What is the difference between input referred and output referred P1dB and IP3?
Input vs Output Referred Linearity
Understanding the difference between input-referred and output-referred linearity metrics is essential for correctly cascading component specifications in a system design.
| Parameter | Class A | Class AB | Class F/Doherty |
|---|---|---|---|
| Max Efficiency | 50% | 50-78% | 70-90% |
| Linearity | Excellent | Good | Moderate (needs DPD) |
| P1dB Backoff | 0-3 dB | 3-6 dB | 6-10 dB |
| Complexity | Low | Low | High |
| Common Use | Test, small signal | General PA | Base station, broadcast |
- 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
What is the typical IP3 to P1dB ratio?
For most amplifiers: IIP3 ≈ IP1dB + 9.6 dB (theoretically, for a memoryless polynomial nonlinearity). In practice: IIP3 = IP1dB + 8 to 12 dB (depends on the amplifier technology and topology). GaAs pHEMT: IIP3 ≈ IP1dB + 10-12 dB. GaN HEMT: IIP3 ≈ IP1dB + 8-10 dB (GaN has a harder compression characteristic). CMOS: IIP3 ≈ IP1dB + 9-11 dB.
Why do some datasheets only give one reference?
Convention varies by device type: LNA/receiver front end: IIP3 almost always (because engineers need it for the cascade formula). PA: OP1dB and OIP3 almost always (because the output power is the key specification). Mixer: both IIP3 and OIP3 are often given (because the conversion gain makes the relationship explicit). If only one is given: use the gain to convert to the other reference.
How do I handle variable-gain amplifiers?
For a variable-gain amplifier (VGA) or AGC: the IP3 and P1dB change with the gain setting. High gain setting: IIP3 degrades (the amplifier compresses earlier because the internal signal level is higher). Low gain setting: IIP3 improves. The OIP3 remains approximately constant (it tracks the output stage capability, which does not change with gain setting). This is why AGC is typically placed after the LNA: adjusting the gain of a stage after the first amplifier changes the IIP3 of that stage without affecting the first-stage NF.