Why not use a standard Wilkinson combiner for unequal signals?

A standard Wilkinson combiner is designed to sum two signals of exactly equal amplitude and phase. When identical signals enter the two ports, the voltage across the 100-ohm isolation resistor is zero, so no current flows through it and no power is lost. However, if you feed a 10 Watt signal into port 1 and a 5 Watt signal into port 2, a voltage differential exists across the isolation resistor. Current flows through the resistor, dissipating RF power as heat. Not only does this reduce the total output power and degrade system efficiency, but the isolation resistor (often a tiny surface-mount component) will quickly overheat and burn out.

How does the compensated combiner solve this problem?

An amplitude-compensated combiner modifies the physical structure of the combiner to match the expected power ratio (K squared). Instead of both quarter-wave arms being 70.7 ohms, the arm carrying the higher power is designed with a lower characteristic impedance (wider trace), and the arm carrying the lower power is designed with a higher characteristic impedance (narrower trace). The isolation resistor value is also scaled. By adjusting these impedances based on the K squared ratio, the voltage at both ends of the isolation resistor is forced to be equal despite the unequal input powers. Because the voltage differential is zero, no current flows through the resistor, and the unequal signals are combined with 100 percent theoretical efficiency.

Where are amplitude-compensated combiners used?

They are primarily used in two applications: asymmetric Doherty power amplifiers and phased array antenna feeds. In an asymmetric Doherty PA, the peaking amplifier might be twice the size of the carrier amplifier (a 1:2 ratio). To combine their outputs efficiently at peak power, an amplitude-compensated combiner must be used. In phased arrays, designers often use amplitude tapering (e.g., Taylor or Chebyshev tapers) where the center elements transmit high power and the edge elements transmit low power to reduce side lobes. An amplitude-compensated corporate feed network routes the exact required power to each element without burning the difference as heat in attenuators or isolation resistors.

Passive Components

Amplitude-Compensated Combiner

An RF engineer designs an asymmetric Doherty amplifier where the peaking stage produces twice the power of the carrier stage (100W vs 50W). If they combine these outputs using a standard 50/50 Wilkinson combiner, a severe voltage differential occurs across the isolation resistor. RF energy flows through the resistor and turns into heat, destroying the resistor and ruining the amplifier's efficiency. The solution is an amplitude-compensated combiner. By calculating the power ratio (K² = 2) and adjusting the characteristic impedances of the two quarter-wave arms—making the high-power arm wider and the low-power arm narrower—the engineer equalizes the voltage at the isolation resistor. No current flows across it, no power is wasted, and the unequal signals are summed with near-perfect theoretical efficiency.

Category: Passive Components

Design Variable: K² (Power Ratio)

Key Application: Asymmetric Doherty PAs

Unequal Wilkinson Combiner Equations

For a compensated combiner splitting power by a ratio of K² (where P₂ / P₃ = K²), the physical impedances must be scaled relative to the system impedance (Z₀ = 50 Ω).

Component	Equation (Normalized to Z₀)	Example: 2:1 Ratio (K² = 2)
Arm 1 Impedance (Z₂)	Z₀ · √(K · (1 + K²))	Z₂ = 50 · √(1.414 · 3) = 103.0 Ω
Arm 2 Impedance (Z₃)	Z₀ · √((1 + K²) / K)	Z₃ = 50 · √(3 / 1.414) = 72.8 Ω
Isolation Resistor (R_iso)	Z₀ · (K + 1/K)	R_iso = 50 · (1.414 + 0.707) = 106.1 Ω
Output Matching (Z_out)	Usually requires a transformer	Must match to Z₀

Why standard combiners fail with unequal power:
P_loss = (V₁ − V₂)² / R_iso
If V₁ ≠ V₂, power is dissipated. The compensated combiner forces V₁ = V₂ by altering the transmission line transformation ratio on each arm.

Common Questions

Frequently Asked Questions

Why not use a standard Wilkinson?

Standard Wilkinson combiners assume equal amplitude. If fed with unequal signals, a voltage differential forms across the isolation resistor. Current flows through it, turning RF power into heat. This ruins efficiency and often burns out the resistor.

How does it compensate?

By scaling the characteristic impedance of the arms. The arm carrying more power is designed with a lower impedance (wider microstrip trace), and the arm carrying less power is designed with a higher impedance (narrower trace). This forces the voltages at the isolation resistor to be equal, meaning no current flows and no power is lost.

Where are they used?

Primarily in asymmetric Doherty amplifiers (where peaking and carrier stages have different power ratings) and in corporate feed networks for phased arrays utilizing amplitude tapering (where edge elements transmit less power than center elements to reduce sidelobes).

Related Terms

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Passive Design

Unequal Split Calculator

Enter your desired power division ratio (K-squared) and system impedance. Instantly calculate the required characteristic impedances for both quarter-wave arms and the optimal isolation resistor value.

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