There is more than one way to combine the outputs of
amplifiers for more power. Perhaps the simplest device, and the one that is used
most often in antenna stacks, is the Wilkinson. However, the one I prefer for
amplifiers is the branchline Hybrid Coupler.
There are excellent write-ups on both types
www.microwaves101.com, but to put it
in a nutshell here, the branchline offers better bandwidth and isolation, and
the required 50 ohm termination is more common and inexpensive.
Isolation is useful to us when combining amplifiers; if one of
the amplifiers should fail, the other(s) will be protected by the coupler, and
will survive.
You can find examples of these couplers being used in several
of the other articles on this site (100w 23cm
"brick", 60w 23 cm Amplifier,120w
13 cm Amplifier, 300w 33cm Amplifier,
600w 23cm LDMOS amplifier, etc.
To
the right is the performance measured on the coupler used in a 33cm amplifier. A
snapshot of the actual coupler is below.
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