via array and large ground plane
- • Impact: Reduced junction temperature by 15°C
Challenge: Component tolerance sensitivity
- • Solution: Broader matching bandwidth and component screening
- • Impact: Improved yield from 85% to 96%
Challenge: EMI/harmonic compliance
- • Solution: Enhanced harmonic filtering and shielding
- • Impact: Passed EMC testing with 10 dB margin
Key Design Insights
- Device selection is critical - consider power density, efficiency, and cost
- Load-pull analysis is essential for optimizing efficiency and output power
- Thermal management becomes dominant at power levels above 5W
- Component tolerances significantly impact yield - design for robustness
- Harmonic suppression requires dedicated filtering networks
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