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Low-Noise Amplifier

Low-Noise Amplifier. RF Receiver. Antenna. BPF1. LNA. BPF2. Mixer. BPF3. IF Amp. Demodulator. RF front end. LO. Low-Noise Amplifier. First gain stage in receiver Amplify weak signal Significant impact on noise performance Dominate input-referred noise of front end

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Low-Noise Amplifier

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  1. Low-Noise Amplifier

  2. RF Receiver Antenna BPF1 LNA BPF2 Mixer BPF3 IF Amp Demodulator RF front end LO

  3. Low-Noise Amplifier • First gain stage in receiver • Amplify weak signal • Significant impact on noise performance • Dominate input-referred noise of front end • Impedance matching • Efficient power transfer • Better noise performance • Stable circuit

  4. LNA Design Consideration • Noise performance • Power transfer • Impedance matching • Power consumption • Bandwidth • Stability • Linearity

  5. Noise Figure • Definition • As a function of device G: Power gain of the device

  6. NF of Cascaded Stages Sin/Nin Sout/Nout G1, N1, NF1 Gi, Ni, NFi GK, NK, NFK • Overall NF dominated by NF1 [1] F. Friis, “Noise Figure of Radio Receivers,” Proc. IRE, Vol. 32, pp.419-422, July 1944.

  7. Simple Model of Noise in MOSFET • Flicker noise • Dominant at low frequency • Thermal noise • g: empirical constant 2/3 for long channel much larger for short channel • PMOS has less thermal noise • Input-inferred noise Vg Id Vi

  8. Noise Approximation Noise spectral density 1/f noise Thermal noise dominant Thermal noise Frequency Band of interest

  9. Power Transfer and Impedance Matching • Power delivered to load • Maxim available power Rs jXs jXL Vs I V RL • Impedance matching • Load and source impedances conjugate pair • Real part matched to 50 ohm

  10. Available Power Equal power on load and source resistors

  11. Reflection Coefficient Rs jXs jXL Vs I V RL

  12. Reflection Coefficient No reflectionMaximum power transfer

  13. S-Parameters • Parameters for two-port system analysis • Suitable for distributive elements • Inputs and outputs expressed in powers • Transmission coefficients • Reflection coefficients

  14. S-Parameters a1 b2 S21 S11 S22 S12 b1 a2

  15. S-Parameters • S11 – input reflection coefficient with the output matched • S21 – forward transmission gain or loss • S12 – reverse transmission or isolation • S22 – output reflection coefficient with the input matched

  16. S-Parameters I1 I2 S Z1 Z2 Vs1 V1 V2 Vs2

  17. Stability Condition • Necessary condition where • Stable iff where

  18. A First LNA Example • Assume • No flicker noise • ro = infinity • Cgd = 0 • Reasonable for appropriate bandwidth • Effective transconductance io Rs Vs Rs 4kTRs Vs Vgs gmVgs 4kTggm

  19. Power Gain • Voltage input • Current output

  20. Noise Figure Calculation • Power ratio @ output • Device noise + input-induced noise • Input-induced noise

  21. Device iout iin Unity Current Gain Frequency Ai fT 0dB f frequency

  22. Small-Signal Model of MOSFET • Cgs • Cgd • rds • Cdb • Rg: Gate resistance • ri: Channel charging resistance i2 i1 V1 V2 i1 i2 Rg Cgd Cdb Cgs V’gs V2 rds V1 ri gmV’gs

  23. i1 i2 Rg Cgd Cgs V’gs gmV’gs V1 ri wT Calculation i1 i2 Rg Cgd Cdb Cgs V’gs gmV’gs rds V1 ri

  24. wT of NMOS and PMOS • 0.25um CMOS Process* Set: Solve for wT [2] Tajinder Manku, “Microwave CMOS - Device Physics and Design,” IEEE J. Solid-State Circuits, vol. 34, pp. 277 - 285, March 1999.

  25. Noise Performance • Low frequency • Rsgm >> g ~ 1 • gm >> 1/50 @ Rs = 50 ohm • Power consuming • CMOS technology • gm/ID lower than other tech • wT lower than other tech

  26. Review of First Example • No impedance matching • Capacitive input impedance • Output not matched • Power transfer • S11=(1-sRCgs)/(1+sRCgs) • S21=2Rgm/(1+sRCgs), R=Rs=RL • Power consumption • High power for NF • High power for S21

  27. Impedance Matching for LNA • Resistive termination • Series-shunt feedback • Common-gate connection • Inductor degeneration

  28. Resistive Termination io Rs 4kTggm 4kT/Rs 4kT/RI Vs RI Is Rs RI Vgs gmVgs • Current-current power gain • Noise figure

  29. Comparison with Previous Example • Previous example • Resistive-termination Introduced by input resistance Signal attenuated

  30. Summary - Resistive Termination • Noise performance • Low-frequency approximation • Input matched Rs = RI = R • Broadband input match • Attenuate signal • Introduce noise due to RI • NF > 3 dB (best case)

  31. Series-Shunt Feedback RF • Broadband matching • Could be noisy RL Rs Vs Ra iout Rs RF RL Cgs Vgs gmVgs Vs Ra

  32. Common-Gate Structure 4kTggm RL Rs RL Rs 4kTRs gmVgs Vs Vgs RL Rs 4kTRs gm Vs Vgs gmVgs 4kTggm

  33. Input Impedance of CG Structure • Input impedance Yin=gm+sCgs • Input-impedance matching • Low frequency approximation • Direct without passive components 1/gm=Rs=50 ohm

  34. Noise Performance of CG Structure Signal attenuated

  35. Power Transfer of CG Structure • Rs = RL = R = 50 ohm • S11=0, S21=1 @ Low frequency

  36. Summary – CG Structure • Noise performance • No extra resistive noise source • Independent of power consumption • Impedance matching • Broadband input matching • No passive components • Power consumption • gm=1/50 • Power transfer • Independent of power consumption

  37. Inductor Degeneration Structure Zin iout Rs Lg Rs Lg iin Cgs Vgs gmVgs Vs Vin Ls Vs Ls Zin

  38. Input Matching for ID Structure Zin • Zin=Rs • IM{Zin}=0 • RE{Zin}=Rs iout Rs Ls Lg Cgs Vgs gmVgs gmLs/Cgs Vs

  39. Effective Transconductance Zin iout Rs Ls Lg Cgs Vgs gmVgs gmLs/Cgs Vs

  40. Noise Factor of ID Structure • Calculate NF at w0 = 0 @ w0

  41. Input Quality Factor of ID Structure I R L C V Rs Ls Lg Cgs gmLs/Cgs Vs

  42. Noise Factor of ID Structure • Increase power transfer gmLs/Cgs = Rs • Decrease NF gmLs/Cgs = 0 • Conflict between • Power transfer • Noise performance

  43. Further Discussion on NF • Frequency @ w0 w2 ~= 1/Cgs/(Lg+Ls) • Input impedance matched to Rs RsCgs=gmLs • Suitable for hand calculation and design • Large Lg and small Ls

  44. Power Transfer of ID Structure • Rs = RL = R = 50 ohm • @

  45. Computing Av without S-Para Rs Lg Vs Ls

  46. Power Consumption

  47. Power Consumption • Technology constant • L: minimum feature size • m: mobility, avoid mobility saturation region • Standard specification • Rs: source impedance • w0: carrier frequency • Circuit parameter • Lg, Ls: gate and source degeneration inductance

  48. Summary of ID Structure • Noise performance • No resistive noise source • Large Lg • Impedance matching • Matched at carrier frequency • Applicable to wideband application, S11<-10dB • Power transfer • Narrowband • Increase with gm • Power consumption • Large Lg

  49. Cascode • Isolation to improve S12 @ high frequency • Small range at Vd1 • Reduced feedback effect of Cgd • Improve noise performance LL Vo Vbias M2 Vd1 Rs Lg M1 Vs Ls

  50. Lg Rs Vo Cgs Vgs gmVgs Vs Ls LL LL Vo Lg Rs M1 Vs Ls

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