What is the difference between a 90-degree and 180-degree hybrid?

A 90-degree hybrid (quadrature coupler) splits the input into two outputs with equal amplitude and 90 degrees of phase difference. The branchline coupler is the most common planar realization: a square ring of quarter-wave transmission lines with impedances of Z0 and Z0/sqrt(2). It is used in balanced amplifiers, where the 90-degree phase relationship causes reflected signals from mismatched amplifiers to cancel at the input port and be absorbed by the termination resistor. A 180-degree hybrid (such as a rat-race ring) splits the input into two outputs with equal amplitude and either 0 or 180 degrees of phase difference, depending on which port is excited. It is used in balanced mixers, push-pull amplifiers, and sum/difference antenna feed networks.

Why is the Lange coupler preferred at microwave frequencies?

A branchline coupler achieves 3 dB coupling (equal split) easily, but its bandwidth is limited to about 10 to 20 percent because the quarter-wave sections are resonant. The Lange coupler uses interdigitated (interleaved) coupled lines to achieve tight coupling (3 dB) in a single section with much wider bandwidth (an octave or more). The coupled lines are only a quarter wavelength long, making the Lange coupler compact. Its challenge is fabrication: the interdigitated fingers must be very narrow (25 to 50 micrometers) with small gaps, requiring thin-film or MMIC processes. Bond wires or air bridges connect alternate fingers. Above 10 GHz, Lange couplers are the standard choice for balanced amplifier input and output networks.

How does a balanced amplifier use hybrid couplers?

A balanced amplifier places two identical amplifiers between a pair of 90-degree hybrid couplers. The input coupler splits the signal into two paths with 90 degrees phase difference, each feeding one amplifier. The output coupler recombines the amplified signals. Any reflections from the amplifier inputs (due to imperfect matching) travel back through the coupler and arrive at the termination resistor with equal amplitude and opposite phase, canceling each other. The result is that the balanced amplifier presents a well-matched 50-ohm input regardless of how poorly the individual amplifiers are matched. This makes the balanced topology essential for wideband amplifiers where achieving a good match from the transistors alone is difficult.

Passive Components

Coupler

A wideband GaN power amplifier has 6 dB return loss at its input: 25% of the incoming power reflects back toward the source. In a conventional amplifier, that reflected power reaches the driver stage and creates gain ripple. In a balanced amplifier, two 90-degree hybrid couplers bracket two identical PAs. The reflected signals from both PAs arrive at the coupler's termination port with equal amplitude and opposite phase, canceling completely. The input return loss improves to better than 20 dB regardless of the individual PA match. This is the power of couplers: they transform imperfect components into well-behaved systems through carefully engineered phase relationships.

Category: Passive Components

Types: 90° hybrid, 180° hybrid, Lange

Key Application: Balanced amplifiers

Choosing the Right Coupler Topology

Coupler Type	Phase Split	Coupling	Bandwidth	Size	Application
Branchline	90°	3 dB	10 to 20%	λ/4 × λ/4	Narrowband balanced amps
Lange	90°	3 dB	Octave+	λ/4 length	MMIC wideband amps, >10 GHz
Rat-race (ring)	0° or 180°	3 dB	10 to 20%	3λ/4 circumference	Balanced mixers, sum/diff
Coupled-line	90°	6 to 20 dB	20 to 40%	λ/4 length	Directional couplers, power monitoring
Riblet (short-slot)	90°	3 dB	15 to 25%	Waveguide	High-power radar, satellite

Branchline coupler (50 Ω):
Series arms: Z₀ = 50 Ω, length = λ/4
Shunt arms: Z₀/√2 = 35.4 Ω, length = λ/4

Balanced amplifier return loss improvement:
RL_balanced = Isolation + RL_amp (in dB)
Coupler isolation 20 dB + amp RL 6 dB = balanced RL > 20 dB

Lange coupler coupling:
C = 10·log[(Z_0e − Z_0o) / (Z_0e + Z_0o)]²

Common Questions

Frequently Asked Questions

90° vs. 180° hybrid?

90° (branchline, Lange): equal split with quadrature phase. Used in balanced amps where reflections cancel. 180° (rat-race): 0/180° split. Used in balanced mixers and sum/difference antenna feeds. Different phase relationships serve different architectures.

Why Lange at microwave?

Branchline bandwidth: 10 to 20%. Lange: octave+. Interdigitated coupled lines achieve tight 3 dB coupling in a single λ/4 section. Requires thin-film fabrication (25 to 50 μm fingers). Standard above 10 GHz for balanced MMIC amps.

How does a balanced amplifier work?

Two 90° couplers bracket two identical PAs. Reflections from PA inputs arrive at the termination with opposite phase and cancel. Result: excellent input match regardless of individual PA matching. Essential for wideband designs.

Related Terms

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