How do I design a balanced LNA for improved input match and linearity?
Balanced Amplifier Architecture
The balanced amplifier is the standard topology for broadband, well-matched LNAs and gain blocks from 1 to 40 GHz. Its key advantage is decoupling the amplifier's impedance match from its intrinsic S-parameters, allowing the designer to optimize the individual LNA stages for noise or gain without worrying about input/output VSWR.
| 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 |
Frequently Asked Questions
When should I use a balanced LNA?
When broadband impedance match is critical: wideband receivers, test equipment, multi-octave systems. When driving a load with poor match: the balanced amplifier maintains good performance regardless of load VSWR. When graceful degradation is needed: if one LNA stage fails, the other continues to operate with 6 dB less gain.
What coupler should I use?
Lange couplers for multi-octave bandwidth (typically 1-20 GHz). Branch-line couplers for narrower bandwidth (10-20%). At mmWave frequencies: integrated coupled-line or Lange couplers on the MMIC. Below 1 GHz: balun transformers or lumped-element quadrature hybrids.
What about odd-mode oscillation?
If the two LNA stages couple through power supply or ground connections, they can oscillate in odd mode (opposite phases). Resistors between the gate bias feeds and between the drain bias feeds suppress odd-mode oscillation. Always include 50-100 Ω odd-mode stabilization resistors in balanced amplifier designs.