What is the effect of feedback on the noise figure of a common source FET amplifier?
Feedback Effects on FET LNA Noise Figure
Understanding how different feedback types affect noise is critical for LNA design: the wrong feedback choice can easily double the amplifier's noise figure, while the right choice (inductive degeneration) can improve the system noise figure by enabling SNIM (simultaneous noise and impedance match).
| Parameter | Superheterodyne | Direct Conversion | Digital IF |
|---|---|---|---|
| Image Rejection | 60-90 dB (filter) | 30-50 dB (mismatch) | N/A (digital) |
| DC Offset | No issue | Major issue | No issue |
| LO Leakage | Low | High | Low |
| Integration | Difficult | Easy (single chip) | Moderate |
| Dynamic Range | 80-120 dB | 60-90 dB | 70-100 dB |
- 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
Frequently Asked Questions
Why does inductive degeneration not significantly degrade noise figure?
An ideal inductor is a lossless, noiseless element (no thermal noise). Adding L_s in the source does not introduce any new noise sources. The small NF penalty (0.05-0.2 dB) comes from: the slight shift of Gamma_opt away from the true NF_min source impedance, the reduction in gain (which slightly increases the Friis noise contribution of stage 2), and practical inductor losses (finite Q). The net effect is much smaller than the benefit of enabling impedance match at the input.
Can I use inductive degeneration at millimeter-wave frequencies?
Yes, but the required inductance values are very small (0.01-0.1 nH at 30-100 GHz). At these values, the inductance is often realized by the bond wire or via inductance connecting the FET source to ground, or by a short transmission line stub. MMIC designs use thin-film spiral inductors or shorted transmission line sections. Precise control of sub-0.1 nH inductance is challenging and requires careful EM simulation.
What is the optimal amount of feedback for noise figure?
For resistive shunt feedback: the minimum noise figure occurs with the smallest feedback resistor that still provides the required bandwidth and stability (larger R_f means less noise but less feedback effect). For inductive degeneration: the optimal L_s provides R_in = R_s (50 ohms), which simultaneously optimizes input match and noise. Too much inductance (R_in > R_s) degrades both noise and match. The optimal value is uniquely determined by the transistor's omega_T and C_gs.