What is the difference between input VSWR and output VSWR on an amplifier datasheet?
Input and Output VSWR Explained
Every two-port RF component has both input and output impedance, characterized by S11 and S22 respectively. For amplifiers, these two parameters serve different roles and are optimized for different criteria.
| Parameter | L-Network | Pi/T-Network | Transmission Line |
|---|---|---|---|
| Bandwidth | Narrow (<10%) | Moderate (10-30%) | Broad (>30%) |
| Components | 2 (L, C) | 3 (L, C, C or C, L, C) | Stubs, lines |
| Q Control | Fixed by impedance ratio | Adjustable | Set by line length |
| Frequency Range | DC-6 GHz | DC-6 GHz | 1-100+ GHz |
| Design Complexity | Low | Medium | Medium-high |
- 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
Can the amplifier oscillate with high VSWR?
Yes. If the product of forward gain (S21) and reflected power exceeds unity due to poor output match and insufficient reverse isolation (S12), the amplifier can become conditionally unstable and oscillate. Check the stability factor (K > 1 and |Δ| < 1) under actual load conditions.
Which VSWR matters more for LNAs?
Input VSWR matters more because it affects the source impedance seen by the transistor, which impacts noise figure. However, the LNA input is often deliberately mismatched (noise match rather than conjugate match) to minimize NF, resulting in relatively poor input VSWR (8-12 dB RL).
How does a pad improve VSWR?
An attenuator pad between stages improves the effective VSWR at both interfaces because the pad absorbs reflected signals. A 3 dB pad improves the effective return loss by 6 dB (round trip through the pad). This is the simplest way to improve inter-stage VSWR at the cost of gain and noise figure.