What is the role of source inductance in LNA noise matching?
Source Degeneration for LNA
In a common-source (or common-emitter) LNA without degeneration, the input impedance is predominantly capacitive (gate-source capacitance) while the optimum noise impedance has a significant real part. This means the matching network cannot simultaneously present the optimum noise impedance and the conjugate match impedance to the transistor, forcing a compromise between NF and gain.
| Parameter | LNA | Driver | Power Amplifier |
|---|---|---|---|
| Noise Figure | 0.3-2.0 dB | 3-8 dB | 5-15 dB (not specified) |
| Gain | 10-25 dB | 10-20 dB | 8-15 dB |
| P1dB | -10 to +10 dBm | +15 to +25 dBm | +30 to +50 dBm |
| OIP3 | +5 to +25 dBm | +25 to +40 dBm | +40 to +55 dBm |
| DC Power | 10-100 mW | 0.5-5 W | 5-500 W |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
- Margin allocation: include sufficient design margin to account for manufacturing tolerances and aging effects
Frequently Asked Questions
How much inductance do I need?
Typically 0.1-0.5 nH at 1-5 GHz. The required value depends on the transistor's gm and Cgs. A rough estimate: Ls ≈ 50Ω × Cgs/gm. At higher frequencies, the required Ls decreases. The value is optimized by simulation using the transistor's noise parameters.
How do I implement sub-nH inductance?
Short wire bonds (0.5-1 nH/mm), via holes through a ground plane (0.1-0.3 nH per via), or a short microstrip trace to ground. In MMIC designs, the source via inductance itself provides the degeneration. In hybrid designs, bond wire length controls the inductance.
Does source degeneration affect linearity?
Yes, positively. Source degeneration improves IIP3 by 3-6 dB because the feedback linearizes the transconductance. This is a secondary benefit of the technique. The improved linearity comes at the cost of reduced gain, which is the same tradeoff as any negative feedback.