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How do I design a balanced power amplifier using Lange couplers for improved linearity?

Designing a balanced power amplifier using Lange couplers creates a PA with improved input and output VSWR (return loss), inherent stability, and the ability to absorb reflected power gracefully. The balanced amplifier consists of: two identical PA devices connected between two quadrature (90-degree) hybrid couplers, typically Lange couplers for microwave frequencies. The input Lange coupler splits the input signal into two equal-amplitude, 90-degree phase-shifted paths feeding the two PA devices. The output Lange coupler recombines the amplified signals at the output. The key advantages are: perfect input and output match (even if the individual PAs have poor match); any reflection from one PA is 180 degrees out of phase with the reflection from the other PA at the combined port, so reflections cancel and appear at the isolated (terminated) port, and the overall amplifier presents a well-matched impedance (VSWR less than 1.5:1 across the coupler bandwidth). Improved stability: because the input and output are well-matched regardless of the PA's impedance, the balanced amplifier is inherently more stable than a single-ended PA. Graceful failure: if one PA fails, the amplifier continues to operate at 6 dB lower gain (3 dB from the lost PA path, 3 dB from the coupler's reduced combining efficiency), and the surviving PA remains matched and stable. The Lange coupler is preferred for this application because: it provides 3 dB coupling with tight amplitude and phase balance over a multi-octave bandwidth (2:1 or wider), and it is compact and integrable on MMIC and PCB substrates.

Category: Power, Linearity, and Distortion

Updated: April 2026

Product Tie-In: Power Amplifiers, Combiners, Loads

Balanced PA with Lange Couplers

The balanced amplifier is one of the most widely used RF amplifier topologies, especially for wideband applications where maintaining a good match over the full bandwidth is challenging with a single-ended design.

Design Considerations

Coupler performance: The Lange coupler provides approximately 3 dB coupling, less than 0.5 dB amplitude imbalance, and less than ±3 degrees phase imbalance over a 2:1 bandwidth. This amplitude and phase balance determines the reflection cancellation quality. Amplitude imbalance of 0.5 dB: reflection cancellation approximately 15-20 dB improvement over single-ended
Termination: The isolated ports of both couplers must be terminated in accurate 50-ohm loads. These loads absorb the reflected power from the PAs. The load power rating must handle the maximum expected reflected power (up to half the PA output power in the worst case)

Balanced PA Parameters

Balanced amp gain: G_bal = G_PA - 0.3 dB (coupler insertion loss × 2)
Balanced amp return loss: RL_bal ≈ |S₁₁_PA| + coupling × 2 + balance
For S₁₁_PA = -10 dB, coupling balance ±0.5 dB: RL_bal > -25 dB
Bandwidth: determined by the Lange coupler (typically 2:1)
PA failure: gain drops 6 dB, output match maintained

Common Questions

Frequently Asked Questions

When should I use a balanced PA vs. single-ended?

Use balanced when: wideband operation is required (the balanced topology maintains matched impedance over the coupler's bandwidth, typically 2:1 or better), unconditional stability is needed (the balanced topology provides inherent stability margin), graceful degradation is valued (one PA can fail without total loss), and input/output VSWR specification is tight (return loss better than -15 dB across a wide band). Use single-ended when: narrow bandwidth (matching networks can be optimized for a narrow band), minimum loss (no coupler loss), minimum size (no couplers), and maximum efficiency (the 0.3 dB coupler loss reduces efficiency).

How do I design the Lange coupler?

The Lange coupler is an interdigitated microstrip structure. The design parameters are: substrate dielectric constant and thickness (determines the line widths and spacings), number of fingers (4 or more for tight coupling), finger width and spacing (calculated to achieve 3 dB coupling at the center frequency), and finger length (quarter-wave at the center frequency). Use electromagnetic simulation tools (ADS Momentum, Sonnet, HFSS) to optimize the coupler dimensions. The tight finger spacings (10-50 um for MMIC, 100-200 um for PCB) require precise fabrication.

What about using a Wilkinson instead of Lange?

A Wilkinson divider/combiner can replace the Lange coupler but: the Wilkinson splits signals in-phase (not quadrature), so the reflection cancellation does not occur (reflections add in-phase at the combined port). The Wilkinson provides matched, isolated splitting but does not improve the PA's return loss. The quadrature (90-degree) phase relationship of the Lange coupler is essential for the reflection cancellation that makes the balanced amplifier work. Alternative quadrature couplers: branchline coupler (narrower bandwidth than Lange, simpler fabrication), rat-race coupler (180-degree hybrid, different topology), and broadside coupler (stripline implementation, 2:1+ bandwidth).

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