What is the noise measure of a transistor and how does it help in LNA design?
Noise Measure Concept
The noise measure was introduced by Haus and Adler as a more meaningful comparison metric than minimum noise figure alone. Two transistors may have the same NFmin at a given frequency, but the one with higher gain will produce a lower noise figure in a multi-stage amplifier because its gain better suppresses the noise of subsequent stages. The noise measure captures both noise and gain in a single parameter.
| 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 |
Bias and Operating Point
For a cascade of n identical stages, each with noise factor F and available gain Ga: F_cascade = 1 + (F-1)(1 + 1/Ga + 1/Ga² + ... + 1/Ga^(n-1)). As n approaches infinity: F∞ = 1 + Mmin, where Mmin is the minimum noise measure of each stage. This shows that M determines the ultimate noise floor of any cascaded amplifier system built from a given transistor technology.
- 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
Stability Considerations
The noise measure is particularly useful for comparing transistor technologies at high frequencies (near fT) where the gain per stage is low. A technology with slightly higher NFmin but significantly higher gain can have a lower noise measure (and better cascade performance) than a technology with lower NFmin but lower gain.
Frequently Asked Questions
How is noise measure used in practice?
Compare transistor technologies at the operating frequency by plotting Mmin versus frequency. The technology with the lowest Mmin at your frequency provides the best noise performance for a multi-stage design. This is more meaningful than comparing NFmin alone because it accounts for the gain available.
Is M frequency-dependent?
Yes. M increases with frequency because the available gain decreases faster than the noise figure. Near fmax, the gain approaches 0 dB and M approaches infinity. The useful operating range is where M remains below a target value (typically < 1 dB).
How many stages should I use?
Enough that the cascade noise figure is within 0.5 dB of the single-stage NFmin. With 15 dB gain per stage, two stages achieve noise within 0.1 dB of the single-stage NFmin. With 10 dB gain per stage, three stages may be needed.