How does an outphasing (LINC) amplifier work?

Linear Amplification using Nonlinear Components (LINC) takes a complex, varying-amplitude signal (like 5G OFDM) and mathematically splits it into two signals that have constant amplitude but varying phase. Because these two signals never change amplitude, they can be amplified by highly efficient, non-linear switch-mode amplifiers (like Class E). At the output, the two signals are combined. If they are perfectly in phase, they add up to maximum power. If they are pulled out of phase (outphasing), they partially cancel each other out, creating the lower amplitude needed for the complex waveform.

Why is a standard Wilkinson divider useless for outphasing?

A Wilkinson power combiner contains an isolation resistor between its two inputs. When you combine two out-of-phase signals in a Wilkinson, the out-of-phase energy does not go to the antenna; it gets dumped directly into the isolation resistor as heat. This completely destroys the efficiency of the outphasing architecture. The Chireix combiner is a 'non-isolating' combiner. It forces the two amplifiers to interact directly with each other, triggering active load modulation instead of burning the cancelled energy as heat.

What is the purpose of the reactive components in a Chireix combiner?

When two amplifiers interact out of phase, they see a highly reactive (imaginary) impedance. One amplifier sees a massive inductive load, and the other sees a massive capacitive load. This reactive mismatch kills amplifier efficiency. The Chireix combiner adds a specific shunt inductor to one branch and a shunt capacitor to the other. These static components mathematically cancel out the dynamic reactance at a specific 'back-off' power level, allowing both amplifiers to see a purely real (resistive) impedance and operate at peak efficiency exactly where the signal spends most of its time.

PA Architecture

Chireix Combiner

To achieve maximum electrical efficiency, an engineer decides to use highly non-linear Class E amplifiers for a 5G basestation. Because Class E amplifiers cannot amplify varying amplitudes, the engineer uses the "outphasing" (LINC) technique: splitting the complex signal into two constant-amplitude, phase-shifting vectors. However, when these two vectors are combined out of phase to recreate the low-amplitude dips, standard combiners burn the cancelled energy as heat, ruining the efficiency. The solution is the Chireix combiner. Invented in 1935, this non-isolating combiner forces the two amplifiers to interact. As their phases diverge, they actively modulate each other's load impedance. To fix the imaginary reactance caused by this interaction, the Chireix architecture adds carefully tuned shunt compensation components (one inductor, one capacitor). This ensures the amplifiers see purely resistive loads at deep power back-off, preserving ultra-high efficiency for modern complex waveforms.

Category: PA Architecture

Topology: Non-isolating active load modulation

Compensation: Static shunt reactive elements

Combiner Isolation Comparison

Combiner Type	Isolation Resistor	Out-of-Phase Behavior	Primary Application
Wilkinson Divider	Yes (100Ω)	Burns out-of-phase energy as heat	Standard linear power combining
Quadrature Hybrid	Yes (Dump Port)	Dumps out-of-phase energy to 50Ω load	Balanced amplifiers
Chireix Combiner	None	Reflects energy to actively modulate impedance	Outphasing (LINC) Amplifiers

Chireix Load Modulation:
Z₁ = 2R_L · (1 + j · cot(θ))
Z₂ = 2R_L · (1 - j · cot(θ))
Where θ is the outphasing angle between the two amplifiers. As they outphase to lower the output power, one amplifier sees a massive inductive load (+j), and the other sees a massive capacitive load (-j).

Shunt Compensation:
By adding a fixed shunt capacitor to branch 1 and a fixed shunt inductor to branch 2, the designer can perfectly cancel the +j and -j reactance at one specific outphasing angle θ_c. This creates a massive efficiency peak at that specific back-off power level.

Common Questions

Frequently Asked Questions

Why is LINC/Outphasing so highly efficient?

Because it completely removes amplitude modulation from the amplification stage. High-efficiency switch-mode amplifiers (like Class D or Class E) can only operate at a constant, maximum power level. Outphasing splits a varying signal into two constant-power signals. The amplifiers operate at peak 80% efficiency at all times. The amplitude variation is recreated purely by phase cancellation in the Chireix combiner at the very end.

Why does lack of isolation cause load modulation?

In a non-isolating combiner, there is no resistor to absorb reflected energy. When Amplifier 1 pushes voltage into the central node, Amplifier 2 "feels" that voltage. Because their phases are shifting relative to each other, the voltage Amplifier 2 sees is constantly changing. Ohm's Law (Z = V/I) dictates that if the voltage changes while the current remains constant, the apparent impedance (Z) must be changing. This dynamic impedance shift is the core mechanism of active load modulation.

Is Chireix better than Doherty?

They are competing architectures. Doherty is simpler to implement because it relies on amplitude-dependent splitting, making it the dominant architecture in current 5G hardware. Chireix outphasing can theoretically achieve higher efficiency over broader bandwidths, but it requires incredibly precise digital signal processing to perfectly control the phase vectors of the two amplifiers, making it much more sensitive to manufacturing tolerances.

Related Terms

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PA Architecture

Outphasing Efficiency Simulator

Input your target peak-to-average power ratio (PAPR) and compensation angle. Visualize the active load modulation trajectory on the Smith Chart and calculate the theoretical efficiency improvement of the Chireix architecture.

Simulate Chireix Outphasing