Power, Linearity, and Distortion Practical Power Topics Informational

How do I design a power combiner for combining the output of multiple power amplifiers?

Designing a power combiner for combining the output of multiple power amplifiers creates a single higher-power output from N lower-power PA modules, overcoming the output power limitation of individual transistors or MMICs. The combiner must: maintain impedance matching to each PA (so all PAs see 50 ohms at their output), provide isolation between PAs (so failure of one PA does not affect the others or reflect power back into the remaining PAs), and combine the signals in-phase (constructive addition requires all PA outputs to be phase-matched). The combiner types are: Wilkinson combiner (a quarter-wave transmission line combiner that provides matched, isolated, and in-phase combining; 2-way Wilkinson: each PA sees 50 ohms, isolation between ports is greater than 20 dB, combining loss is ideally 0 dB (in practice 0.1-0.3 dB per stage); N-way Wilkinson combiners can be constructed by cascading 2-way stages (for 4-way: two levels of 2-way Wilkinson stages)), balanced combiner (quadrature hybrid or Lange coupler: provides combining with inherent impedance match and isolation; suitable for broadband combining), radial combiner (a cylindrical structure where N transmission lines feed into a single output through a radial waveguide; provides very low-loss combining for 4-32 PAs; used in: high-power military transmitters and satellite HPAs), and spatial (quasi-optical) combiner (the PA outputs are radiated from antenna elements and combined spatially in free space; eliminates the combiner's physical loss; used for mmW power combining where transmission line losses are prohibitive).

Category: Power, Linearity, and Distortion

Updated: April 2026

Product Tie-In: Power Amplifiers, Combiners, Loads

RF Power Combiner Design

Power combining is the standard technique for generating RF power levels beyond what a single transistor or MMIC can produce. The combiner's efficiency directly determines the system's overall power efficiency.

Efficiency Considerations

Combiner loss: Each combining stage has insertion loss of 0.1-0.5 dB (depending on the combiner type and frequency). For a 16-way combiner using 4 levels of 2-way Wilkinson: total loss = 4 × 0.2 = 0.8 dB. This 0.8 dB loss means 17% of the combined power is wasted as heat in the combiner
Graceful degradation: When one PA fails (open or short circuit): the combiner should minimize the impact on the remaining working PAs. A Wilkinson combiner with good isolation: a failed PA reduces the output power by 1/N (for N-way) and the reflected power from the failed port is absorbed by the isolation resistors, protecting the other PAs

Power Combiner Parameters

Combined power: P_out = N × P_PA - L_combiner [linear] or:
P_out [dBm] = P_PA + 10log₁₀(N) - L_combiner [dB]
For 8 PAs at 10W each, L=1dB: P_out = 8×10 × 10^(-0.1) = 63.5 W
Efficiency: η = P_out / (N × P_PA) = 10^(-L/10)
For L=1dB: η = 79.4%
Failed PA impact: P_out = (N-1)²/N × P_PA (with good isolation)

Common Questions

Frequently Asked Questions

How many PAs can I combine?

The practical limit depends on the combiner loss per stage: for Wilkinson (0.2 dB per stage): 2-way: 0.2 dB loss. 4-way: 0.4 dB. 8-way: 0.6 dB. 16-way: 0.8 dB. 32-way: 1.0 dB. Beyond 16-32 PA combining: the combiner loss becomes significant relative to the power gained by adding more PAs. At 64-way: the combiner loss is approximately 1.2 dB, meaning 24% of the power is wasted. For N greater than 16-32: consider a radial combiner (0.3-0.5 dB for 16-32 way) or spatial combining.

What about phase matching between PAs?

All PA outputs must be phase-matched for efficient combining. Phase error reduces the combining efficiency: for phase error delta_phi: combining loss = -10log10(cos²(delta_phi/2)). For 10 degrees phase error: loss = 0.07 dB (negligible). For 30 degrees: loss = 0.6 dB (significant). For 90 degrees: loss = 3 dB (half the power is wasted). Phase matching is achieved by: using equal-length transmission lines from all PAs to the combiner, matching the PA modules' group delay, and phase-trimming individual channels if needed.

What happens when a PA fails?

In a well-designed Wilkinson combiner: when one PA of N fails: the output power drops by more than 1/N because: the failed PA contributes zero power, and the power from the working PAs that would have combined with the failed PA's signal is absorbed by the isolation resistors. For a 4-way combiner with one PA failed: P_out = (3/4)² × 4 × P_PA = 9/4 × P_PA = 2.25 × P_PA (compared to 4 × P_PA for all working). The output drops by 2.5 dB (not 1.25 dB). Graceful degradation is better with: more PAs (the impact of one failure is smaller) and higher isolation (less interaction between working and failed PAs).

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