Search for quadrature
couplers on EverythingRF.com
here to go to our main page on couplers and splitters
here to go to our page on balanced amplifiers
here to go to our page on power combining
here to go to our page on reflection phase shifters
See below for our discussion
of the disappearing reflection coefficient
in a balanced amplifier.
A quadrature coupler is one in
which the input is split into two signals (usually with a goal of
equal magnitudes) that are 90 degrees apart in phase. Types of quadrature
couplers include branchline couplers (also known as quadrature hybrid
couplers), Lange couplers and overlay couplers.
Here's a clickable index to out
material on quadrature couplers and splitters:
coupler (separate page)
combining advantages on load pull effects (tres important!)
Balanced amplifier VSWR (New for March 2014!)
One excellent use of quadrature
couplers is to impedance match pairs of devices. The devices are
arranged so that reflections from them are terminated in a load
that is isolated from the quadrature coupler's input. This trick
is possible only because of the 90 degree (quadrature) phase difference
of the coupled and through arms.
When transistors are combined using quadrature couplers, this is called
a balanced amplifier. Quadrature couplers
are also used to make reflective attenuator devices (such as shunt
PIN diodes) become absorptive.
Another use for Lange or overlay
couplers is to form a diplexer, where
one port passes DC while the other port passes RF. This structure
can be used as a bias tee.
were very popular back in the 1970s within soft substrate stripline
boards. Recently they are being used again, in some of the new
multilayer media such as LTCC. The
problem with overlay couplers is that none of the CAD software
packages provides an easy way to design them. So you end up spending
a week with electromagnetic simulations in what would take five
seconds if it was a planar quadrature coupler.
Our page on coupled
line couplers continues this discussion.
Was Smitty really marooned
on the island all week long?
are often used to power combine
amplifiers, such a structure is called a balanced
amplifier. This allows an important degree of freedom, so
long as the amplifiers have the matched reflection coefficients
on inputs and outputs, the network will have good impedance input
and output impedance matches. What's going on? The figure below
will help you visualize it.
A signal of amplitude
"V" at phase angle zero enters the network, and splits.
The upper amplifier is excited with half of the signal (0.707
x voltage) at phase angle zero, while the lower amp see the same
signal but phase shifted -90 degrees.
happens in the ideal case where your two amplifiers have the
same reflection coefficient? The signal returned to the input
port cancels out, and all of the reflected power goes to the
See our page on balanced amplifier VSWR for an analysis of this phenomenon using Microwave Office.