Noise Match
Understanding Noise Match
Every transistor has an optimum source impedance for minimum noise figure, specified by the manufacturer as Gamma_opt (or Y_opt). This impedance is NOT the conjugate of S11; it is determined by the internal noise sources of the device. LNA design is fundamentally about presenting this optimum impedance while achieving acceptable gain and stability.
Noise Parameters
- NF_min: Minimum achievable noise figure when Gamma_opt is presented.
- Gamma_opt: Optimum source reflection coefficient for NF_min.
- R_n: Noise resistance. Sensitivity of NF to source impedance deviation from Gamma_opt.
Design Trade-offs
- Noise match: Present Gamma_opt. Achieves NF_min but not maximum gain. May not be well-matched (S11).
- Gain match: Present conjugate of S11. Maximum gain but NF higher than NF_min.
- Compromise: Choose a source impedance between Gamma_opt and conjugate-S11 for acceptable NF and gain.
Frequently Asked Questions
What is noise match?
Noise match is presenting the optimum source impedance (Gamma_opt) to an amplifier for minimum noise figure. This impedance differs from the conjugate match for maximum gain. LNA design involves optimizing the source impedance between noise and gain objectives.
Why is noise match different from impedance match?
The optimum impedance for minimum noise depends on the internal noise sources of the transistor, which are independent of the S-parameters. The gain match (conjugate of S11) maximizes power transfer; the noise match minimizes amplifier noise contribution.
What is noise resistance?
Noise resistance R_n determines how sensitive the noise figure is to deviations from Gamma_opt. Low R_n means NF is insensitive to source impedance (forgiving). High R_n means even small deviations from Gamma_opt significantly increase NF.