Load-Pull PA Design Methodology

What Is Load-Pull?

Load-pull is a large-signal measurement technique where the PA's load impedance is systematically varied while measuring output power, PAE (Power-Added Efficiency), and/or linearity. This maps contours of constant performance on the Smith chart, revealing the optimum load impedance for maximum power or efficiency.

Load-Pull Contour Data

  At each load Γ_L: measure P_out (dBm)
  Repeat for many Γ_L values → power contours on Smith chart
  Power contour: locus of Γ_L giving P_out = P_max − ΔdB
  
  Example GaN PA (10W, 3.5 GHz):
  Γ_L for P_max=40.5 dBm: |Γ_L|=0.78 ∠−70°
  → Z_L = 50×(1+Γ_L)/(1−Γ_L) = 5.2 + j4.8 Ω (optimum load)

  3 dB power contour: slightly larger circle around optimum point
  1 dB PAE contour: often slightly shifted toward lower |Γ_L|

Designing Output Matching from Load-Pull Data

  Target: transform 50Ω system → Z_L_opt = 5.2 + j4.8 Ω
  (matching network presents Z_L_opt to PA, 50Ω to antenna)

  Step 1: Load load-pull data Z_L_opt as target in RF View
  Step 2: Auto Match → synthesizes network from 50Ω to 5.2+j4.8Ω
    Series C to cancel +j4.8Ω at 3.5 GHz: C = 9.5 pF
    Transform 5.2→50Ω: L-network Q=√(50/5.2−1)=3.07
    Shunt C: X=50/3.07=16.3Ω → C=2.78pF
    Series L: X=3.07×5.2=15.9Ω → L=0.72nH

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